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| application note tea7540 - speakerphone circuit for handsfree telephone set AN372/1197 by paolo aymon, claude ferry i introduction ....................................................... 1 ii hands-free system architecture .................................. 3 ii.1 reference schematics . . . . . . . . . . . . . . . . . . . . . . . ...................... 3 ii.1.1 tea7540 pin description . . . . . . . . . . . . . . . . . . . . . . . ......................... 3 ii.1.2 tea7540 block diagram. . . . ............................................. 4 ii.1.3 typical application . .................................................... 5 ii.2 main characteristics of a telephone set using tea7540 . . . . . . . . . . . . 6 ii.3 hands-free telephone set architecture. . . . . . . . . . . . . . . . . . . . . . . .... 6 ii.4 schematic blocks and operating modes. . . . . . . . . ................... 6 ii.5 additional features of tea7540. . . . ................................. 7 ii.5.1 low voltage operation and power saving with tea7540 . . . . . . ................. 7 ii.5.2 automatic level control with tea7540. ..................................... 7 ii.5.3 self adaptive compression with tea7540 . .................................. 8 ii.5.4 detection of typical noise conditions with tea7540 . . . . . . . . . . . . . . . . . . . . . . . .... 9 ii.5.5 flexible applications . . . . ................................................ 9 iii loop gain principle ................................................ 10 iii.1 voice-path loop-gain calculation . . . . . . . . . ......................... 10 iii.2 examples. .......................................................... 11 iii.3 volume control . . . . . . . . . . . . . . . . . . . . . . ............................. 12 iii.4 tx/rx switching control. ........................................... 13 iii.4.1 the idle/tx/rx comparator principle . . . . . . . . . . . . . . . . . . . . . . . ................ 13 iii.4.2 the selection of the hysteresis value . ..................................... 13 iii.5 timer. ........................................................... ... 14 iv circuit description ................................................ 15 iv.1 main functional blocks . ........................................... 15 iv.1.1 signal compressors . . . . ................................................ 15 iv.1.2 peak detector and noise monitor . . . . . . . . . . . . .............................. 15 iv.1.3 comparators . . . . ...................................................... 16 iv.1.4 attenuation control . .................................................... 17 iv.1.5 noise threshold hysteresis . . . . .......................................... 17 iv.1.6 chip select circuitry . . . . ................................................ 19 iv.2 pin description. .................................................... 20 iv.3 block diagram . .................................................... 21 iv.4 functional description of the pin allocation. . . . . . . . . ............. 22 v influence of the extermal components ........................... 26 v.1 gains and dynamics of the compressors. . . . . . . . . . . . . . . . . . . . . . . .... 26 v.1.1 gain and dynamic of tx compressor. . . . . . . . . . . . . . . . . . . . . . . ................ 26 v.1.2 gain and dynamic of rx compressor. . . . . . . . . . . . . . . . . . . . . . . ................ 26 v.1.3 tx/rx gain in handset mode . . . . . . . . . . . . . . . . . . . . . . . ...................... 29 1/44
v.2 attenuators and the volume control . . . . . . ....................... 29 v.2.1 programmable attenuation of the signal path (as). . . . . . . . . ................... 29 v.2.2 programmable attenuation of the tx/rx control path (hytx/hyrx). . . . . . ........ 33 v.3 rise and fall time . ................................................. 33 v.3.1 rise and fall time of the tx compressor . .................................. 33 v.3.2 rise and fall time of the rx compressor. .................................. 33 v.3.3 rise aand fall time of the tx peak detect . . . . . . . . . ......................... 36 v.3.4 rise and fall time of the rx peak detect . .................................. 36 v.3.5 delay time between tx/rx and idle mode transitions. . . . . . .................... 36 v.3.6 delay time between tx and rx mode transitions . . . . . . ....................... 36 vi applications ....................................................... 40 vi.1 hands-free particular effects . . . . . . . ............................. 40 vi.1.1 the olarseno effect . .................................................... 40 vi.1.2 ochopped speecho effect . . . . . . . . . . . . . . . . . . . . . . . ......................... 40 vi.1.3 owalky-talkyo effect . .................................................... 40 vi.1.4 volume control influence . . . . . . . . . . . . . . . . . . . . . . . ......................... 40 vi.1.5 examples . . . . . . . . . . . . . . . . . . . . . . . . .................................... 40 vi.2 basic hands-free telephone set application . . . . . . ................. 41 vi.3 gains compatibility in monitoring and hands-free mode . . . . . . . . . . . . 42 vi.4 hands-free option . . . . ............................................. 44 contents (continued) page i - introduction this note discusses the principles and feature of a hands-free telephone set using tea7540, the de- scription of tea7540 ic, design applications, and the set-up procedure. supply : tea7540 can be : - powered directly from the telephone line due to its low consumption (2.1ma typically) and mini- mum operating voltage (2.5v). - powered from a stabilized voltage source from 2.5v to 7v. the current consumption is from 2.1ma up to 2.8ma when the voltage supply varies from 2.5v to 7v. duplex voice paths : the handset allows 2-way simultaneousconversa- tions (transmit and receive). this is commonly called the ofull duplex operationo. in the hands-free telephone set case, the loop that exists within the system forcesthe introduction of an attenuation into the path which is detected as the dominated voice path. only one person speaks at a time, while the other listens. this is the case of current speakerphones available on the market. they are called ohalf duplex operating systemso. tea7540 performs the switching function needed for speakerphone operation. it allows a good ap- proximation of full duplex operation by attenuation switching optimization and by gain control for the transmit and receive voice paths. tea7540 has also a system which enables con- stant backgroundnoise to be ignoredin makingthe transmit/receive switching decision. tea7540 voice-switched speakerphone circuit in- corporates compressor-amplifiers, attenuators, level detectors, and a switching control system. it can also be operated in the handset mode with a chip select command which bypasses the switching functions associated with the hands-free mode. each input of tea7540 includes a compressor pre-amplifier. this feature allows the transmitted line level to be independent of the distance be- tween the speaker and the microphone. similary, the received level does not depend on the tele- phone line charcateristics. specially designed for basic low cost hands-free telephone set applications, tea7540 provides a control interface between the speech ic, the hands-free microphone, and the loudspeaker ic. the interface can also be extended to a microcom- puter with more sophisticated operating facilities and controls. tea7540 application note 2/44 ii - architecture description ii.1 - reference schematics ii.1.1 - tea7540 pin description no pin description 1 gnd ground 2 cs chip select 3 oute transmit attenuator output 4 atte transmit attenuator input 5 hyste transmit hysteresis 6 hystr receive hysteresis 7 tim timer 8 cce transmit compressor time constant 9 ce3 transmit background noise memorization 10 ce2 transmit peak detector 11 ce1 transmit rectifier input 12 micout transmit compressor output 13 ine transmit compressor input 14 vref reference voltage 15 inr receive compressor input 16 recout receive compressor output 17 cr1 receive rectifier input 18 cr2 receive peak detector 19 cr3 receive background noise memorization 20 ccr receive compressor time constant 21 amp2 microphone pre-amplifier input 22 amp1 microphone pre-amplifier supply 23 vol volume control 24 atr attenuator control 25 attr receive attenuator input 26 outr receive attenuator output 27 iref reference current 28 v+ power supply input tea7540 application note 3/44 ii.1.2 - tea7540 block diagram ii - architecture description (continued) v ref c4 8v+ c10 cee v ref r9 c12 15 20 2 3 4 7910 16 17 18 19 27 23 11 14 22 24 r2 p1 vol atr r45 v+ transmit attenuator microphone amplifier atte 21 26 power supply amp1 outr handset microphone v ref transmit receive speech/noise speech/noise comparator speech/noise comparator transmit/receive comparator 8mv 25 5 16 28 gnd v+ attr hyste hystr r5 r6 noise memo peak detector preamp preamp noise memo peak detector 13 12 ine handsfree microphone c3 ccr inr from 2/4 wires recout c1 cr1 cr2 r3 v+ c15 v ref v ref r7 tim c9 ce3 ce2 c11 r8 v ref v+ c5 micout ce1 tea7540 cs oute amp2 i ref cr3 differential hysteresis hyt hyr timer c0 r1 v+ 8mv attenuation control receive attenuator noise threshold hysteresis hysth logic reference 100mv transmit compressor receive compressor r4 AN372-01.eps figure 1 tea7540 application note 4/44 ii.1.3 - tea7540 typical application for more information, see application section. ii - architecture description (continued) c5 c8 c12 r6 1 2 3 4 5 6 7 8 9 10 16 17 18 19 20 26 27 28 11 12 13 14 15 21 22 23 24 25 r2 r1 p1 r45 r5 c10 r7 c9 r8 c11 cs r9 hands-free microphone handset microphone v ol atr i ref gnd cs hyste hystr cce tim ce3 ce2 ine v re f amp2 v+ oute outr amp1 inr attr cr1 recout atte ce1 micout ccr cr3 cr2 t e a 7 5 4 0 c1 c3 r3 c15 r4 c4 to speech circuit to loudspeaker amplifier up. supply from speech circuit v+ AN372-02.eps figure 2 ii.2 - main characteristics of a telephone set using tea7540 tea7540 is easily supplied from the line. tea7540 has an automatic level control, called compressor, in each channel : tea7540 is self- adaptivedependingon line (receive line losses due to line length variations) and room conditions (dis- tance variations between microphone and speaker). tea7540is not affected by typical noise conditions in each path. programmable attenuato rs are provided sepa- rately in each voice path. chip select allows hands-free and monitoring op- eration separately. microphone preamplifier for handset mode is pro- vided on-chip. different national standards