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| UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 1 voice switched speakerphone circuit description the UTC34018 speakerphone integrated circuit incor- porates the necessary amplifier, attenuators, and control functions to produce a high quality hands-free speakerphone system. included are a microphone amplifier, a power audio amplifier for the speaker, transmit and receive attenuators, a monitoring system for background sound level, and an attenuation control system which responds to the relative transmit and receive levels as well as the background level. also included are all necessary regulated voltages for both internal and external circuitry, allowing lining powered operation(no additional power supplies required).a chip select pin allows the chip to be powered down when not in use. a volume control function may be implemented with an external potentiometer. the UTC34018 applications include speakerphones for household and business use, intercom systems, automotive telephones, and others. features *all necessary level detection and attenuation controls for a hand-free telephone in a single integrated circuit *background noise level monitoring with long time constant *wide operating dynamic range through signal compression *on-chip supply and reference voltage regulation dip-28 *typical 100mw output power(into 25 ohms)with peak limiting to minimize distortion *chip select pin for active/standby operation *linear volume control function *standard 28-pin plastic dip package absolute maximum ratings (ta=25 c voltages referred to pin 22 ) characteristics value units v+ terminal voltage (pin 16) +12,-1.0 v cs (pin 18) +12,-1.0 v speaker amplifier ground(pin 14) +3.0,-1.0 v vlc(pin 24) +vcc,-1.0 v storage temperature -65~+150 c recommended operation conditions characteristics value units v+ terminal voltage (pin 16) +6.0 to +11 v cs (pin 18) 0to+11 v icc(pin 20) 0 to 3.0 ma vlc(pin 24) 0.55vb to vb v receive signal(pin 27) 0 to 250 mvrms microphone signal(pin 9) 0 to 5.0 mvrms speaker amplifier ground (pin 14) -10 to +10 mvdc ambient temperature -20 to +60 c
UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 2 block diagram mic amp transmit attenuator signal/noise detector attenuator control transmit/ receive comparator transmit level detector receive attenuator receive level detector speaker amp peak limiter vcc regulator side tone netw ork receive channel transmit channel transmit output receive input dc input receive volume control enable input telephone line side tone mic speaker pin description pin name description 1 rr a resistor to ground provides a reference current for the transmit and receive attenuators 2 rtx a resistor to ground determines the nominal gain of the transmit attenuator ,the transmit channel gain is inversely proportional to rtx resistance. 3 txi input to the transmit attenuator. input resistance is nominally 5.0k ohms. 4 txo output of the transmit attenuator. the txo output signal drivers the input of transmit level detectors well as the external circuit which drivers the telephone line. 5 tli input of the transmit level detector. an external resistor ac coupled to the tli pin sets the detection level. decreasing this resistor increases the sensitivity to transmit channel signals. 6 tlo output of the transmit level detector. the external resistor and capacitor set the time the comparator will hold the system in the transmit mode after speech ceases. 7 rli input of the receive level detector. an external resistor ac coupled to the rli pin sets the detection level. decreasing this resistor increases the sensitivity to receive channel signals. 8 rlo output of the receive level detector. the external resistor and capacitor set the time the comparator will hold the system in the receive mode after speech ceases. 9 mci microphone amplifier input. input impedance is nominally 10k ohms and the dc bias voltage is approximately equal to vb 10 mco microphone amplifier output. the mic amp gain is internally set at 34db(50v/v). 11 cp1 a parallel resistor and capacitor connected between this pin and vcc holds a voltage corresponding to the background noise level. the transmit detector compares the cp1 voltage with the speech signal from cp2. 12 cp2 a capacitor at this pin peak detects the speech signals for comparison with the background noise level held at cp1. 