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| ts34118 voice switched speakerphone circuit 1/21 version: a13 sop - 2 8 general description the ts34118 voice switched speakerphone circuit inc orporates the necessary amplifiers, attenuators, le vel detectors, and control algorithm to form the heart of a high quality hands-free speakerphone system. i ncluded are a microphone amplifier with adjustable gain and mut e control, transmit and receive attenuators which o perate in a complementary manner, level detectors at both inp ut and output of both attenuators, and background n oise monitors for both the transmit and receive channels . a dial tone detector prevents the dial being atte nuated by the receive background noise monitor circuit. also included are two line drive amplifiers which can be used to form a hybrid network in conjunction with an extern al coupling transformer. a high-pass filter can be used to filter out 60hz noise in the receive channel, or for other filtering functions .a chip disable pin permits po wering down the entire circuit to conserve power on long loops where loop current is at a minimum. the ts34118 may be operated from a power supply, or it can be powered from the telephone line, requiri ng typically 5.0ma. the ts34118 can be interfaced dire ctly to tip and ring (through a coupling transforme r) for stand-alone operation, or it can be used in conjunc tion with a handset speech network and/or other fea tures of a feature phone features improved attenuator gain range: 52db between transmit and receive. low voltage operation for line-power application (3v~6.5v). 4-point signal sensing for improved sensitivity back ground noise monitors for both transmit and receive paths. compatible with ts34119 speaker amplifier. microphone amplifier gain set by external resistor s- mute function included. chip disable for active/standby operation. ordering information part no. package packing ts34118cs28 c8 sop-28 25pcs / tube ts34118cs28 rd sop-28 1.5kpcs / 13 reel absolute maximum rating description value unit supply voltage (pin 4) -1.0, +7.0 v voltage at cd (pin 3), mut (pin 12) -1.0,vcc +1.0 v voltage at vlc (pin 13) -1.0,vcc +0.5 v voltage at txi (pin 9),rxi (pin 21),fi(pin 2) -0.5, vcc +0.5 v storage temperature range -65 to +150 oc pin definition : 1. fo 8. txo 15. v b 22. gnd 2. fi 9. txi 16. cpt 23. tli1 3. cd 10. mco 17. tli2 24. tlo1 4. vcc 11. mci 18. tlo2 25. rlo1 5. hto+ 12. mut 19. rlo2 26. rli1 6. hto- 13. vlc 20. rli2 27. cpr 7. hti 14. c t 21. rxi 28. gnd
ts34118 voice switched speakerphone circuit 2/21 version: a13 recommended operating conditions description min. typ. max. units supply voltage (pin 4) (see text) 3.5 -- 6.5 v cd input (pin 3), mut input (pin 12) 0 -- vcc v i vb current (pin 15) - -- 500 a vlc (pin 13) 0.3xv b -- v b v attenuator input signal voltage (pin 9,21) 0 -- 350 mvrms microphone amplifier, hybrid amplifier gain 0 -- 40 db load current @rxo, txo (pins 8,22) @mco (pin 10) @hto-, hto+ (pin 6,5) 0 0 0 -- -- -- 2.0 1.0 5.0 ma ambient operating temperature range -20 -- +60 oc electrical characteristics (t a =25oc, vcc=5.0v, cd 0.8v, unless noted) parameter symbol min typ max unit power supply vcc supply current (vcc=6.5v, cd=0.8v) (vcc=6.5v, cd=2.0v) icc -- -- 5.5 600 8.0 800 ma a cd input resistance (vcc=v cd =6.5v) cd input voltage - high - low r cd v cdh v cdl 50 2.0 0 90 -- -- -- vcc 0.8 k ? v v v b output voltage (vcc=3.5v) (vcc=5.0v) v b -- 1.8 1.3 2.1 -- 2.4 v v b output resistance (i vb =1.0ma) r ovb -- 400 -- ? v b power supply rejection ratio (c vb =220 f,f=1.0khz) psrr -- 54 -- db attenuators (t a =+25 oc ) receive attenuator gain (f=1.0khz, v lc =v b ) rx model, rxi=150mvrms (vcc=5.0v) rx model, rxi=150mvrms (vcc=3.5v) gain change Cvcc=3.5v versus vcc=5.0v agc gain change Cvcc=2.8v versus vcc=5.0v* idle model, rxi=150mvrms range (rx to tx model) g rx g rx ? g rx1 ? g rx2 g rx1 ? g rx3 +4.0 +4.0 -0.5 -- -22 49 +6.0 +6.0 0 -25 -20 52 +8.0 +8.0 +0.5 -15 -17 54 db volume control range (rx model, 0.3v b ts34118 voice switched speakerphone circuit 5/21 version: a13 pin function description pin name description 1 fo filter output. output impedance is less than 5 0 ohms. 2 fi filter input. input impedance is greater than 1.0mohm. 3 cd chip disable. a logic low(<0.8v) sets normal operat ion. a logic high (>2.0v) disables the ic to conserve power. input impedance is norminally 90 k ? . 4 vcc a supply voltage of +2.8 to +6.5 volts is required, at 5.0ma. as vcc falls from 3.5 to 2.8volts, an agc circuit reduces the receive attenu ator gain by 25db (when in the receive mode). 5 hto+ output of the second hybrid amplifier. the gain is internally set at C1.0 to provide a differential output, in conjunction with hto-, to the hybrid transformer. 6 hto- output of the first hybrid amplifier. the ga in of the amp is set by external resistors. 