are easily matched by varying the values of a small number of external components. tea7540 application note 5/44 ii.3 - hands-free telephone set architecture a complete hands-free telephone set design incor- porates the following circuits : - tea7540 - speech circuit - loudspeaker amplifier ii - architecture description (continued) speech line rx tx cs loudspeake r amplifier hands-free tea7540 loudspeaker hands-free microphone handset earpiece microphone c b a a : handset a + c : monitoring b + c : hands-free AN372-03.eps figure 3 : simplified schematic of an hands-free telephone set the chip select switch (pin2) is used to select one of the two basic operating modes of tea7540 : handset mode a) for monitoring operation in the receive path is introduced a programmable gain and in the transmit one a fixed gain (20db). b) the handset microphone is enabled and the hands-free microphone is disabled. hands-free mode a) the handset microphone is off and the handsfree microphone is on. b) the attenuation controls are operative in the transmit and receive paths. ii.4 - schematic blocks and operative modes the circuit basically contains 5 blocks : (figure 4) - tx compressor : transmit pre-amplifier with vari- able gain - tx attenuator: programmable transmit attenuator - rx compressor : receive pre-amplifier with vari- able gain - rx attenuator: programmablereceive attenuator - control block : attenuation control system the control block monitors the level of transmit/re- ceive signals and provides a set of control signals which switch the system into one of the three following operational modes : transmit mode - the tx attenuatorhas a constant attenuation (0db), - the tx compressor maintains constant its output v1 (200mv pp ), - the rx attenuation provides the programmed attenuation, - the rx compressor is disabled. receive mode - the rx attenuatorhas a constantattenuation(0db), - the rx compressor maintains constant v2 (200mv pp ), - the tx attenuation provides the programmed attenuation, - the tx compressor is disabled. idle mode - the loop attenuation (as, programmable by pin 24) is distributed equally (as/2) between the transmit and receive attenuators. 3 26 13 15 23 24 compressor attenuator g1 at attenuation control ar g2 ras gl loudspeaker amplifier tea7540 acoustic coupling transmit gain gt 2/4 wires gr a1 sidetone coupling line receive again speech circuit vol a2 attenuator compressor AN372-04.eps figure 4 : loop gain principle tea7540 application note 6/44 telephone housing balance network volume control room a a a e rx tx a complete hands-free system stability condition (gr + gt) < (aa + ae) exchange line room line telephone housing gt gr AN372-05.eps figure 5 : howling effect due to acoustic coupling and to receive sidetone ii.5 - additional features of tea7540 ii.5.1 - low voltage operation and power saving with tea7540 a) efficient operation over a wide range of supply voltage (2.5v to 7v) to fulfil different national standards. the consumption is at low current even at maximum supply voltage ensuring low power consumption. in some countries the voltage available over the telephone set is as low as 5v and the minimum line current is lower than 20ma. tea7540 allows line powered application due to its minimum operating voltage, 2.5v, and operating current, 2.1ma. b) background noise immunity : tea7540 has a system which enables constant background noise to be ignored in making the selection of the appropriate operating mode : transmit/ receive/ idle (see ii.5.4 section). the backgroundnoise controlof tea7540keeps the system in idle mode until the level of the signal is higher than the noise threshold, in this case the system switches in receive or transmit mode. c) half duplex/full duplex : tea7540 allows a good approximation of full duplex operation by attenuation switching optimization and by gain control for the transmit and receive paths. d) stability : the attenuationcontrol of tea7540 distributes the required loop attenuation between the transmit or receive channels according to the mode of operation: transmit receive or idle. attenuationis required to avoid instability which appears when the total loop gain (electronic gain + transducer + acoustic coupling gains) is higher than 0db. stability condition : (gr+gt)<<(aa+ae) (see figure 5) ii.5.2 - automatic level control with tea7540 tea 7540 has two signals compressors which per- form an automatic level control of the tx and rx signals and provide constant amplitude voltage outputs (200mv peak-peak). a) the transmit compressor compresses the hands-free microphone signal in such a way that the distance between the speaker and the microphonecan be from 0.5 up to 3 meters.this implies : see figure 6. b) the line level speech depends on the line length. the receive compressor compresses the receive signal from speech circuit to compensate the looses due to line length. this implies : see figure 7. ii - architecture description (continued) tea7540 application note 7/44 0.5m 3m v 0 = 200mv pp v 1 = 11.2mv pp v 1 = 1.7mv pp g 1 =g 1(min.) = 25db v 0 = 200mv pp g 1 =g 1(max.) = 41.5db AN372-06.eps figure 6 : transmit compressor range g 1max -g 1min = 16.5db speech circuit line speech circuit line v 2 = 32mv pp v 2 = 3mv pp v 0 = 200mv pp v 0 = 200mv pp g 2 =g 2 (max.) = 36.5db g 2 =g 2 (min.) = 16db AN372-07.eps figure 7 : receive compressor range g 2max -g 2min = 20.5db ii - architecture description (continued) ii.5.3 - self-adaptative compression with tea7540 the compressors in the inactive (dominated) chan- nel works at a fixed gain which can never increase but may decrease following an increase of domi- nated channel signal. a) behaviourof a dominantchannel compressors: see figure 8 250mv pp 200mv pp 1.7mv pp 11.2mv pp input tx compressor output signal tx 250mv pp 200mv pp 3mv pp 32mv pp input rx compressor output signal rx dynamic = 20.5db dynamic = 16.5db a n372-08.eps figure 8 : compressor gain versus input signal b) behaviour of a dominated channel compressors : see figure 9. as shown in figure 9 when the rx (or tx) compres- sor input voltage decreasesthe gain is always held at its previous value. when this voltage increases to a value higher than previous value, the gain decreases. gain g (max.) input level level before switching g (min.) a n372-09.e ps figure 9 tea7540 application note 8/44 compressor tx background noise envelope smoothing speech envelope tracking attenuator tx mode selection block subtractor background noise envelope smoothing speech envelope tracking subtractor control attenuation block attenuator rx compressor rx AN372-10.eps figure 10 : background noise/speech discrimination ii.5.4 - detection of typical noise condition with tea7540 (figure 10) tea7540 is able to : - recognize the presence of a speech over a wide range of noise levels in both transmit and receive channels. - select the appropriate mode, transmit, receive or idle, by extracting and comparing the burst signal component (speech), of both the tx and rx chan- nels, while rejecting the long-term steady-state noise components. - switch , such as appropriate mode, in the trans- mit, receive or idle. this involves appropriate switching of tx and rx channel attenua tion values. ii.5.5 - flexible application - all the different time constants are externally programmable. - attenuation valuesare externallyprogrammable. - the same application can be used in monitoring mode (group listening) via the chip select pin. ii - architecture description (continued) tea7540 application note 9/44 iii - loop gain principle (see also section v.2) iii.1 - the voice-path loop-gain calculation the hands-free telephone circuit contains a loop comprising the transmit and receive path amplifiers and attenuators, in association with sidetone and acoustic coupling attenuation (figure 4). the loop- gain,lg, must be lower than 0db to avoid instability. loop gain = lg = g1 - at +gt - a1 + gr + g2 - ar + gl - a2 (db) <0db the compressor gains, g1 and g2, may be ex- pressed in the form: g1 = g1max - dg1 (db) g2 = g2max - dg2 (db) intheseequations,g1maxand g2maxare constant gain values, and dg1 and dg2 represent effectively attenuationsintroduced by the compressor amplifi- ers to handle signals larger than minimum. 0 < dg1 < 16.5db 0 < dg2 < 20.5db in the basic loop-gain equation at represents the attenuationintroduced in the transmission channel when the circuit is in either the receive or idle modes. similarly ar represents the attenuation introduced in the receive channel when the circuit is in the transmit or idle modes. note : in receive mode ar = 0 in transmit mode at = 0 in idle mode the total attenuation is equally shared between two channels. conceptually, the circuit of figure 4 may be re- drawn as shown in figure 11 where g1max, gt, gr, g2max, and gl represent amplifiers of fixed gain, and the dynamic components dg1, at, dg2, and ar consitute an attenuatorblock (as). the loop-gain equation may now be written in the form : lg = (g1max+g2max+gt+gr+gl)-(a1+a2)- (at+ar+dg1+dg2) hence : loop gain = const(gain) - const(att) - (at+ar+dg1+dg2) or, alternatively : loop gain = constant - (at+ar+dg1+dg2) let the total controlled attenuation be represented by : as = at + ar for dg1 and dg2 #0 loop gain = lg = constant - (as+dg1+dg2) ..