13 xdi input to the transmit detector system. the microphone amplifier output is ac coupled to the xdi pin through an external resistor. 14 skg high current ground pin for the speaker amplifier output stage. the skg voltage should be within 10mv of the ground voltage at pin 22. UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 3 (continued) pin name description 15 sko speaker amplifier output. the sko pin will source and sink up to 100ma when ac coupled to the speaker. the speaker amplifier gain is internally set at 34db(50v/v). 16 v+ input dc supply voltage. v+ can be powered from tip and ring if an ac decoupling inductor is used to prevent loading ac line signals. the required v+ voltage is 6.0v to 22v(7.5v nominal ) at 7.0ma. 17 agc a capacitor from this pin to vb stabilizes the speaker amplifier gain control loop, and additionally controls the attack and decay time of this circuit. the gain control loop limits the speaker amplifier input to prevent clipping at sko. the internal resistance at agc pin is nominally 110k ohms. 18 cs digital chip select input. when at a logic "0" (<0.7v) the vcc regulator is enabled. when at a logic "1" (>1.6v),the chip is in the standby mode drawing 0.5ma.an open cs pin is a logic "0".input impedance is nominally 140k ohms. the input voltage should not exceed 11v. 19 ski input to the speaker amplifier. input impedance is nominally 20k ohms. 20 vcc a 5.4v regulated output which powers all circuits except the speaker amplifier output stage. vcc can be used to power external circuitry such as a microprocessor(3.0ma max).a filter capacitor is required.the UTC34018 can be powered by a separate regulated supply by connecting v+ and vcc to a voltage between 4.5v and 6.5v while maintaining cs at a logic "1". 21 vb an output voltage equal to approximately vcc/2 which series as an analog ground for the speakerphone system. up to 1.5ma of external load current may be sourced from vb. output impedance is 250 ohms. a filter capacitor is required. 22 gnd ground pin for the ic(except the speaker amplifier) 23 xdc transmit detector output. a resistor and capacitor at his pin hold the system in the transmit mode during pauses between words or phrases. when the xdc pin voltage decays to ground, the attenuators switch from the transmit mode to idle mode. the internal resistor at xdc is nominally 2.6k ohms(see fig.1). 24 vlc volume control input>connecting this pin to the slider of a variable resistor provides receive mode volume control. the vlc pin voltage should be less than or equal to vb. 25 acf attenuator control filter. a capacitor connected to this pin reduces noise transient as the attenuator control switches level of attenuation. 26 rxo output of the receive attenuator. normally this pin is ac coupled to the input of the speaker amplifier. 27 rxi input of the receive attenuator. input impedance is nominally is 5.0k ohms. 28 rrx a resistor to ground determines the nominal gain of the receive attenuator. the receive channel gain is directly proportional to the rrx resistance. electrical characteristics (referred to fig. 1) parameter symbol min typ max unit supply voltages v+ supply current v+=11v,pin 18=0.7v v+=11v,pin 18=1.6v i v+ 9.0 800 ma m a vcc voltage(v+=7.5v) line regulation(6.5v UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 6 design guidelines (ref to fig. 1) attenuators the transmit and receive attenutors are complementary in function, i.e., when one is at maximum gain the other is at maximum attenuator, and vice versa. they are never both on or both off. their main purpose is to control the transmit and receive paths to provide the half-duplx operation required of a speakerphone. the attenuators are controlled solely by the voltage at the acf pin(pin 25).the acf voltage is provided by the attenuator control block, which receives 3 inputs: a)the rx-tx comparator, b)the transmit detector comparator, and c)the volume control. the response of the attenuators is based on the difference of the acf voltage from vcc, and therefore a simple method for monitoring the circuit operation is to monitor this voltage difference(referred to as vacf).if d vacf is approximately 150 mv the circuit is in the receive mode. if d vacf is approximately 75 mv the circuit is in the idle mode, and two attenuators are at gain setting approximately half way(in db) between their fully on and fully off positions. the maximum gain and attenuation values are determined by the three resistors rr,rtx,rrx(referred to fig2,3,4).rr affects both attenuators according to its value relative to rtx and rrx, which is why fig 4 indicates the variations versus the ratio of the other resistors to rr.