7 hti input and summing node for the first hybrid a mplifier . dc level is v b. 8 txo output of the transmit attenuator. dc level i s approximately v b. 9 txi input to the transmit attenuator. max. signal level is 350m vrms . input impedance is 10k ? . 10 mco output of the microphone amplifier. the gain of the amplifier is set by external resistors. 11 mci input and summing node of the microphone amp lifier. dc level is v b. 12 mut mute input. a logic low(<0.8v) sets normal operatio n. a logic high (>2.0v) mutes the microphone amplifier without affecting the rest of the circuit. input impedance is norminally 90k ? . 13 vlc volume control input. when vlc=v b , the receive attenuator is at maximum gain when in the receive mode. when vlc=0.3v b , the receive gain is down 35db. does not affect th e transmit mode. 14 c t an rc at this pin sets the response time for the c ircuit to switch modes. 15 v b an output voltage vcc/2. this voltage is a system ac ground, and bias es the volume control. a filter cap is required 16 cpt an rc at this pin sets the time constant for the transmit background monitor. 17 tli2 input to the transmit level detector on the mike/speaker side. 18 tlo2 output of the transmit level detector on the mike/s peaker side, and input to the transmit background monitor. 19 rlo2 output of the receive level detector on the mike/speaker side. 20 rli2 input to the receive level detector on the mike/speaker side. 21 rxi input to the receive attenuator and dial tone detec tor. max input level is 350mv rms. input impedance is 10k ? . 22 rxo output of the receive attenuator. dc level i s approximately v b . 23 tli1 input to the transmit level detector on the line side. 24 tlo1 output of the transmit level detector on th e line side. 25 rlo1 output of the receive level detector on the line si de, and input to the receive background monitor. 26 rli1 input to the receive level detector on the line side. 27 cpr an rc at this pin sets the time constant for the receive background monitor. 28 gnd ground pin for the entire ic. ts34118 voice switched speakerphone circuit 6/21 version: a13 typical application circuit figure 1. hybrid amplifier distortion test figure 2. ts34118 block diagram ts34118 voice switched speakerphone circuit 7/21 version: a13 application information the fundamental difference between the operation of a speakerphone and a handset is that of half-duple x versus full duplex. the handset is full duplex since con-v ersion can occur in both directions (transmit and r eceive) simultaneously. a speakerphone has higher gain leve ls in both paths and attempting to converse full du plex results in oscillatory problems due to the loop that exists within the system. the loop is formed by the recei ve and transmit paths, the hybrid, and the acoustic coupling (speak er to microphone). the only practical and economica l solution used to data is to design the speakerphone to funct ion in a half duplex mode C i.e. only one person sp eaks at a time, while the other listens. to achieve this requ ired a circuit which can detect who is talking, swi tch on the appropriate path (transmit or receive) and switch o ff (attenuate) the other path. in this way, the loo p gain is maintained less than unity. when the talkers exchan ge function, the circuit must quickly detect this, and switch the circuit appropriately. by providing speech level de tectors the circuit operates in a hands free mode , eliminating the need for a push to talk switch. the handset by the way, has the same loop as the sp eakerphone. but since the gains are considerably lo wer, and since the acoustic compiling from the earpiece to t he mouthpiece is almost non existent (the receiver is normally held against a persons eat.) oscillations dont oc cur. the ts341118 provides the necessary level detectors , attenuators, and switching control for a properly operating speakerphone. the detection sensitivity and timing are externally controllable. additionally, the ts34 118 provides background nodule monitors which make the circuit insensitive to room and line noise, hybrid amplifie r, and other associated functions, please refer to the block dia gram (figure 2) when reading the following sections . attenuators the transmit and receive attenuators are complement ary in function. i .e. when one is at maximum gain (+6.0db), the other is at maximum attenuation (-4.5db), and v ice verse. they sum of their gains remains constant (within a nominal error hand of 0.1db) at a typical value of ~40db (see figure 10). their purpose is to control the transmit and receive paths to provide the half-duplex operat ion required in a speakerphone. the attenuators are non-inverting, and have a C3.0d b (from max gain) frequency of 100khz. the input im pedance of each attenuator (txi and rxi) is nominally 10k v (see figure 3), and the input signal should be lim ited to 350mvrms (990mvp-p) to prevent distortion. that