(1) the total dynamic attenuation, as, is shared be- tween two controlled attenuators, one each in the receive and transmit channels, according to the mode of operation. let as0 = at0 +ar0 for dg1 = dg2 = 0, i.e. for maximum compressor gains. as0 is the total at- tenuation inserted by the attenuation controlblocks in the two paths in function of the operating mode, in particular : tx mode rx mode idle mode at0 0db as0 as0/2 ar0 as0 0db as0/2 theas0valueisprogrammedby an externalresistor between10dband 60dbas table1 shows. the as0 value must be choosen in order to guarantee the stability condition (lg0 < 0db) for all the operating modesand with all the signal levelconditions.it must compensatethe sum of the electrical and acoustical coupling and the constant gains. let lg0 the corre- sponding value of the loop gain. then : loop gain = lg0 = constant - as0 << 0db ...... (2) for any other compressor gain/operating mode conditions we may write, using the expression (1) and (2) : lg - lg0 = (as0-as) - (dg1-dg2) .................. (3) when the as0 value has been choosen tea7540 will control the value of as = at + ar so that lg - lg0 = 0 for all operative conditions of compres- sor gain (i.e. all values of dg1 and dg2), and all modesof operation(i.e.all possiblemodesofsharing the total controlled attenuation,as, betweenthe two paths, in this way the system will approach to a full duplex behaviour because tea7540 will keep al- ways lg at the max value guaranting the stability. from rule 3 we have : as = as0 - dg1 - dg2 as is a parameter time varying in function of level signals and shared between receive and transmit paths in function of the operating modes. dg1 at ar dg2 a2 gt gr as gl g2 max g1 max a1 AN372-11.eps figure 11 : attenuationsblock principle tea7540 application note 10/44 this providesthat lg is alwaysas closeas possible to lg0 value. thus we have: a) in transmit mode we have zero attenuation in the transmit channel, at = 0 and hence : ar = as0 - dg1 - dg2 note : remember that the gain of the maximum receiving compressor (g2) is frozen at its previous value before the switching, and dg2 varyes as explained in section ii.5.3 whereas dg1 is varying accordingtoinputmicrophonesignal.consequently when the system is in tx mode and the level of microphone is increasing the compressor gain decreases. tea7540 will decrease as on the receive path by an equalamount. b) in receive mode we have zero attenuation in the receive channel, ar =0 and hence : at = as0 - dg1 - dg2 note : remember that the gain of the maximum transmitting compressor (g1) is frozen at its previous value before the switching, and dg1 varyes as explained in section ii.5.3 whereas dg2 is varying according to the tx input signal from the line. in the same previous way when the system is in rx mode and the level on the line signal is increasing the compressor gain decreases. tea7540 will decrease as on the transmitting path by an equal amount. c) in idle mode the attenuation is shared equally between the two channels and hence : at = ar = as/2 = ( as0 - dg1 - dg2 )/2 where dg1and dg2 are frozen to them previous values. in tea7540theattenuatorcontrollerprocessesthe compressor signals (which provides a measure of the values of dg1 and dg2) and produces control voltages which set the attenuation levels at (receive mode), ar (transmit mode), or both at and ar (idle mode) so as to ensurestability under all conditions of operation. table 1 : table of programmable attenuationas0 r pin 24 programmable attenuation as0 r pin 24 programmable attenuation as0 0k 10.4db 8k 40.2db 1k 13.2db 9k 44.3db 2k 17.0db 10k 48.0db 3k 20.3db 11k 52.0db 4k 24.3db 12k 55.0db 5k 28.4db 13k 57.7db 6k 32.5db 14k 60.2db 7k 36.4db 15k 62.4db with g1, g2 = max and dg1, dg2 = 0 iii.2 - examples in the following examples (figures 12, 13 and 14) the circuit is in receive mode with required total attenuation values of as0 = 30db, as0 = 37db, and as0 = 50db (depending on different values of the constant). a resistance value of approximately 5.5k w con- nected to pin 24 yields a maximum attenuation of 30db with both compressors operatingunder maxi- mum gain conditions. with the compressor gains reduced by a total of dg1 + dg2 = 30db the attenuation at is reduced to its minimum possible value of 0db. with any further decrease of com- pressor gains the loop gain is reduced by up to 7db (see figure 12). in this case (figure 13) the value of as0 is set to a value of 37db (resistance value of 7.2k w con- nected to pin 24) with both compressors operating at maximum gain. the attenuation is reduced by 1db for each decrease of 1db in total compressor gain up to the maximum of 37db. in this case (figure 14) the value of as0 corre- sponding to the maximum compressor gain setting is adjusted to 50db (using a value of about 10.5k w connected to pin 24). although the available dy- namic range of attenuation values is 52db the actual attenuation range is limited to 37db. iii - loop gain principle (see also section v.2) (continued) 10 20 30 40 50 10 20 30 40 0 at (db) dg1 + dg2 (db) 10 20 30 40 50 dg1 + dg2 (db) 0 0 lg (db) -10 -20 -30 aso lgo AN372-12.eps figure 12 : example 1. as0 = 30db with the const. = 24db tea7540 application note 11/44 10 20 30 40 50 10 20 30 40 0 at (db) dg1 + dg2 (db) 10 20 30 40 50 dg1 + dg2 (db) 0 0 lg (db) -10 -20 -30 aso lgo AN372-13.eps figure 13 : example 2. as0 = 37db with the const. = 31db 10 20 30 40 50 10 20 30 40 0 at (db) dg1 + dg2 (db) 10 20 30 40 50 dg1 + dg2 (db) 0 0 lg (db) -10 -20 -30 aso lgo 50 AN372-14.eps figure 14 : example 3. as0 = 50db with the const. = 44db iii - loop gain principle (see also section v.2) (continued) iii.3 - volume control the receive channel contains a volume control which is active only in receive mode. tea7540 attenuatorcontrolunitadjusts theattenuationsin the transmit and receive channels so that the overall loopgain remainsconstant.in the receive mode any additionalattenuationavol added (or substracted) in the rx channel is compensated by an equal attenuationsubstracted (or added) in tx channel. in the transmit mode, however, the attenuation at remainsfixed at 0db and variations in ar are hence not compensated. the loop gain remains < 0db because the stability (see rules in section iii.1) is guarantedwith the volume control attenuatorset to the minimum value (avol = 0db). in the receive mode the rules for at and ar become : ar = 0db + avol at = as - avol an external potentiometerconnected to the pin 23 fixes the value of the attenuation of the volume controlcircuit (avol)in the rangefrom 0db to 33db. tea7540 application note 12/44 iii.4 - tx/rx switching control iii.4.1 - the idle/transmit/receive comparator principle (see section iv.1.3) animportantfunctionalrequirementfor ahandsfree telephone system relates to the switching between idle, transmit, and receive modes. this switching should respond rapidly to the sud- den increase of signal which occurs with the onset of speech, without reacting improperly to the brief periods of interword silence which occur in normal connected speech. the associated control is effected by comparing the amplitudes of the signals produced by the two com- pressor/amplifier units for the receive and transmit channels.for each channel,twosignals arederived, thefirst being proportionaltothe peakamplitude,and the second to the mean-squared amplitude, of the signal produced at the compressor output. the peak amplitude signal, vpeak, produced by a fast-acting (short time-constant) detector circuit gives a measurement of the speech signal with its burst nature. the mean-squared amplitude signal, v noise , produced by a long time-constant circuit, gives a measure of the noise signal which is of an essentially constant nature. the role of the comparator circuit is to recognize the channel in which a burst-type (speech) signal first occurs. a hysteresis component (attenuator)is then switched intothe oppositechannelto effectivelyraise the threshold of the quiscent (dominated) channel with respect to the active (dominating) channel. iii.4.2 - the selection of the hysteresis value simplifying assumptions a) we will consider the case where the noise components in both channels are zero. in practice the comparison circuit works with respect to the differencesignal, v peak -v noise . to cater for this case, the hysteresis value may be increased beyond 6db by externalmeans at the disposal of the designer as appropriate to the needs of his particular application. this is achieved by connecting external resistances between pin 6 and earth (rx channel hysteresis). the user has been offered this option as a preferable alternativeto a built-in larger internal hysteresis, since it allows him to tailor circuit performance closely to the needs of his particular application. by way of example, this facility enables a quasi full-duplex mode to be realized since both the overall loop attenuation and also the individual hysteresis values may be held at the minimum possible values. the completely individual flexibility offered by the sgs-thomson chip-set enablesthe designer to produceeconomical,viabledesignstocovera wide number of cases, e.g. analog telephone systems with loop current power supplies, digital telephone systemswithextremelyhighsidetonerejectionand high loudspeaker efficiences, or long-line systems with low loudspeaker efficiency. all of these examplesmay be satisfactorilyotunedoby external adjustment of the hysteresis values. the value of the external resistor (r5 or r6) required to fine tune the hysteresisvalue for any particular application may be chosen using the following table as a guide. hyt = 6db + h ot , hyr = 6db + h or r pin 5 - r pin 6 h ot -h or 1m 0db 14k 3db 5.6k 6db 3.3k 9db 2.3k 12db b) becauseof the essentialsymmetry of the tx and rx channels we will consider only the case of the tx dominant mode, identical conditions of operation occuring for the rx dominant mode. 1 - normal operating mode (figure 15) in the normal operational mode the input signal in the dominating channel (tx) is assumed to lie within the dynamic operating range of the compressor amplifier, and hence the compressor output voltage is 100mv, independentlyof the actualsignal input. in this case with compressor outputs in both chan- nels of 100mv, a hysteresis value of 6db in the dominatedchannel (rx)will produce a good margin undesired speech chopping produced by improp- erly mode switches. accordingly sgs-thomson has placed a fixed value of 6db in the circuit. 2 - abnormal range of operation (figure 16) this case relates to the situation where the input signal is beyond (below) the dynamic operating range of the compressor and the tx compressor output is less than the normal value of 100mv. this may arise in two cases : a) there is relatively large separationbetween the sender's mouth and the handsfree system microphone. b) in the intersyllabe and interword periods of silence in normal connected speech. in a typical example, with a dominant channel output of 50mv a hysteresis value of 6db may be not adequate. iii - loop gain principle (see also section v.2) (continued) tea7540 application note 13/44 g1 at v1 100mv pp tx compressor hyt hyt 0db 6db 100mv 100mv 100mv 50mv(max.) g2 100mv pp (max.) rx compressor tx/rx tx rx speech sidetone line a n372-15.eps figure 15 g1 at v1 50mv pp tx compressor hyt hyt 0db 6db 50mv 100mv 50mv 50mv(max.) g2 100mv pp (max.) rx compressor tx/rx tx rx speech sidetone line AN372-16.eps figure 16 iii - loop gain principle (see also section v.2) (continued) iii.5 - timer the timer in figure 17 is designed to allow rapid transitions to the transmit or receive modes from any operating mode (transmit/receive/idle), and relatively slow transitions to the idle mode from either transmit or receive mode. the rapid transition to one of the active modes, transmit or receive, is required in order to avoid the loss of the beginning of a segment of speech. this is accomplished by the timer providing, as appro- priate, a charge/discharge current to the external capacitor (t = cv/i). the slow transition is required to avoid switch in the idle mode in the short periods of