(grx and gtx are the maximum gains, and arx and atx are maximum attenuations).rtx affects the gain and attentuator according to fig 3.as can be seen from the figures, the gain difference(from on to off) is a reasonably constant 45db until the upper gain limit is approached. a value of 30k is recommended for rr as a starting point, and then rtx and rrx selected to suit the particular design goals. the input impedance of the attenuators(at txi and rxi) is typically 5.0k ohms, and the maximum input signal which will not cause output distortion is 250mvrms(707mvp-p).the 4300 ohms resistor and 0.01 m capacitor at rxo(in fig 1)filters out high frequency components in the receive path. this helps minimize high frequency acoustic feedback problems which may occur if the filter were not present. the filter's insertion loss is a 1.5db at 1.0khz.the outputs of the attenuators are inverted from their inputs. referring to the attenuator control block, the d vacf voltage at its output is determined by three inputs. the relationship of the inputs and outputs is summarized in the following truth table. tx-rx comp transmit det comp volume control vacf mode transmit transmit no effect 6.0mv transmit transmit idle no effect 75mv idle receive transmit affects d vacf 50-150mv receive receive idle affects d vacf 50-50mv receive as can be seen from the truth table, the tx-rx comparator dominates. the transmit detector comparator is effective only in the receive mode. the tx-rx comparator is in the transmit position when there is sufficient transmit signal present over and above any receive signal. the transmit detector comparaotr then determined whether the transmit signal is a result of background noise(1 relatively stable signal),or speech which consists of bursts. if the si gnal is due to background noise, the attenuators will be put into the idle mode( d vacf=75mv).if the signal consists of speech, the attenuator will be switched to the transmit mode( d vacf=6.0mv).a further explanation of this function will be found in the section on the transmit detector circuit. the rx-tx comparator is in the receive position when there is sufficient receive signal to overcome the background noise and any speech signals. the d vacf voltage will now be 150mv if the volume control is at the maximum position, i.e. vlc(pin 24)=vb .if vlc is less than vb, the gain of the receive attenuator, will vary in a complementary manner as shown in fig 5.it can be seen that at the minimum recomm ended operating level(vlc=0.55vb) the gain of the transmit atternuator is actually greater than that of the receive attenuator.the effect of varying vlc is to vary d vacf, with a resulting variation in the gains of the attenuators, fig 6 shows the gain variation with d vacf. UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 7 the capacitor at acf(pin 25) smoothes the transition between operating modes. this keeps down any "clicks" in the speaker or transmit signal when the acf voltage switches. the gain separation of the two attenuators can be reduced from the typical 45db by adding a resistor between pin 20(vcc) and pin 25(acf). the effect is a reduction of the maximum d vacf voltage in receive mode, while not affecting d vacf in the transmit mode. as an example, adding a 12 k ohms resistor will reduce d vacf by approximately 15mv(to 135mv). decrease the gain of the receive attenuator by approximately 5. 0db, and increase the gain of the transmit attenuator by a similar amount. if the circuit requires the receive attenuator gain to be +6. 0db in the receive mode, rrx must be adjusted (to 27k ohms) to re-establish this value. this change will also increase the receive attenuator gain in the transmit mode by a similar amount. the resistor at tli may also require changing to reset the sensitivity of the transmit level detector. log amplifiers(transmit and receive level detectors) the logs amps monitor the levels of transmit and receive signal, so as to tell the tx-rx comparator which mode should be in effect. the input signals are applied to the amplifiers(at tli and rli) through ac coupling capacitors and current limiting resistors. the value of these components determines the sensitivity of the respective