max imum recommended input signal is independent of the volume control setting. the diode clamp on the inpu ts the input swing, and therefore the maximum negat ive output swing. this is the reason for vrxol and vtxol speci fication being defined as they are in the electrica l characteristics. the output impedance is 10 v until the output current limit typically 2.5ma) is reached. figure 3. attenuator input stage ts34118 voice switched speakerphone circuit 8/21 version: a13 application information (continue) attenuators the attenuators are controlled by the single output of the control block, which is measurable at the c t pin (pin 14). when the ct pin is at +240 millivolts with respect to vb, the circuit is in the receive mode (receive attenuator is at +6.0db). when the ct voltage is at +240 millivolts with respect to vb, the circuit is in the transmit mode (transmit attenuator is at +6.0db). the circuit is in an idle mode when the ct voltage is equal to vb. causing t he attenuators gain to be halfway between their fully on and fully off positions (-20db each). monitoring the ct volt age (with respect to vb) is the most direct method of monitor ing the circuits mode. the inputs to the control block are seven, 2 from t he comparators operated by the level detectors, 2 f rom the background noise monitors, the volume control, the dial-tone detector, and the agc circuit. these seve n inputs are described below. level detectors there are four level detectors-two on the receive s ide and two on the transmit side. refer to figure 4 . the terms in parentheses from one system, and the other terms fr om one system. each level detector is a high gain a mplifier with back-to-bank diodes in the feedback path, resu lting in non-linear gain, which permits operation o ver a wide dynamic range of speech levels. the sensitivity of each level detector is determined by the external r esistor and capacitor at each input (tli1, tli2, rli1 and rli2) . each output charge an external capacitor through a diode and limiting resistor, thus providing a do representati on of the input as signal level. the outputs have a quick rise time (determined by the capacitor and an internal 350 v resistor), and a slow decay time set by an interna l current source and the capacitor. the capacitors on the four outpu ts should have the same value (10%) to prevent tim ing problems. referring to figure 2, on the receives si de, one level detector (rli1) is at the receive inp ut receiving thus same signal as at tip and ring, and the other (pli2 ) is at the output of the speaker amplifier. on the transmit side, one level detector (tli2) is at the output of the m icrophone amplifier, while the other (tli1) is at t he hybrid output. outputs rlo1 and tlo1 feed a comparator, the output of which goes to the attenuator control block. lik ewise, outputs rlo2 and tlo2 feed a second comparator whic h also goes to the attenuator control block. the to uch table for the effects of the level detectors on the control block is given in the section describing t he control block. figure 4. level detector ts34118 voice switched speakerphone circuit 9/21 version: a13 application information (continue) background noise monitors the purpose of the background noise monitors is to distinguish speech (which consists of bursts) from background noise (a relatively constant signal level). there a re two background noise monitors-one for the receiv e path and one for the transmit path. referring to figure 4, t he receive background noise monitor is operated on by the rli1- plo1 level detector, while the transmit background noise monitor is operated on by the tli2-tlo2 level detector. they monitor the background noise by storing a dc v oltage representative of the respective noise level s in capacitors at cpr and cpt. the voltages at these p ins have slow rise times (determined by the externa l rc), but fast decay times. if the signal at rli1 (or tli2) c hanges slowly, the voltage at cpr (or cpt) will rem ain more positive than the voltage at the non-inverting inpu t of the monitors output comparator. when speech i s present, the voltage on the non-inverting input of the comparato r will rise quicker than the voltage at the inverti ng input (due to the burst characteristic of speech), causing its ou tput to change. this output is sensed by the attenu ator control block. the 36mv offset at the comparators input keeps the comparator from changing state unless the speech l evel exceeds the background noise by 4.0db. the time constant of the external rc ( 4.7 seconds) determines the response time to background noise variations. volume control the volume control input at vlc (pin 13) is sensed as a voltage with respect to vb. the volume contro l affects the attenuators only in the receive mode. it has no eff ect in the idle or transmit modes. when in the receive mode, the gain of the receive a