silence between individual spoken words. in the idle mode the timer provides no charge/discharge current, and the ex- attenuation control timer vt vr r7 v ref comparator c9 7 a n372-17.eps figure 17 : timer simplified schematic ternal capacitor is charged/discharged via the ex- ternal resistor with an effective time constant of about 1 second (t = 2.2 rc). tea7540 application note 14/44 iv - circuit description iv.1 - main functional blocks iv.1.1 - signal compressors the signal compressors are managed by the cir- cuitry shown in figure 18. the microphone input signal plus the transmit channelinput noise are amplified by a variablegain agc amplifier (labelled compressor in figure 16) to yield an output signal with a mean envelope amplitude of 100mv pp . the peak detector and noise/speechcomparator are used to compare the noise/speechamplitude to a reference dc voltage of 100mv, and to generate a difference voltage which controls the switch labelled k in figure 16. the position of switch k controls the discharge or charge of external capacitor c10 by currents of 100 m a or 1.25 m a respectively. the integrated volt- age developed across c10 is amplified by the v/i converter and produces an agc voltage to rapidly (typically within 22ms) decrease the compressor gain, and to slowly (typically within 1.8s) increase the compressor gain (see section iv.1). the agc system has hence a dynamic response which reacts quickly to the increase of amplitude at the beginning of each word, but decays slowly at the end of a word. this feature prevents the system from increasing the gain, and hence noise-level, in the periods of silence between successive words in a passage of connected speech. without this feature, the variations of noise level which could occur within a passage of connectedspeech would be extremely disagreeable to the listener. the behaviour of a disabled compressor is de- scribe in section ii.5.3. the compressor of inactive part holds its previous gain and the capacitor (c9 in tx path, c10 in rx path)is disconnectedfromthe currentsources.the voltagedecreasing(pin 8 txor pin 20 rx) depends on the quality of the capacitor. use a capacitor with as low leakage current as possible. iv.1.2 - peak detector and noise monitors the circuit of figure 19 shows the method of gen- eration of two signals, vtx and vntx (or vrx and vnrx). signal vtx represents the envelope ampli- tude of the combined noise and useful signal at the compressor output, whereas vntx represents the envelope amplitude of the noise component only. hence if vtx = vntx the compressor output is a noise output solely (no transmit signal present). however, if vtx > vntx the transmit channel com- pressor output contains a useful signal component, either speech or dialling tone. output vtx is equal to the voltage across capacitor c12 which may either be charged rapidly by a current of 20 m a if transistor t1 is cut off, or discharged slowly viar1 when transistort1 isdriven into saturation.the output of comparator amplifier a1 is used to drive t1 intocut-offwhentheinstantaneouscompressoroutput isgreaterthanvtx anddrives t1 intosaturationwhen the compressor output falls below the value of vtx. hencevtxisa voltagethatrapidlyfollowsanyincrease of the compressor's noise plus signal output and fol- lows any decrease of that outputwith an time constant fixed by the external r1 c1 values (typically 100ms). the signal vtx will hence contain a long-term steady- state componentequivalentto backgroundnoise, plus short-termtransient (burst) componentswhich follows thevariationsof amplitude dueto speechsignals in the compressor output. these latest componentsmay be eliminatedbythe similar circuit comprising t2,a2,and timing componentsr8 and c11. these timing compo- nents define a much longer time-constant of approxi- mately 10s resulting in an output vntx which effectively contains only the long-term components of the compressor amplifier output. the additionalcomponentscomprising the compa- rator amplifier a3 and transistor t3 limit the maxi- mum value of vntx to 36mv. this feature enables the circuit to identify the case where the useful signal is a dialling tone. in this case vntx will be limited to a value of 36mv whereas vtx will in- crease to a greater value. the amplitude relationship, vtx > vntx, corre- sponding to the presence of a useful signal com- ponent in the compressor output, is hence satisfied in this case also. 11 12 13 microphone attenuation control hyst1 c5 attenuator at agc v/i converter peak detector comparator 100mv 8 v+ c10 k v+ 100 m a 1.25 m a v+ compressor AN372-18.eps figure 18 : signal compressors tea7540 application note 15/44 9 10 36mv t3 t2 noise limiter vntx (vnrx) a2 a3 noise v ref v+ r8 (r3) (19) c11 (c15) vtx (vrx) speech + noise (18) r9 (r4) c12 (c11) v+ 20 m a t1 a1 11 (17) peak detector signal from compressor v ref AN372-19.eps figure 19 : peak detector and noise monitors (see also section iv.3.3) iv - circuit description (continued) iv.1.3 - comparators the comparator circuits in figure 20 determine the specific modes of operation : speech/noise and transmit/receive. if vtx exceeds vtnx by more than 8mv, a useful signal componentis consideredto be present in the transmit channel compressor output. similary if vrx exceeds vnrx by the same margin a signalcomponentispresentin thereceive channel. if either or both of these conditions are true the or gate output is driven high (positive logic true condition) and the speech/noise (i.e. speech/noise mode) output is set to 1. idle mode is set when both vstx and vsrx are less than 8mv. the transmit/receive mode signal output is deter- mined by a comparison of the transmit channel signal vstx = vtx - vntx with the receive channel signal vsrx = vrx - vnrx. before this comparison the voltage representing the useful signal in the dominated channel is at- tenuated by either hyt (transmit channel) or hyr (receive channel), whereas the signal in the domi- nating channel is passed to the comparator input without attenuation (i.e. hyt = 0db for tx domi- nant ; hyr = 0db for rx dominant). the status of tea7540 ic's can be resume by the following true table : noise/speech transmit comparator 1 x 1 0 0 noise/speech receive comparator x 1010 transmit/receive comparator 1 0 1 0 x tea7540 status t r t r i t : transmit r : receive i : idle tea7540 application note 16/44 8mv speech/noise timer (tea7540 status) transmit/receive vtx vntx vrx vnrx 5 noise/speech transmit comparator noise/speech receive 8mv vstx = vtx - vntx comparator vsrx = vrx - vnrx hyt* hyr* vt vr 6 vstx r5 vsrx r6 comparator * hyt = 6db+hot * hyt = 6db+hor a n372-20.e ps figure 20 iv - circuit description (continued) iv.1.4 - attenuation control the organization of the attenuationcontrolcircuitry is shown in figure 21. as described in sectionsiii.1 and iii.2. the attenuation is required to control the transmit and receive channel attenuators, a t and a r ,in function of operating mode, the compressor ampli- fier gains, and the receive channel volume control setting. in transmit mode we have zero attenuation in the transmit channel, a t = 0, and hence : a r = as0 - dg1 - dg2 in receive mode we have zero attenuation in the receive channel, a r = 0, and hence : a t = as0 - dg1 - dg2 in idle mode, the attenuation is shared equally between the two channels and hence : a r =a t = as/2 = (as0 - dg1 - dg2)/2 the controller block in figure 21 develops the required outputs to control a t and a r , in terms of the agc inputs dg1 and dg2 (which depend on the compressor amplifier gains), the mode control sig- nals, the receive channel volume control setting, the maximum value of total attenuationas0 (deter- mined by the externalresistor) and the timeroutput voltage, v tim . the timer is required to control the speed of transi- tion between idle, transmit, and receive modes. the transition into transmit or receive (i.e. speech) mode must be quick enough in order not to miss the beginning of a segment of speech. transition from transmit or receive mode to the idle mode must be slow in order to avoid an increase of noise level between successive words of a continuous passage of speech. these changes have an appropriate time constant by discharging the capacitor c9 by acurrent of 50 m a or by charging the same capacitor through the resistor r7 in figure 21. this selection is con- trolled by the timer circuit by setting the switch k. the timing diagram of figure 22 illustrates the dynamic response of the attenuatorvalues, a t and a r , in the two cases of a transitionbetween rx and tx and of transition from rx mode to idle mode. iv.1.5 - noise threshold hysteresis figure 23 shows the noise threshold hysteresis circuit which affects the dynamic response for changes from an active mode to the idle mode (the parameters of this circuit are definied internally and are not programmable by external components. this circuit inserts additionalattenuation into either vstx or vsrx, according to the appropriate oper- ating mode. this attenuation acts in conjunction with the timer signals to slightly delay any change from an active to the idle mode and leads to an optimization of the dynamic response of the system. tea7540 application note 17/44 at at transmit/receive speech/noise dg1 dg2 timer ras (r2) v ol (p1) transmit attenuator receive attenuator v+ 50 m a v ref 220mv k 7 v tim v ref c9 r7 attenuation control 24 23 220mv 50 m a AN372-21.eps figure 21 : attenuationcontrol tx mode rx mode v ref v ref + 220mv vtim v ref - 220mv at 0db attenuation as ar idle as/2 tx/rx 1 0 1 0 speech/noise AN372-22.eps figure 22 : transmition diagram among tx-rx, idle mode (see also section iv.3) speech/noise transmit/receive or hystn vstx t from timer vsrx AN372-23.eps figure 23 : noise threshold hysteresis iv - circuit description (continued) tea7540 application note 18/44 15 to rx attenuator g2 cs3 rx compressor from 2/4 wires chip select logic 2 3 4 12 13 21 22 28 hands-free microphone c5 tx compressor a to 2/4 wires external supply tx attenuator cs1 cs power supply rcs (r45) v + handset microphone cs2 AN372-24.eps figure 24 : chip select circuitry iv.1.6 - chip select circuitry iv - circuit description (continued) figure 24 illustrates, conceptually, the functions of the two transmit and the receive channels and their control by means of the external switch cs. there are basically two operating modes : a) cs = on : handsfree mode. in this mode the handset microphone pre-amplifier is inactive and the two handsfree channels of tea7540,transmit and receive, are both active. b) cs = off : monitoring mode. in this mode the transmit compressor is inactive and the pre-amplifiers a is active with a fixed gain of 20db. the voltage of