amplifiers , and has an effect on the switching times between transmit and receive modes. the feedback elements for the amplifier are back-to-back diodes which provide a logarithmic gain curve, thus allowing operation over a wide range of signal levels. the outputs of the amplifiers are rectified, having a quick rise time and a slow decay time. the rise time is determined by the external resistor and capacitor, and is on the order of a fraction of a second. the switching time is not fixed, but depends on relative values of the transmit and receive signals, as well as these external components. fig 7 indicates the dc transfer characteristics of the log amps, and fig 8 indicates the transfer characteristics with respect to an ac input signal the dc level at tli, rli, tlo, and rlo is approximately vb. the tx-rx comparator responds to the voltages at tlo and rlo, which in turn are functions of the currents sourced out of tli and rli, respectively. if an offset at the comparator input is desired, e. g. to prevent noise from switching the system, or to give preference to either the transmit or receive channel, this may be achieved by biasing the appropriate input(tli or rli). a resistor to ground will cause a dc current to flow out of that input, thus forcing the output of that amplifier to be biased slightly higher than normal. this amplifier then becomes the preferred one in the system operation. resistor values from 500k to 10m ohms are recommended for this purpose. speaker amplifier the speaker amplifier hac a fixed gain of 34db(50v/v), and is non-inverting. the input impedance is nominally 22 k ohms as long as the output signal is below that required to activate the peak limiter. the fig 9 indicates the typical output swing available at sko(pin 15). since the output current capability is 100ma, the lower curve is limited to 1 5. 0volt swing. the output impedance depends on the output signal level and is a relatively low as long as the signal level is not near maximum limits. at 3 vp-p the output impedance is less than 0. 5 ohms, and at 4. 5vp-p it is less than 3 ohms. the output is short circuit protected at approximately 300 ma. when the amplifier is over driven, the peak limiter causes a portion of the input signal to be shunted to ground, in order to maintain a constant output level. the effect is that of a gain reduction caused by a reduction of the input impedance (at ski) to a value not less than 2. 0k ohms. the capacitor at pin 17(agf) determines the res ponse time of the peak limiter circuit. when a large input signal is applied to ski, the voltage at agc(pin 17) will drop quickly as a current source is applied to the external capacitor. when the large input signal is reduced, the current source is turned off, and an internal 110k ohms resistor discharges the capacitor so that the voltage at agc can return toots normal value(1. 9vdc). the capacitor additionally stabilizes the peak limiting feedback loop. if there is a need to mute the speaker amplifier without disabling the rest of the circuit, this may be accomplished by connecting a resistor from the agc pin to gnd. a 100k ohms resistor will reduce the gain by 34db (0db from ski to sko), and a 10k ohms resistor will reduce the gain by almost 50db. UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 8 transmit detector circuit the transmit detector circuit, also known as the background noise monitor, distinguishes s peech(which consists of bursts) from the background noise( a relatively constant signal). it does this by storing a voltage level, representative of the average background noise, in the capacitor at cp1(pin 11). the resistor and capacitor at this pin have a time constant approximately 5 second(in fig 1). the voltage at pin 1 is applied to the inverting input of the transmit detector comparator. in the absence of speech signal, the non-inverting input receiving the same voltage level minus an offset of 36mv. in this condition, the output transistor turned off, and the voltage at xdc(pin 23) will be at gnd. if the tx-rx comparator is in the transmit position, the attenuators will be in the idle mode( d vacf=75mv). when speech is presented to the microphone, the signal burst appearing at xdi reaches the non-inverting input of the transmit detector comparator before the voltage at the inverting i nput can change, causing the output to switch high, driving the voltage ad xdc up to approximately 