ttenuator will be +6.0db, and the gain of the trans mit attenuator will be C46db only when vlc is equal to vb. as vlc is reduced below vb, the gain of the receive attenu ator is reduced (see figure 14), and the gain of the transm it attenuator is increased such that their sum rema ins constant. changing the voltage at vlc changes the voltage at ct (see the attenuator control block section), whic h in turn controls the attenuators. the volume control setting does not affect the maxi mum attenuator input signal at which noticeable dis tortion occurs. the bias current at vlc is typically 60na o ut of the pin, and does not vary significantly with the vlc voltage or with vcc. dial tone detectors the dial tone detector is a comparator with one sid e connected to the receive input (rxi) and the othe r input connected to vb with a 15mv offset (see figure 5). if the circuit is in the receive mode, and the inco ming signal is greater than 15mv (10mvrms), the comparators outpu t will change, disabling the receive idle mode. the receive attenuator will then be at a setting determined sol ely by the volume control. the purpose of this circuit is to prevent the dial tone (which would be considered as continuous noise ) from fading away as the circuit would have the tendency to swit ch to the idle mode. by disabling the receive idle mode, the dial tone remains at the normally expected full level. figure 5. dial tone detector ts34118 voice switched speakerphone circuit 10/21 version: a13 application information (continue) attenuator control block the attenuator control block has the seven inputs d escribed above: -the output of the comparator operated by rlo2 and tlo2 (microphone/speaker side) - designated c1. -the output of the comparator operated by rlo1 and tlo1 (tip/ring side) C designated c2. -the output of the transmit background noise monito r C designated c3. -the output of the receive background noise monitor -designated c4. -the volume control. -the dial tone detector. -the agc circuit. the single output of the control block controls the two attenuators. the effect of c1-c4 is as follows : input output c1 c2 c3 c4 mode tx tx 1 x transmit tx rx y y fast idle rx tx y y fast idle rx rx x 1 receive tx tx 0 x slow idle tx rx 0 0 slow idle rx tx 0 0 slow idle rx rx x 0 slow idle x=dont care; y=c3 and c4 are not both 0. a definit ion of the above terms: 1) transmit means the transmit attenuator is full y on (+6.0db), and the receive attenuator is at max . attenuation (- 46db). 2) receive means both attenuators are controlled by the volume control. at max. volume, the receive attenuator is fully on (+6.0db), and the transmit attenuator is at max. attenuation (-46db). 3) fast idle means both transmit and receive spee ch is present in approximately equal levels. the at tenuators are quickly switched (30ms) to idle until one speec h level dominates the other. 4) slow idle means speech has ceased in both tran smit and receive paths. the attenuators are then sl owly switched (1 second) to the idle mode. 5) switch to the full transmit or receive modes fro m any other mode is at the fast rate ( 30ms). a summary of the truth table is as follows: 1) the circuit will switch to transmit if: a) both transmit level detectors sense higher signal levels relative to the respective receive level detectors (tli1 versus rli1, tli2 versus rli2), and b) the transmit backg round noise monitor indicates the presence of speech. 2) the circuit will switch to receive if: a) both r eceive level detectors sense higher signal levels r elative to the respective transmit level detectors, and b) the receive background noise monitor indicates the pre sence of speech. 3) the circuit will switch to the fast idle mode if the level detectors disagree on the relative stren gths of the signal levels, and at least one of the background noise mo nitors indicates speech. for example, referring to the block diagram (figure 2), if there is sufficient signal a t the microphone amp output (tli2) to override the speaker signal (rli2), and there is sufficient signal at the recei ve input (rli1) to override the signal at the hybri d output (tli1), and either or both background monitors indicate spe ech, then the circuit will be in the fast idle mode . two conditions which can cause the fast idle mode to oc cur are a) when both talkers are attempting to gain control of the system by talking at the same time, and b)when one talker is in a very noisy environment, forcing the other talker to continually override that noise level. in general, the fast idle mode will occur infrequentl y. ts34118 voice switched speakerphone circuit 11/21 version: a13 attenuator control block - continue 4) the circuit will switch to the slow idle mode wh en a) both talkers are quiet (no speech present), o r b) when one talkers speech level is continuously overridde n by noise at the other speakers