the timerisforcedtov ref - 220mv and so the attenuation control block see 0db on rx attenuator(a r ). the dc supply required by the micro-phone pre- amplifier may be provided by a separate power supply, e.g. the one available in the dialling circuit. in this case of the power supply requirements are so small that the pre-amplifier is active, and the handset must be kept active, even under the worst conditions of low available supply current due to long-line connections. in the monitoring mode the automatic control of the compressor amplifier in the receive channel is in- operative. the gain of this compressor may however be ad- justed by selection of a suitable value of the exter- nal resistance r cs (r45) connectedbetween pin 2 and pin 28. tea7540 application note 19/44 iv.2 - pin description no pin description 1 gnd ground 2 cs chip select 3 oute transmit attenuator output 4 atte transmit attenuator input 5 hyste transmit hysteresis 6 hystr receive hysteresis 7 tim timer 8 cce transmit compressor time constant 9 ce3 transmit background noise memorization 10 ce2 transmit peak detector 11 ce1 transmit rectifier input 12 micout transmit compressor output 13 ine transmit compressor input 14 vref reference voltage 15 inr receive compressor input 16 recout receive compressor output 17 cr1 receive rectifier input 18 cr2 receive peak detector 19 cr3 receive background noise memorization 20 ccr receive compressor time constant 21 amp2 microphone pre-amplifier input 22 amp1 microphone pre-amplifier supply 23 vol volume control 24 atr attenuator control 25 attr receive attenuator input 26 outr receive attenuator output 27 iref reference current 28 v+ power supply input iv - circuit description (continued) tea7540 application note 20/44 iv.3 - block diagram iv - circuit description (continued) v ref c4 8v+ c10 cee v ref r9 c12 15 20 2 3 4 7910 16 17 18 19 27 23 11 14 22 24 r2 p1 vol atr r45 v+ transmit attenuator microphone amplifier atte 21 26 power supply amp1 outr handset microphone v ref transmit receive speech/noise speech/noise comparator speech/noise comparator transmit/receive comparator 8mv 25 5 16 28 gnd v+ attr hyste hystr r5 r6 noise memo peak detector preamp preamp noise memo peak detector 13 12 ine handsfree microphone c3 ccr inr from 2/4 wires recout c1 cr1 cr2 r3 v+ c15 v ref v ref r7 tim c9 ce3 ce2 c11 r8 v ref v+ c5 micout ce1 tea7540 cs oute amp2 i ref cr3 differential hysteresis hyt hyr timer c0 r1 v+ 8mv attenuation control receive attenuator noise threshold hysteresis hysth logic reference 100mv transmit compressor receive compressor r4 AN372-01.eps figure 25 tea7540 application note 21/44 iv.4 - functional description of the pin allocation iv - circuit description (continued) pin 1 : ground pin 2 : chip select (see section iv.1.6) there are two possibilities to force the system in monitoring or hands-free modes : a) by an external switch (see figure 26) when the pin 2 is connected to gnd (switch close) the system is set in hands-free mode (v pin2 = 0v). when the pin 2 is floating the system is set in monitoring (v pin2 =v cc -400mv). in this way the receive amplifier is set to its maximumvalue. it's possible to connect pin 2 with pin 28 (supply voltage) by a resistor to program the gain of the receive amplifier how the figure 26 shows. this resistor programs an internal bias current (i = 400mv/r) for the amplifier. b) by external command ( m p)(see figure 26) it depends on the output stage of the gates. 1) open collector t on = hands-free mode, 0v < v pin2 pin 6 : receive hysteresis (see section iii.4.2.a) this hysteresis is inserted in the receive hysteresis block hyr when tea7540 is in transmit mode. it is programmable by an external resistor as in the following table : r pin 6 h 0t 1.0m 0db 14.0k 3db 5.6k 6db 3.3k 9db 2.3k 12db pin 7 : timer this pin provides informationon the state of opera- tionand assumes three values (see section iv.1.4). -v ref + 220mv for transmit mode, -v ref - 220mv for receive mode, -v ref for idle mode. the switching time constants from idle to transmit (or receive) are externally programmed by an rc network. the output current at this pin is i = 50 m a: see figure 27. iv - circuit description (continued) idle tx mode t1 t2 rx mode t3 v ref + 220v v ref v ref - 220v AN372-26.eps figure 27 the voltage transition (v1) at pin 7 is 220mv from idle to transmit (or receive) and the time constant is t1 (recommended value : t1 = 2.2ms) the voltage transition (v2) from receive (or trans- mit) to transmit (or receive) modes is v2 = 2 ? v1 the correspondently time constant is t2 = 2 ? t1 (recommended value : t2 = 4.4ms). from transmit (or receive) to iidle modes is : this time is longer and controlled by the discharge of capacitor c via the parallel connected resistor. t3 = 2.2 ? r ? c (recommended value: t3 = 800ms). pin 8 : cce transmit compressor time constant the capacitor connected to this pin controls the rate at which the transmit compressor adapts its gain so as to maintain an output voltage level of 200mv peak-to-peak (see section iii.4.1). therefore the ratio between the time constants is internally fixed at a value of 80. the output current during the increment gain phase i1 = 1.25 m a and during the decrement gain phase is i2 = 100 m a. these current sources are used to charge and discharge the external capacitor with the voltage range of 220mv when the incoming signal is de- creasing the gain increases with a charge time t1 equal to : t1 = (v ? c)/i1 (recommended value 1.8s) (where v = 0.22v, i1 = 1.25 m a, and c = 10 m f) when the incoming signal is increasing the gain decreases with a discharge time t2 equal to : t2 = t1/80 (recommended value 22ms) pin 9 : ce3 transmit background noise this is used to extract the amplitude of the transmit channelbackgroundnoise signal. thecomponents connected to this pin determine the transmit chan- nel background noise detector rise time. the maxi- mum voltage drop on the external capacitor is internally clamped at 36mv. - the rise time is defined by : t r = (36e-3 ? c ? r)/(v cc /2) (recommended value : t r = 9.2s) - the decay time is the same as the decay time of the peak detector signal (pin 10) (t d = 80ms). v cc r8 9 c11 v ref d v = 36mv a n372-27.eps figure 28 10 r9 c12 20 m a v ref 100mv pp AN372-28.eps figure 29 tea7540 application note 23/44 pin 10 : ce2 transmit peak detector - rise time : this is the time required to charge the capacitor c to the maximum value of the ampli- tude of the compressor output voltage (100mv pp ). t r =(c ? v)/i = c5 ? 10 3 (recommended value : t r = 2.3ms) - decay time : this is the time for the discharge of the capacitor c in the resistor r. t d = 2.2 ? r ? c (recommended value : t d = 80ms) pin 11 : ce1 transmit rectifier input this pin is connected to the transmit compressor output by a capacitive coupling (typical value: 100nf). the input impedance = 10k w . note : by connecting in series an external resistor between pins 12 and 11 it is possible to increase the output voltage of the compressor within the range of its dynamic. pin 12 : micout transmit compressor output the maximum output current is 25 m a by an exter- nal resistor connected between this pin and gnd it is possible to increase the threshold of no dis- torted signal due to : v out ( mv pp )= ? ? ? 25 m a + v ref r ext ? ? ? ? 5k pin 13 : ine transmit compressor input thegain variation is adjustedaccording to the input signal amplitude - the gain is between 25 and 41.5db. - the compressor output voltage is 200mv pp . - the input impedance is 10k w . hence the input signal voltage range can be calcu- lated to be between 1.7mv pp and 11.2mv pp . pin 14 : reference voltage v ref =v cc /2. the decoupling capacitor is typically 100 m f. pin 15 : inr receive compressor input - the gain is between 16db and 36.5db. - the compressor output voltage is 200mv pp . - the input impedance is 10k w . hence the input signal voltage can be calculated to be between 3mv and 32mv. pin 16 : recout receive compressor output this output drives the receive rectifier and the receive attenuator input. the maximum output current is 25 m a. it's possible to increase the no distorted output signal (see pin 12). pin 17 : cr1 receive rectifier input the input impedance is 10k w . this input is con- nected to the receive compressor output by a capacitor coupling (typical value : 100nf). note : by connecting in series a resistor between this pin and pin 16 it is possible to increase the output level of the compressor within its dynamic (like pin 11). pin 18 : cr2 receive peak detector the rise and decay times are determined by the same rules of the tx peakdetectoranddependson : - a 20ma internal source current, - an external capacitor, - an external resistor, - the compresor output voltage. the suggested value of rise time is t r = 11ms and decay time equal to 145ms (c = 22 m f, r = 30k w ). pin 19 : cr3 receive background noise memorization the components connected to this pin determine the receive background noise rise time. the maxi- mum voltage drop across the external capacitor is also internally clamped to 36mv (see pin 9). - the rise time is given by: t r = (36e-3 ? c ? r)/(v cc /2) (recommended value : t r = 9.2s). - the decay time of this signal is the same as the decay time of the receive peak detector signal (t d = 145ms). pin 20 : ccr receive compressor time constant control this functionis based on the same principle as that explianed previously with respect to pin 8 for the transmit compressor. pin 21 : amp2 handset microphone pre-amplifier input this pre-amplifier is enabled in the monitoring mode (pin 2 connected to pin 28 through an exter- nal resistor). it enables the hands-set microphone to be used without an external analog switch. the gain is 20db so as to be compatible with the required output voltage and the associated input impedanceis 50k w . the output of the pre-amplifier is internally connected to the transmit attenuator output. iv - circuit description (continued) tea7540 application note 24/44 pin 22 : amp1 microphone pre-amplifier supply (see section iii.1.6) this supply has been separated from the main supply (pin28) to enable it to be supplied by an- other source (i.e. the same of the dialling circuit) in order to save as much as possible supply current in hands-free mode. pin 23 : vol ( active only in receive mode ) using a potentiometer connected to this pin it is possibleto introducea maximumof 32db of attenu- ation in the receive channel. the following table shows the relationshipbetween resistance value and attenuation. r pin 23 attenuation 0k 0db 1k 3.3db 2k 6.7db 3k 10.0db 5k 16.7db 10k 33.4db pin 24 : atr attenuation as0 it should be recalled that this attenuation