4volts. this high level causes the attenuator control block to switch the attenuators from the idle mode to the transmit mode(assuming the tx-rx comparator is in the transmit mode). as long as the speech continues to arrive, and is maintained at a level above the background, the voltage at xdc will be maintained at a high level, and the circuit will remain in transmit mode. the time constant of the components at xdc will determine how much time the circuit requires to return to the idle mode after the cessation of microphone speech signals, such as occurs during the normal pauses in speech. the series resistor and capacitor ax xdi(pin 13) determine the sensitivity of the transmit detector circuit. fig 10 indicates the change in dc voltage levels at cp2 and cp1 in response to a steady state sine wave applied at the input of 0. 068 m f capacitor and 4700 ohms resistor( the voltage change at cp1 is 2. 7 times greater than the change at p2). increasing the resistor, or lowing the capacitor, will reduce the response at these pins. the first amplifier(between xdi and cp2) is logarithmic in order that this circuit be able to handle a wide range of signal levels(or in other words, it responds equally well to people who talk quietly and to people who shout). fig 7 indicates the dc transfer characteristic of the log amp. fig 11 indicates the response at pins 11, 12, and 23 to a varying signal at the microphone. the series of events in fig 11 is as follows: 1)cp2 follows the peaks of speech signals, and decays at a rate determined by the 10 a current source and the capacitor at this pin. 2)cp1 increases at a rate determined by the rc at this pin after cp2 has made a positive transition. it will follow the decay pattern of cp2. 3)the noninverting input of the transmit detector comparator follows cp2, gained up by 2. 7, and reduced by an offset of 36mv. this voltage, compared to cp1, determines the output of the comparator. 4)xdc will rise quickly to 4vdc in response to a positive transition at cp2, but will decay at a rate determined by the rc at this pin. when xdc is above 3. 25vdc, the circuit will be in transmit mode. as it decays towards gnd, the attenuators are taken to the idle mode. microphone amplifier the microphone amplifier is noninverting, has an internal gain of 34db(50v/v), and a nominal input impedance of 10k ohm. the output impedance is typically less than 15 ohms. the maximum p-p voltage swing available at the output is approximately more than what is required in most applications. the input at mci should be ac coupled to the microphone so as to not upset the bias voltage. generally, microphone sensitivity may be adjusted by varying the 2k microphone bias resistor, rather than by attempting to vary the gain of the amplifier. power supply the voltage supply for the UTC34018 at v+ should be in the range of 6. 0 to 11 volts, although the circuit will operate down to 4. 0volts>the voltage can be supplied either from tip and ring, or from a separate supply. the required supply current, with no signal to the speaker, is shown in fig 11. the upper curve indicates the normal operating current when cs is at a logic "0". fig 13 indicates the average dc current required when supplying UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 9 various power levels to a 25 ohms speaker. fig 13 also indicates the minimum supply voltage required to provide the indicated power levels. the peak in power supply current at 5. 0-5. 0 volts occurs as the vcc circuit coms into regulation. it is imperative that the v+ supply be a good ac gnd for stability reasons. if this pin is not well filter( by a 1000 f capacitor at the ic), any variation at v+ caused by the required speaker current flowing through this pin can cause a low frequency oscillation. the result is usually that the circuit will cut the speaker signal on and off at the rate of a few hertz. experiments have shown that only a few inches of wire between the capacitor is not physically adjacent to the ic. it is equally imperative that both ground pins(pin 14 and 22) have a low loss connection to the power supply ground. vcc vcc is a regulated output voltage of 5. 4 0. 