location. the time required to switch the circuit between tra nsmit, receive, fast idle and slow idle is determin ed in part by the components at the ct pin (pin 14). (see the sec tion on switch time for a more complete explanation of the switching time components). a schematic of the ct circuitry is shown in figure 6, and operates as fol lows: - rt is typically 120k ? , and ct is typically 5.0 f. - to switch to the receive mode, i1 is turned on (i 2 is off), charging the external capacitor to +240m v above vb. (an internal clamp prevents further charging of the capacitor.) - to switch to the transmit mode, i2 is turned on ( i1 is off) bringing down the voltage on the capacit or to C240mv with respect to vb. - to switch to idle quickly (fast idle), the curren t sources are turned off, and the internal 2.0k ? resistor is switched in, discharging the capacitor to vb with a time constant = 2.0kct. - to switch to idle slowly (slowly idle), the curre nt sources are turned off, the switch at the intern al 2.0k ? resistor is open, and the capacitor discharges to vb through the external resistor rt with a time constant = rt ct. figure 6. ct attenuator control block circuit microphone amplifier the microphone amplifier (pins 10,11) has the non-i nverting input internally connected to vb, while th e inverting input and the output are pinned out. unlike most op -amps, the amplifier has an all-npn output stage, w hich maximizes phase margin and gain-bandwidth. this fea ture ensures stability at gains less than unity, as well as with a wide range of reactive loads. the open loop gain is typically 80db (f<100hz), and the gain-bandwidth is typically 1.0mhz (see figure 16). the maximum p-p output swin g is typically 1.0 volt less than vcc with an outpu t impedance of <10 w until current limited is reached (typically 1.5ma) .input bias current at mci is typically 40na out of the pin. the muting function (pin 12), when activated, will reduce the gain of the amplifier to -39db (with rmi=5.0k ? ) by shorting output to the inverting input (see figure 7). the mute input has a threshold of 1.5 volts, and the voltage at this pin must be kept within the range of ground an d vcc (see figure 17). if the mute function is not used, the pin should be grounded. ts34118 voice switched speakerphone circuit 12/21 version: a13 microphone amplifier C continue figure 7. microphone amplifier and mute hybrid amplifiers the two hybrid amplifiers (at hto+, hto-, and hti), in conjunction with an external transformer, provi de the two-to four wire converter for interfacing to the telephon e line. the gain of the first amplifier (hti to hto -) is set by external resistors (gain=-rhf/phi in figure 2), and its output drives the second amplifier, the gain o f which is internally set at C1.0. unlike most op-amps, the am plifiers have all-npn output stage, which maximizes phase margin and gain-bandwidth. this feature ensures sta bility at gains less than unity, as well as with a wide range of reactive loads. the open loop gain of the first amp lifier is typically 80db , and the gain bandwidth o f each amplifier is 1.0mhz (see figure 16). the maximum p-p output swin g of each amplifier is typically 1.2 volts less tha n vcc with an output impedance of <10 ? until current limiting is reached (typically 8.0ma) . the output current capability is guaranteed to be a minimum of 5.0ma. the bias curre nt at hti is typically 30na out of the pin. the connections to the coupling transformer are sho wn in the block diagram (figure 2). the block label ed zbal is the balancing network necessary to match the line i mpedance. filter the operation of the filter circuit is determined b y the external components. the circuit within the t s34118, from pins fi to fo is a buffer with a high input impedan ce (>1.0m w ), and a low output impedance (<50 w ). the configuration of the external components determines whether the circuit is a high-pass filter (as show n in figure 2), a low-pass filter, or a band-pass filter. as a high pass filter, with the components shown in figure 8, the filter will keep out 60hz (and 120hz ) hum which can be picked up by the external telephone lines. figure 8. high pass filter ts34118 voice switched speakerphone circuit 13/21 version: a13 filter C continue as a low pass filter (figure 9), it can be used to roll off the high end frequencies in the receive ci rcuit, which aids in protecting against acoustic feedback problems. with an appropriate choice of an input coupling capacit or to the low pass filter is formed figure 9. low pass filter power supply, v b , and chip disable the power supply voltage at vcc (pin 4) is to be be tween 3.5 and 6.5 volts for normal operation, with reduced operation possible down to 2.8 volts. the power sup ply current is shown in figure 18 for both the powe r-up and power-down mode. the output voltage at vb (pin 15) is (vcc-0.7)/2, and