is fully introduced into the inactive channel in the speech mode, and that an attenuation of a/2 is introduced into both channels in the noise (idle) mode. as discussed previously (see section ii.1), it is important, when implementing a high quality hand- set design, to ensure that the loop gain is as close as possible to zero db. this condition is chosen to avoid the howoling effect otherwise experienced with full-duplex op- eration. the following table shows the relationshipbetween the resistance value and attenuationvalue, as0. r pin 24 attenuation 0k 10.4db 1k 13.2db 2k 17.0db 3k 20.3db 4k 24.3db 5k 28.4db 6k 32.5db 7k 36.4db 8k 40.2db 9k 44.3db 10k 48.0db 11k 52.0db typically, for classical coupling efficiencies, the circuit can operate with a programmed attenuation as0 equal to 45db. pin 25 : attr receive attenuator input this input is normally connected to the receive compressor output by a capacitive coupling. it is useful to choose this capacitor value to perform a high pass filter with its input impedance (10k w )so as to reduce the input noise. pin 26 : outr receive attenuator output the output voltage is 200 mv pp when the incoming signal is in the compressing range. this pin is connected to the loudspeaker amplifier input. its maximum output current is 60 m a. pin 27 : iref external resistor the external resistor connected to this pin enables the bias current source of the circuit to be adjusted. a typical value for this component is 3.57k w . pin 28 : vcc positive supply voltage the minimum working voltage is 2.5v for a current consumption of 2.3ma, and hence line-powered applications can be achieved. this supply can be typically provided by the speech circuit in the same way as the loudspeaker amplifier. iv - circuit description (continued) tea7540 application note 25/44 v - influence of the external components v.1 - gain and dynamic of tx compressor v.1.1 - gain and dynamic of tx compressor gain - pin 13 is the output of tx compressor. - pin 12 is the output of the tx compressor. the chip is set in the hands-free mode note : the figure shows the minimum and the maximum value of the gain. the typical range between values is about 17db. dynamic this particular behaviourallows to compensatethe distance between the speaker and hands-free mi- crophone from 0.5 meters to 3 meters. we can divide the dynamic curve (figure 31) in three areas : 1) the first area shows a linear increase of output signal according to the increase of input signal. the agc of the compressor in this region is not active. the gain is maximum (see also figure 30). 2) the second area is characterizedby a constant output level and in this region the agc of the compressor is active, its output is kept at 200mv pp . the gain is changing from the maximum to the minimum value (see also figure 30). 3) the third part shows again an increase of the ouput level and in this part the agc of the compressor is no more active. the gain is now minimum (see also figure 30). note : the figure shows that within 0.9 an d 5.7mv rms the compressor keeps the output level constant and about equ al to 220mv pp . if we apply a modulated signal at the input of the microphone we can see the particular behaviour shown in figure 32. as the input signal is increasing the compressor output follows it lineary up to the agc threshold (0.9mv rms ), then the output is maintained at con- stant value (200mv pp ). when the input signal is higher than 5mv rms the output signal starts to increase. for outputsignal higher than250mv pp its distortion increases, by an external resistor between pin 12 to gnd it is possible to increase this thereshold up to 400mv pp (100k). v.1.2 - gain and dynamic of rx compressor for this analysis we have to consider as the meas- urement pins : - pin15 as input of rx compressor, - pin 26 as output of rx compressor. previous comments are still truth. the only different characteristic is the value of the dynamic range, 20db in this case. we can divide the dynamic curve (figure 34) in three areas : 1) the first area shows a linear increase of output signal according to the increase of input signal. the compressor in this region is not active. the gain is maximum (see also figure 33) 2) the second area is characterizedby a constant output level and in this region the compressor is active, its output is kept at 200mv pp . the gain is changing from the maximum to the minimum value (see also figure 33). 3) the third part shows again an increase of the ouput level and in this part the agc of compressor is no more active. the gain is now minimum (see also figure 33). if we apply a modulated signal at the input of the compressor we can see the particular behaviour shown in figure 35. as the inputsignalis increasing thecompressoroutput follows it lineary up to the agc threshold (1mv rms ), then the output is maintained at constant value (200mv pp ). when the input signal is higher than 11mv rms the output signal starts to increase. for output signal higher than 250mv pp its distortion in- creases, but by an external resistor between pin 16 and gnd it is possible to increase this thereshold up to 400mv pp (by 100k). by another external resistor between pins 16 and 17 it is possible to increase the output level within the range of the dynamic. finally the figure 35 shows the output of rx com- pressor with a modulated signal in the input of compressor. this behaviour assures a compensa- tion of the line losses. note : the gain is fixed internally in orderto clarify the utility of these compressorswe cansay that thesecompressorsworkwith maximum gain when the level of input signal is low and add a variable attenuation (dg), up to reach minimum gain, when the level of the output signal is high. this value of attenuationis an important parameter for a correct behaviour of hands-free system as you'll see later when we'll speak about attenuators. tea7540 application note 26/44 AN372-31.tif trace 1 : modulated tx compressor input (pin 13) 3mv/div, t = 2s trace 2 : output tx compressor (pin 11) 50mv/div, t = 2s (rext pin 12 to gnd = 100k) figure 32 : dynamic of tx compressor 0.9 2.7 4.5 6.3 8.1 130 190 250 310 370 mic. input signal pin 13 (mv rms ) output signal (mv pp ) AN372-30.eps figure 31 : dynamic of tx compressor 0.9 2.7 4.5 6.3 8.1 23 29 35 41 47 gain (db) input signal pin 13 (mv rms ) AN372-29.eps figure 30 : gain of tx compressor v - influence of the external components (continued) tea7540 application note 27/44 AN372-34.tif trace 1 : input rx compressor 18mv/div, t = 2s trace 2: output of rx compressor (pin 17) 50mv/div, t = 2s (rext pin 16 to gnd = 100k) figure 35 : dynamic of rx compressor output signal (mv pp ) input signal pin 15 (mv rms ) 020 4 8 12 16 280 200 120 240 160 80 40 AN372-33.eps figure 34 : dynamic rx compressor gain (db) input signal pin 15 (mv rms ) 020 4 8 12 16 40 32 24 36 28 20 16 12 AN372-32.eps figure 33 : gain of rx compressor v - influence of the external components (continued) tea7540 application note 28/44 36 32 28 24 20 16 0 2.5 i l = 30ma in handset mode 0.5 input signal pin 15 (mv rms ) rx gain (db) r (pin 2/pin 28) = 100k w r (pin 2/pin 28) = 75k w r (pin 2/pin 28) = 56k w 1 1.5 2 AN372-36.eps figure 37 : gains of rx compressor in handset mode gain (db) 012345 mic. input signal pin 15 (mv rms ) 24 22 20 18 16 AN372-35.eps figure 36 : gain of tx amplifier in handset mode v.1.3 - tx / rx gain in handset mode this system can switch in handset mode by a chip select switch at pin 2. tx path in this case the measurement points are : - pin 21 as input of the microphone in handset mode, - pin 3 as output of the transmit path. the following figure shows the gain of transmit preamplifier in handset mode. in this operating mode the tx compressor is completely disabled and the microphone input signal is amplified by a constant value. note : it's not possible to change externally this value (see figure 36) . rx path in handset mode the pins of receiving path are the same of the hands-free case, that is, pins 15 and 26 respectivelyas input and as output of the signal. in this operating mode the compressor doesn't worklike beforebut its gain is kept at constantvalue indipendentlyof the input signal. this constant value can be changedexternally with a resistor connects between pin 2 and pin 28. the figures 36-37 show three differentbehaviours of the rx gain with three different values of the external resistor. this resistor doesn't affect the tx gain. v.2 - attenuation and volume control v.2.1 - programmable attenuation on the paths (as) refering to the block diagram (see also sec- tion iii.2) both channel have an attenuation block after the compressors,a t for tx path and a r for rx path.these blocks are controlledby an attenuation control block that manages the correct value of an attenuation to insert in them. there are three factors that affects this value, that is to say : - the external resistor (pin 24) that establishes a particular value of attenuation as0 when the gains of the compressors are at the maximum value (see also section iii.2). - the value of the attenuation introduced by the comporessors when are not working at maximum gain (see also section iii.1). - the external potentiometer that manages the volume control in rx path. - the operatingmodes (tx,rx or idle) that manage how to share the total attenuationas betweenthe a t and a r blocks. v - influence of the external components (continued) taking in account these factors the conditions of the measurement the figure shows are : set up condition a) gain of the compressors g1 and g2 at the maximum value (attenuationdg1 = dg2 = 0db). b) operatingmode is hands-freereceiving.in thisway the attenuatoractive is only the transmitting one. in fact like we havesaid before(see also section iii.1) when the system is in rx mode, a r = 0db and vice versawhenit'sintxmodea t =0db. c) volume is at maximum value in figure 38 and it is at its minimum figure 39 in order to demonstrated its influence on the attenuation. d) the external resistor, connected between pin 24 and gnd, is equal to 7.4k w in figures 38 - 39 and it is equal zero in figure 40 in order to insert two differentvalues of attenuationsas0 (see table 1). tea7540 application note 29/44 v - influence of the external components (continued) rx path 372-37a.tif / 372-37b .tif figure 38 trace 1 : input of tx attenuator (pin 4) 100mv/div, t = 10ms trace 2 : output of tx attenuator (pin 3) 100mv/div, t = 10ms trace 3 : input of rx attenuator (pin 25) 100mv/div, t = 10ms trace 4 : output of rx attenuator (pin 26) 100mv/div, t = 10ms v ol max aso = 37db the figures 38 and 39 show that when the volume control circuit adds attenuation in the rx path (vol = min. figure 