5 volts. regulation will be maintained as long as v+ is typically 80 mv greater than the regulated value of vcc. up to 3 mas can be sourced from this supply for external use. the output impedance is less than 20 ohms. the 47 m f capacitor indicated for connection to pin 20 is essential for stability reasons, it must be located adjacent to the ic. if the circuit is deselected (see section on chip select), the vcc voltage will go to 0 volts. if UTC34018 is to be powered from a regulated supply( not the tip and ring lines) of less than 6. 5 volts, the configuration of fig 14 may be used so as to ensure to that vcc is regulated UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 11 19 26 28 27 4 1 2 3 25 21 8 7 6 5 9 10 13 12 11 23 18 16 20 22 17 15 14 24 vb regulator attenuator control receive/transmit comp transmit attenuator receive attenuator speaker amp peak limiter vcc regulator transmit detector comp vth (36mv) receive amp transmit log amp mic amp vb vb vb vb vb vcc vcc rlo vb vlc acf txi rr txo rrx rxi rtx rxo ski skg sko agc gnd vcc 5.4v v+ cs xdc cp1 cp2 xdi mco mci tli tlo rti 25 4.3k 18k 30k 91k 47 f 0.05 f 0.01 f 0.1 f 0.1 f 4.7 f 47 f 1.0 f vcc 24k 20k 2.2m vb 0.068 f 4.7k 1.0 f 2.2m 0.068 f 3.3k 2k 0.1 f 0.05 f 0.068 f 4.7k 4.7 f47 f 100k 47 f 200k 1000 f 47 f 1.0 f vb vcc 10k 12k 59k 33k 56k 26k 500 500 dc line input receive input mic transmit output fig 1 test circuit max gain vacf=6.0mv max attenuation vacf=150mv rr=30k ohms vlc=vb fig 2 transmit attenuator versus rtx fig 3 receive attenuator versus rrx -70 -60 -50 -40 -30 -20 -10 0 +10 +20 db -70 -60 -50 -40 -30 -20 -10 0 +10 +20 db 10k 100k 1m rtx (ohms) 1k 10k 100k rrx (ohms) max gain vacf=6.0mv max attenuation vacf=150mv rr=30k ohms vlc=vb usable range fig 4 gain and attenuation versus resisrtor ratio -70 -60 -50 -40 -30 -20 -10 0 +10 +20 db 0.1 1.0 10 ratio grx vs rrx/rr vacf=150mv gtx vs rtx/rr vacf=6.0mv atx vs rtx/rr vacf=150mv arx vs rrx/rr vacf=6.0mv vlc=vb fig 5 attenuation gain versus vlc +10 0 -10 -20 -30 -40 0.2 0.6 0.4 0.8 1.0 0 rrx=18k ohms rtx=91k ohms rr=30k ohms circuit in receive mode gtx grx mimimum recommended level UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 12 0 20 40 60 80 100 120 140 160 -40 -30 -20 -10 0 +10 fig 6 attenuator gain vs vacf rr=30k ohms rtx=91k ohms rrx=18k ohms gtx grx vacf(mv) db 0 -20 -40 -60 -80 0 50 100 150 200 250 input current output voltage vb (rli,tli,xdi) (rlo,tlo,cp2) dc input current( a) output channel(mv) fig 7 log amp transfer characteristics fig 8 log amp transfer characteristics output channel(mv) 020406080 0 30 60 90 120 150 vb (rli,tli) (rlo,tlo) input votage (@ 1.0khz) r=4700 ohms c=0.068 f r=4700 ohms c=0.068 f r=2700 ohms c=0.1 f cr input voltage(mvp-p) fig 9 speaker amp output vs supply voltage 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 3.0 2.0 2.0 4.0 6.0 8.0 10.0 0 no load 25 ohms load v+(volts) vsko max(p-p volts) input signal (@ mci) cp2(pin 12) solid line=cp1(pin11) dotted line=voninverting input of transmit detectot comparator xdc(pin 23) v1=200mv 2.7 x v1 slope=0.5v/sec 4volts 36mv fig 11 transmit detector operation UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 13 0 50 100 150 250 200 0 100 200 300 400 500 cp1,cp2 voltage channel(mv) vmco(mvrms) fig 10 response at cp1 and cp2 ? vcp1 ? vcp2 fig 12 supply current vs supply voltage v+(volts) iv+(ma) 4.0 5.0 6.0 7.0 8.0 9.0 10 11 1.0 2.0 5.0 6.0 4.0 7.0 8.0 3.0 0.0 3.0 9.0 cs=0 cs=1 vsko=0mvrms v+(volts) 4.0 5.0 6.0 7.0 8.0 9.0 10 11 3.0 iv+(ma) 10 15 20 25 30 35 100mw 80mw 50mw 20mw 10mw 5mw fig 13 supply current vs supply voltage vs speaker power fig 15 supply current vs supply voltage 5.0 6.0 7.0 8.0 5 15 20 25 4.0 3.0 0 10 vs(volts) is(ma) available operating range 18 20 16 22 vs (regulated supply) UTC34018 100 f fig 14 alternate power supply configuration UTC34018 li near i ntegrated ci rcui t youw ang electroni cs co. ltd 14 9 15 14 txi mco xdi txo tli cp1 acf agc v+ cs rli rxi gnd rlo tlo xdc cp2 ski rxo rrx rr rtx vcc vb vlc skg sko mci vb 20k 24k 91k 30k 18k 4.3k 200k 47 f47 f 0.01 f 0.05 f 4.7 f4.7 f 1.0 f 1.0 f 2.2m 2.2m 47 f 0.05 f 2k vb 4.7k 0.1 f 0.068 f 0.068 f 3.3k 100k 1.0 f 47 f 4.7 f vcc vb 0.068 f 3.3k 0.1 f 0.01 f 22k 47k 470k 4700pf 470k 4700pf 0.05 f 15k 1000 f 47k 8.2k 0.068 f 0.01 f 2.2k 470k 10 f 33k 47k 8.2k 8.2k 47k 6.8k 2.7k 220k 68k 0.01 f 0.01 f 4.7k 0.1 f mic speaker volume control hook switch s1 t r s2 UTC34018 3 10 13 4 5 11 25 17 16 18 7 27 24 21 20 2 1 28 26 19 12 23 6 8 22 fig 16 basic line powered speakerphone 8 6 20 21 23 + - + - 56k 56k rx tx/idle tx idle UTC34018 fig 17 digital transmit/idle/receive indication |
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