provides the ac ground for the sy stem. the output impedance at vb is 400 ? and in conjunction with the external capacitor at vb, forms a low pass filter for power supply rejection with different capacitor s. the choice of capacitor is application dependent base on whether the circuit is powered by the telephone lin e or a power supply. since vb biases the microphone and hybrid amplifiers, the amount of supply rejection at their outputs is directly related to the rejection at vb , as well as their respective gains. depicts this graphically. the chip disable (pin 3) permits powering down the ic to conserve power and/or for muting purposes. wi th cd 0.8 volts, normal operation is in effect. with cd 2.0 volts and vcc, the ic is powered down. in the powered down mode, the microphone and the hybrid amplifiers are disable, and their outputs go to a high impeda nce state. additionally, the bias is removed from the level de tectors. the bias is not removed from the filter (p ins 1,2). the attenuators (pin 8,9, 21, 22), or from pin 13,14, a nd 15 (the attenuators are disabled, however, and w ill not pass a signal). the input impedance at cd is typically 90k ? , has a threshold of 1.5 volts, and the voltage at this pin must be kept within the range of ground and vcc. if cd is not used, the pin should be grounded. switching time the switching time of the ts34118 circuit is domina ted by the components at ct (pin 14, refer to figur e 6), and secondarily by the capacitors at the level detector outputs (rlo1, rlo2, tlo1, tlo2). the time to switch to receive or to transmit from i dle is determined by the capacitor at ct, together with the internal current sources (refer to figure 6). the switching time is: ? t= ? v ct / i for the typical cause where ? v=240mv, i=60 a. and ct is 50 f, ? t=20ms. if the circuit switches directly from receive to transmit (or vice-versa), the total swit ching time would be 40ms. the switching time from either receive or transmit to idle depends on which type of idle mode is in ef fect. if the circuit is going to fast idle, the time constant is determined by the ct capacitor, and the internal 2.0k ? resistor (figure 6). with ct = 5.0 f, the time constant is 30ms (for 95% change). fast idle is an infrequent o ccurrence, however, occurring when both speakers are talking a nd competing for control of the circuit. the switch ing time from idle back to either transmit or receive is describe d above. ts34118 voice switched speakerphone circuit 14/21 version: a13 switching time - continue if the circuit is switching to slow idle, the tim e constant is determined by the ct capacitor and rt , the external resistor (see figure 6). with ct = 5.0 f, and rt = 120k ? , the time constant is 600ms, giving a switching time of 1.8 seconds (for 95% change). the switching period to slow idle begins when both speakers have stoppe d talking. the switching time back to the original mode will d epend on how soon that speaker begins speaking agai n. the sooner the speaking time starts during the 1.8 seco nds period, the quicker the switching time since a smaller voltage excursion is required. that switching time is determined by the internal current sources as de scribed above. the above switching times occur, however, after the level detectors have detected the appropriate sign al levels, since their outputs operate the attenuator control block. referring to figure 4, the rise time of the level detectors outputs to new speech is quick by comparison ( 1.0ms), determined by the internal 350 ? resistor and the external capacitor (typically 2.0 f). the outputs decay time is determined by the ex ternal capacitor, and an internal 4.0 f current source giving a decay rate of 60ms for 120mv excursion at rlo or tlo. however, th e overall response time of the circuit is not a constant since it depe nds on the relative strength of the signals at the different level detectors, as well as the timing of the signals wit h respect to each other. the capacitors at the four outputs (rlo1, rlo2, tlo1, tlo2) must be equal value (10%) to pre vent problems in timing and level response. the rise time of the level detectors outputs is no t significant since it is so short. the decay time, however, provides a significant part of the hold time necessary to hold the circuit during the normal pauses in speech . the components at the inputs of the level detectors (rli1, rli2, tli1,tli2) do not affect the switchin g time, but rather affect the relative signal levels required t o switch the circuit, as well as the frequency resp onse of the detectors. design equations referring to figure 10(the coupling capacitors have been omitted for simplicity), the following defini tions will be used (all measurements are at 1.0khz) - gma is the gain of the microphone amplifier meas ured from the microphone output to txi (typically 3 5v/v, or 31db); - gtx is the gain of