39) the attenuation inserted in the tx path decreases (see traces 2;4). note : rext (pin 24) = 7.4k w ; volume max. the figure 38 and 40 show that when the external resistor programs a lower value of as0 (figure 40) the tx attenuator a t is reduced and this not affect the rx attenuation (a r = 0db). the figures show three different operating states : - it's possible to note that the input signal of rx attenuation(pin 25 ) is the same of the output one (pin 26) while the output signal (pin 3) of tx attenuatoris attenuated(see the table) respect to the input signal (pin 4). - the value of the attenuation introduced on tx path is reduced when ras, connected from pin 24 to gnd, is a short circuit (see table). note : rext (pin 24) = short circuit volume max. the value of the attenuation introduced on tx path is again reduced when the volume has its minimum value (potentiometer at pin 23). tea7540 application note 30/44 372-38a.tif / 372-38b.tif figure 39 v - influence of the external components (continued) trace 1 : input of tx attenuator (pin 4) 100mv/div, t = 10ms trace 2 : output of tx attenuator (pin 3) 100mv/div, t = 10ms trace 3 : input of rx attenuator (pin 25) 100mv/div, t = 10ms trace 4 : output of rx attenuator (pin 26) 100mv/div, t = 10ms (v ol min aso = 37db) tea7540 application note 31/44 372-39a.tif / 372-39b.tif figure 40 v - influence of the external components (continued) trace 1 : input of tx attenuator (pin 4) 100mv/div, t = 10ms trace 2 : output of tx attenuator (pin 3) 100mv/div, t = 10ms trace 3 : input of rx attenuator (pin 25) 100mv/div, t = 10ms trace 4 : output of rx attenuator (pin 26) 100mv/div, t = 10ms (v ol max aso = 0db) tea7540 application note 32/44 v.2.2 - programmable attenuation of the tx/rx hysteresis control blocks (hyt, hyr) (see also section iii.4.2) refering to the block diagram both channels have a hysteresis block that manages the speech threshold for a correct switch of the system. the measurement points are : - pin 11 as output of tx compressor - pin 17 as output of rx compressor - pin 7 as the timing to observe the correct transi- tion from tx to rx and vice versa there are two factors which influence this behav- iour, that is to say : - operating mode (tx or rx) because in function of the status the correspondent hysteresis block is disabled (see also section iii.4.3) - external resistors at pin 6 for receiving path and at pin 5 for transmitting path that establishes the value (ho) to add to default one (- 6db) (see also section iii.4.3) in fact the final value of the attenuation, that the hysteresis block of the inactive part inserts, de- pends on these parameters as the general rule shows : hys = h0 + 6db the figure 41 shows how this block acts on the switch of the system. the h0 value was set to 0db both for tx and rx paths.in fact we have the switch when the signals are each other different about 6db. depending on the part we wish to favour, the attenuation inserted (h0#0db) can be changed by external resistors (at pin 5 and pin 6). v.3 - rise and fall times v.3.1 - rise and fall times of tx compressor (see also section iv.4) the value of the time constant depends on the value of an external capacitor at pin 8 that is chargedby an internal current source of 1.25 m a(t r ) and discharged by another internal current source of 100 m a(t f )). the ratio is equal to 80 and so : 80 t f =t r during t r the compressor increases its gain and viceversa during t f the compressor decreases its gain in order to maintain a constant output voltage level. the figure shows a slow rise time and a very fast fall time, corresponding to the peak of the output compressor signal trace. from an application point of view this timing avoids that the compressor increases its gain quickly when there is only envi- ronment noise during pauses within spoken words. in fact when at the input there is a very low signal the compressorworks with its maximumgain as we have said before, such a choice of the time con- stants avoids an unpleasant fast amplification of the noise when nobody is speaking. the external capacitor can increase or decrease the value of the time constants but not the ratio between them. the pins of the measurements are respectively : - pin 11 as output of compressor of transmit path - pin 8 as pin of time constants of tx compressor - pin 13 as input of compressor of transmit path the figure 42 shows the rise time and fall time of the compressor in transmit path. no signals are connected to the receive input dur- ing the measurements. v.3.2 - rise and fall time of rx compressor in the same way of the previous analysis it's pos- sible to investigate the constant time of the rx compressor. the philosophy of the design is the same, but the external capacitor is connectedto pin 20. the ratio between rise time and fall time is 80 independently of the value of the capacitor. the figures 43 and 44 enphasizedthe fall time and the rise time with their ratio. in fact it is shown an expandedtime scale to note as well as possiblethe fast fall time (figure 43). the pins of the measurements are respectively : - pin 17 as outputof thecompressorof receivepath - pin 18 as output of the peak detector of receive path v - influence of the external components (continued) tea7540 application note 33/44 trace 1 : timing pin (pin 7) 500mv/div, t = 500ms trace 2 : output of tx comparator (pin 11) 100mv/div t: 500ms trace 3 :output of rx comparator (pin 17) 100mv/div, t = 500ms v - influence of the external components (continued) AN372-40.tif figure 41 AN372-41.tif trace 1 : time constants of tx comparator (pin 8) trace 2 : input of tx comparator (pin 13) 5mv/div, t = 200ms trace 3 : output of tx comparator (pin 11) 100mv/div, t = 200ms figure 42 tea7540 application note 34/44 v - influence of the external components (continued) AN372-42.tif trace 1 : input of rx comparator (pin 15) 16mv/div, t = 100ms trace 2 : timeconstants ofrx comparator (pin 20) 200mv/div, t = 100ms trace 3 : output of rx comparator (pin 17) 100mv/div, t = 100ms figure 43 AN372-43.tif trace 1 : input of rx comparator (pin 15) 8mv/div, t = 10ms trace 2 : output of rx comparator (pin 17) 50mv/div, t = 10ms figure 44 tea7540 application note 35/44 v.3.3 - rise and fall time of tx peak detector v - influence of the external components (continued) the measurement points are respectively : - pin 10 as output of tx peak detector, - pin 11 as output of tx compressor. the constant times are fixed by an external rc network at pin 10 as below : t a = cv i ,t d =2rc v = 100mvdc fixed internally, i=20 m a fixed internally, c = 470nf, r = 75k w . the input signal has been modulated to obtain a fastincrease of the outputsignalof the compressor as the figures 45-46 show. the figure 46 enpha- sized, with an expanded time scale, the rise time. v.3.4 - rise and fall time of rx peak detector in the same way of the previous analysis the rise time and the fall time of rx peak detectorare shown in the figure 47. the measurement points are respectively : - pin 18 as output of rx peak detector, - pin 17 as output of rx compressor. the constant times are fixed by an external rc network at pin 18 as the rules above. also in this case the input signal has been modu- lated to obtain a fast increase of the output signal of compressor as the figure shows. v.3.5 - delay time between tx/rx and idle mode transitions the three general modes of the system (transmit, receive, idle) are managed depending on the infor- mation from three comparators that is : (see also the block diagram) - transmit/receive comparator, - speech/noise comparator on the tx path, - speech/noise comparator on the rx path. depending on the outputs of these blocks the attenuation block manages the channels attenu- ations. the constant times are fixed by an external rc network at pin 7 as the rule below. t rise =t r = cv i and t fall =t f = 2.2 rc v = 220mvdc fixed internally, i=50 m a fixed internally, c = 1 m f, r = 430k w . the figure 48 shows the transition between idle to tx modes and the figure 49 from rx to idle. v.3.6 - delay time between tx and rx mode transitions the figure 50 reasumes the behaviour of the tran- sitions of the system the measurements points are : - pin 7 timing pin - pin 17 output of rx compressor - pin 11 output of tx compressor the tx and rx input signals was choosed in order to show the rx to tx, tx to idle and idle to rx transitions. tea7540 application note 36/44 v - influence of the external components (continued) AN372-44.tif trace 1 : output of tx peak detector (pin 10) trace 2 : output of tx comparator (pin 11) figure 45 AN372-45.tif trace 3 : output of tx peak detector (pin 10) 200mv/div, t = 10ms trace 4 : output of tx comparator (pin 11) 100mv/div, t = 10ms figure 46 tea7540 application note 37/44 v - influence of the external components (continued) AN372-46.tif trace 1 : output rx peak detector (pin 18) 100mv/div, t = 200ms trace 2 : output of rx comparator (pin 17) 200mv/div, t = 200ms figure 47 AN372-47.tif trace 1 : timing pin. (pin 7) tx - idle transition 200mv/div, t = 500ms trace 2 : output of tx comparator (pin 11) 100mv/div, t = 500ms figure 48 tea7540 application note 38/44 v - influence of the external components (continued) a n372-48.tif trace 2 : timing pin (pin 7) rx - idle trace 1 : output of tx comparator (pin 18) 50mv/div, t = 10ms figure 49 AN372-49.tif trace 1 : output tx compressor (pin 11) trace 2 : output of rx comparator (pin 17) 10mv/div, t: 50ms trace 3 : timing pin (pin 7) figure 50 tea7540 application note 39/44 vi - applications vi.1- hands-free particular effects vi.1.1 - the larsen effect this effect occurs in applicationswhen the acoustic and sidetone coupling effects are enough to pro- duce a loop gain greater than unity, resulting in a ohowling effecto in the loudspeaker. usually the acoustic coupling from the loudspeaker to the mi- crophone is the principal cause of this instability. to avoid this condition the value of the resistance con- nected to pin 24 has to be increased until the instability disappears,due to the increase of the as0 value. vi.1.2 - ochopped speecho effect during a two-way conversation two effects may be encountered : - chopping of the received signal, - chopping of the transmitted signal. this effect is produced by the loss of parts of a spoken word due to transitions from one active mode to the other (transmit to receive, or vice- versa), caused by incorrect contol of the signals level in input of the comparators, bearing in mind the hysteresis effect which controls the speech level in the not active channel (see section iii.4). when, in the receive mode, the received signal is chopped the effect is due to insufficient hysteresis in the transmit