the transmit attenuator, meas ured from txi to txo; - gha is the gain of hybrid amplifiers, measured f rom txo to the hto-/hto+ differential output (typic ally 10.2v/v, or 20.1db); - ght is the gain from hto-/hto+ to tip/ring for t ransmit signals, and includes the balance network ( measured at 0.4v/v, or C8.0db); - gst is the side tone gain, measured from hto-/ht o+ to the filter input (measured at 0.18v/v , or C1 5db); - ghr is the gain from tip/ring to the filter inpu t for receive signals (measured at 0.833v/v or C1.6 db); - gfo is the gain of the filter stage, measured fr om the input of the filter to rxi, typically 0db at 1.0khz; - grx is the gain of the receive attenuator measur ed from rxi to rxo; - gsa is the gain of the speaker amplifier, measur ed from rxo to the differential output of the ts341 19 (typical 22v/v or 26.8db); - gac is the acoustic coupling, measured from the speaker differential voltage to the microphone outp ut voltage. transmit gain the transmit gain, from the microphone output (vm) to tip and ring, is determined by the output charac teristics of the microphone, and the desired transmit level. for example, a typical electret microphone will produc e 0.35mvrms under normal speech conditions. to achiev e 100mvrms at tip / ring, an overall gain of 285v/v is necessary. the gain of the transmit attenuator is f ixed at 2.0 (+6.0db), and the gain through the hybr id normally 0.4 (-8.0db). therefore a gain of 357v/v is required of the microphone and hybrid amplifiers. it is desira ble to have the majority f that gain in the microphone amplifier fo r three reasons: 1. the low level signals from the microphone should be amplifier as soon as possible to minimize signa l/noise problems. ts34118 voice switched speakerphone circuit 15/21 version: a13 design equations - continue 2. to provide a reasonable signal level to the tli2 level detector; and 3)to minimize any gain applied to broadband noise generated within the attenuator. ho wever, to cover the normal voice band, the microphone amplifiers gain should not exceed 48db. for the circuit of figure 10, the gain of the microphone amplifier was set at 35v/v (31db), and t he differential gain of the hybrid amplifiers was s et at 10.2v/v (20.1db). receive gain the overall receive gain depends on the incoming si gnal level, and the desired output power at the spe aker. normal receive levels (independent of the peaks) at tip/ring can be 35mvrms (-27dbm). although on long lines that level can be down to 8.0mvrms (-40dbm).the spe aker power is: pspk = rs 0.6 10dbm/10 (equation 1) where rs is the speaker impedance, and the dbm term is the incoming signal level increased by the gain of the receive path. experience has shown that 30db gain is a satisfactory amount for the majority of applications. using the above numbers and equation 1, it would appear t hat the resulting power to the speaker is extremely low. however, equation 1 does not consider the peaks in normal speech, which can be 10 to 15 times the rms value. considering the peaks, the overall average power ap proaches 20-30mv on long lines, and much more on sh ort lines. referring to figure 10, the gain from tip/ring to t he filter input was measured at 0.833v/v (-1.6db), the filters gain is unity, and the receive attenuators gain is 2.0v /v (+6.0db) at maximum volume. the speaker amplifie rs gain is set at 22v/v (26.8db), which puts the overall gain at 31.2db. loop gain the total loop gain must add up to less than zero d b to obtain a stable circuit. this can be expressed as: g ma +g tx +g ha +g st +g fo +g rx +g sa +g ac <0 (equation 2) using the typical numbers mentioned above, and know ing that gtx+grx=-40db, the required acoustic coupl ing can be determined: g ac <-[31+20.1+(-15)+0+(-40)+26.8]=-22.9db (equation 3) an acoustic loss of at least 23 db is necessary to prevent instability and oscillations, commonly refe rred to as singing. however, the following equations show th at greater acoustic loss is necessary to obtain pro per level detection and switching. switching threshold to switch comparator c1, currents i1 and i3 need to be determined. referring to figure 11, with a rece ive signal vl applied to tip/ring, a current i3 will flow thro ugh r3 into rli2 according to the following equatio n: i3 = r3 v l ?? ? ?? ? 2 g g g g sa rx fo hr (equation 4) where the terms in the brackets are the v/v gain te rms. the speaker amplifier gain is divided by two s ince gsa is the differential gain of the amplifier, and v3 is o btained from one the side of the output. the curren t i1, coming from the microphone circuit, is defined by: i1 = r1 g v ma m (equation 5) where vm is the microphone voltage. since the switc hing threshold occurs when i1=i3, combining the abo ve two equations yields: v m = v l r3 r1 [ ] 2 g g g g g ma sa rx fo hr (equation 6) ts34118 voice switched speakerphone circuit 16/21 version: a13 design equations - continue figure 10. basic block diagram for design purpose this is the general equation defining the microphon e voltage necessary to switch comparator c1 when a received signal vl is present. the highest vm occurs when th e receive attenuator is at maximum gain (+6.0db). u sing the typical numbers for equation 6 yields: v m = 0.52vl (equation 7) to switch comparator c2, currents i2 and i4 need to be determined. with sound applied to the microphone, a voltage vm is creased by the micropho ne, resulting in a current i2 into tli1: i2 = r2 v m ?? ? ?? ? 