channel.the level of any speech signal present in the transmit signal is not sufficiently re- duced to maintain the dominance of rx channel. in this case the level of the programmable attenuation h ot must be raised, by decreasing the value of the resistance connected to pin 5 (see section iii.4.3). in a similar way when the system is in transmit mode, if the chopping effect occurs in the transmit channelthe level of the programmableattenuation, hor has to be raised in the receive channel, by increasing the value of the resistor connected to pin 6 (see section iii.4.3). take note an excessive value of hysteresis can hold up the switching. vi.1.3 - owalky-talkyo effect under certain conditions the switching of attenu- ation between the transmit and receive channels may produce an unpleasant effect. a sudden ap- parently total loss of reception each time speech beginning, produces a half-duplex effect similar to that obtained with walky-talky sets. the basic cause of this effect is an excessive value of attenu- ation switched from one channel to the other when the active mode switches from receive to transmit. to optimize any particular implementation based on tea7540 chip set the following adjutsments should be made : - the attenuation as0 should first be adjusted to have the minimum value necessaryto respect the loop gain stability requirement (see section vi.1.1). as0 should then be increased by up to a further 3db in order to reduce the distortion pro- duced by the system close to the larsen effect limiting condition. - the average speech level at the compressor input should then be adjusted to obtain the half value of compressing range. to carry out this adjustment connect an oscillo- scope probe to both the compressor gain control inputs, cce and ccr (pins 8 and 20 respectively). adjust the level of the speech signal inputs to the compressors such that, for a typical average speech level, an average level of v ref -110mv is obtained on pin 8 and v ref -50mv is obatained on pin 20 of tea7540. vi.1.4 - volume control influence in the receive mode any additional attenuation added (or sutracted) in the rx attenuator is com- pensated by an equal attenuation subtracted (or added) in tx attenuator : at = as0 - dg1 - dg2 - avol (see section iii.1) in the transmit mode, however, the attenuationat remains fixed at 0db and the attenuationar is still: ar = as0 - dg1 - dg2 in resume, the volume control attenuationimprove the duplex effect only in the receive mode. vi.1.5 - examples a) example 1: in the following example the attenuationas0 is 45db. after optimization, the total attenuation given by the attenuators is : as = as0 - dg1max/2 - dg2max/2 hence: as = 45db - 8.5db - 10db = 26.5db in this case, the speech signal of the inactive path is only attenuatedand not lost for thelistener. take note,as is decreasing while the compressorinput signals is increasing. this is due to dg1 or dg2 increase depending on the operating mode of the system. the attenuationminimum value of as is : as = as0 - dg1max/2 - dg2max/2 hence: as = 45db - 17db - 20db = 8db b) example 2: in the following example the attenuationas0 is 37db. in this case the attenuators give: as = 37db - 8.5db - 10db = 18.5db and for the speech signal maximum amplitude: as = 37db - 17db - 20db = 0db therefore, the presence of this oquasi full duplexo operation provided that the application is carefully done. this is due to the presence of the compressor, the steady level of transmit signal for a distance variation between speaker and microphone from 0.5meters up to 3meters. tea7540 application note 40/44 v 8 (v 20 ) maximum gain (dg1 = dg2 = 0db) t v ref - 110mv minimum gain (dg1 = 16.5db, dg2 = 20.5db) v ref - 220mv v ref AN372-50.eps figure 51 : adjustment of comparator input levels vi - applications (continued) 12 3 4 5 6 7 8 9 10 27 28 11 12 13 15 22 16 17 18 19 20 26 14 21 25 t e a 7 5 4 0 r45 c42 k3 cs r1 r14 (200mv pp ) c18 level adaptor loudspeaker amplifier antilarsen agc (cs-1) cs cs : chip select cs = 0 : hands-free mode c1 c3 r3 c15 c4 r4 c2 c8 c14 r13 c17 handsfree r12 electret microphones c5 r5 c6 peripheral supply ( m p) filter & level adaptor transmit amplifier speech circuit c16 r10 filter & level adaptor receiving amplifier r11 earphone r7 c9 c10 r8 c11 c12 r9 supply (2.5v 7v) microphone supply handset r6 p1 23 24 r2 AN372-51.eps figure 52 : basic hands-free schematic vi.2 - basic hands-free telephone set tea7540 application note 41/44 table ?? : component value (see figure 52) symbol value description c1 c2 c3 c4 c5 c6 c7 c8 c9 c10 c11 c12 c13 c14 c15 100nf 47nf 10 m f 2.2 m f 100nf 33nf 100 m f 100 m f 470nf 10 m f 68 m f 470nf 100 m f 18nf 68 m f coupling and high pass filter coupling and high pass filter rx compressor gain rise and decay time rx peak detector rise and decay time coupling and high pass filter coupling and high pass filter power supply decoupling reference voltage decoupling switching time timer (t1, t2, t3) tx compressor gain rise and decay time tx background noise rise and decay time tx peak detector rise and decay time microphone supply decoupling coupling and high pass filter rx background noise rise and decay time p1 10k rx channel volume control r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 3.57k 9.1k 4.7m 30k 5.6k 1m 820k 4.7m 75k 2.2k 2.2k 56k 1%, internal bias current source adjusting as0 attenuation rx background noise rise time rx peak detector decay time tx hysteresis (attenuation) tx hysteresis (attenuation) switching time (from speech to idle) timer tx background noise rise time tx peak detector decay time tx input level adjusting, hands-free mode tx input level adjusting, hands-free mode rx pre-amplifier gain, monitoring mode vi.3 - monitoring mode and hands-free gains compability vi - applications (continued) 3 26 13 15 21 g1 20db v 3 v 2 hands-free handset tea7540 e v mic v e speech r1 r2 line v l r3 v 4 ls amplifier r1 a v ls rls v 1 a n372-52.e ps figure 53 : interface components calculation tea7540 application note 42/44 a) tx gain in hands-free mode the transmit compressor has been designed to have a steady tx signal level for a distance variation between speaker and microphone from 0.5meter up to 3meters with : 25db g1 41.5db for this gain range the output signal of compressor is stabilized at the vmic = 200mvpp (see section iv.1.2). hence, the input microphone typical signal range can be calculated easily: 1.7mv pp v 3 11.2mv pp b) tx gain in handset mode the handset microphone has to provide a voltage, v 2 =v mic /20db, in normal acoustic levels conditions to have the same tx level on line than in hands-free mode. therefore, the speech stage should be adapted to the compressor output voltage : v mic = 200mv pp hence : v 2 = 20mv pp c) rx gain r1 and r2 evaluation: the receive compressor has been designed to have a steady rx signal level such as v 4 = 200mv pp : the typical range of the rx compressor is : 16db g2 36.5db we can deduce that the input signal range is : 3mv pp v 1 32mv pp let us take one example: for france the mean speech line level is equal to about -15dbm (390mv pp ). to optimize an applicationbasedon tea7540 chip set, the average speech level at compressor input must be adjusted to obtain the middle value of the compressing range (see section v.1.3). that is to say : v 1 = 17.5 mv pp the rx speech gain is x db (according to the country) : 20 log 10 (v e /v l ) = xdb for france xdb = -2db hence v e = 300mv pp hence v e /v 1 =r 2 /(r 2 +r 1 ) = 300mv pp /17.5mv pp =17 we can choose: r 1 = 16k w and r 2 =1k w for france the mf signal specification is : v l = 84mv pp min. vi - applications (continued) this value must be included in the compressing range. checking : v l = 84mv pp ? v e = 66.7mv pp hence: v 1 =v e /17 = 3.9mv pp in practice, this signal can be greater. our example permits : v 1 = 32mv pp ? v l = 800mv pp d) available power for loudspeaker and volume control r3 evaluation : for a standard loudspeaker an average nominal power pls = 10mw. therefore, for each loudspeaker impedance there is an equivalent voltage : r ls v ls 25 w 1.4v pp 50 w 2.0v pp 100 w 2.8v pp 150 w 3.5v pp the sgs thomson loudspeaker amplifier, tea7532, can provide on 50 w of load 25mw maxi- mum with 3v power supply and 100mw with 5v power supply. now, it is easy to calculate r3 in orderto obtain v ls from v 4 = 200mv pp : r 3 =r i ? [a ? 0.2v pp /v ls -1] with r i =1k w , 12db a 32db programmable for example with a = 32db, v ls =3v pp and 50 w as load, we must put : r 3 =1k w [32db ? 0.2v pp /2]= 4k w (see french application circuit) note : it is possible to increase the vl s value up to 8v pp on 150 w of load (54m w ) using r 3 =0 w (see italian application circuit) tea7532 loudspeaker tea7540 hands-free speech line line 0v (3.3v, 60 m a) i ls 2.5ma (3v) power peripheral supply (i l ) AN372-53.eps figure 54 : low cost design solution tea7540 application note 43/44 vi.4 - hands-free option vi - applications (continued) 7 20 2/4 wires speech tx rx line 3.3v stabilized power supply mute supply chip select 22 28 rx 3 15 (60 m a) tea7540 13 21 26 tea7532 4 9 handset microphone hands-free microphone 10 12 antilarsen monitor amplifier agc on/off loudspeaker antidistortion agc handset earphone AN372-54.eps figure 55 : sgs-thomson universal modular hands-free kit (simplified schematic) tea7532 is a 16 pins package which provides : - loudspeaker amplifier : antidistorsion system - shunt regulator : it may be supplied by a current source. the ic has a zener characteristic which can be used as voltage supply for tea7540. - antiacoustic feed-back system (anti-larsen sys- tem). this function is active only in monitoring mode with tea7540. information furni shed is believed to be accurate and reliable. however, sgs-thomson micr oelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no licence is granted by implication or otherwise und erany patent or patent rights of sgs-thomson microelectronics. specifications mentioned in this publication are subject to change without notice. this pu blication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1997 sgs-thomson microelectronics - all rights reserved purchase of i 2 c components of sgs-thomson microelectronics, conveys a license under the philips i 2 c patent. rights to use these components in a i 2 c system, is granted provided that the system conforms to the i 2 c standard specifications as defined by philips. sgs-thomson microelectronics group of companies australia - brazil - canada - china - france - germany - italy - japan - korea - malaysia - malta - morocco the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. tea7540 application note 44/44 |
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