2 g g g ha tx ma (equation 8) since gha is the differential gain of the hybrid am plifiers, it is divided by two to obtain the voltag e v2 applied to r2. comparator c2 switches when i4=i2. i4 is defined by : i4 = r4 v l [ ] fo hr g g (equation 9) setting i4=i2, and combining the above equations re sults in: v l = v m [ ] [ ] 2 g g g g g r2 r4 fo hr ha tx ma (equation 10) this equation defines the line voltage at tip/ring necessary to switch comparator c2 in the presence o f a microphone voltage. the highest vl occurs when the circuit is in the transmit mode (gtx = +6.0db). usi ng the typical numbers for equation 10 yields: v l =840vm (or vm=0.0019 vl) (equation 11) at idle, where the gain of the two attenuators is C 20db (0.1v/v), equations 6 and 10 yield the same re sult: v m =0.024vl (equation 12) equations 7, 11 and 12 define the thresholds for sw itching, and are represented in the following graph : ts34118 voice switched speakerphone circuit 17/21 version: a13 design equations - continue figure 11. switching threshold the m terms are the slopes of the lines (0.52,0.0 24, and 0.0019) which are the coefficients of the t hree equations. the mrx line represents the receive to transmit thr eshold in that it defines the microphone signal lev el necessary to switch to transmit in the presence of a given re ceive signal level. the mtx line represents the tra nsmit to receive threshold, the mi line represents the idle conditio n, and defines the threshold level on one side (tra nsmit or receive) necessary to overcome noise on the other. some comm ents on the above graph: - acoustic coupling and side tone coupling were no t included in equations 7 and 12. those couplings w ill affect the actual performance of the final speakerphone due to their interaction with speech at the microphone , and the receive signal coming in at tip/ring. the effects o f those couplings are difficult to predict due to t heir associated phase shifts and frequency response. in some cases the coupling signal will add, and other times subtr act from the incoming signal. the physical design of the spe akerphone enclosure, as well as the specific phone line to which it is connected , will affect the acoustic an d side tone couplings, respectively. - the mrx line helps define the maximum acoustic c oupling allowed in a system, which can be found fro m the following equation: g ac-max = ma g r3 2 r1 (equation 13) equation 13 is independent of the volume control se tting. conversely, the acoustic coupling of a desig ned system helps determine the minimum slope of that line. usi ng the component values in equation 13 yields a gac -max of C37db. experience has shown, however, that an acous tic coupling loss of >40db is desirable. - the mtx line helps define the maximum side tone coupling (gst) allowed in the system, which can be found from the following equation: g st = fo g r2 2 r4 (equation 14) using the component values in equation 14 yields a maximum side tone of 0db. experience has shown, how ever, that a minimum of 6.0db loss is preferable. the above equations can be used to determine the re sistor values for the level detector inputs. equati on 6 can be used to determine the r1/r3 ratio, and equation 10 can be used to determine the r4/r2 ratio. in figure 10, r1-r4 each represent the combined impedance of the resist or and coupling capacitor at each level detector in put. the magnitude of each rcs impedance should be kept wit hin the range of 2.0k-15k ? in the voice band (due to the typical signal levels present) to obtain the best p erformance from the level detectors. the specific r and c at each location will determine the frequency response of t hat level detector. ts34118 voice switched speakerphone circuit 18/21 version: a13 application information dial tone detector the threshold for the dial tone detector is interna lly set at 15mv (10mvrms) below vb (see figure 5). that threshold can be reduced by connecting a resistor f rom rxi to ground. the resistor value is calculated from: r = ?? ? ?? ? - 1 ? v v 10k b where vb is the voltage at pin 15, and ? v is the amount of threshold reduction. by connecti ng a resistor from vcc to rxi, the threshold can be increased. the resisto r value is calculated from: r = ?? ? ?? ? - - 1 ? v v v 10k b cc where ? v is the amount of threshold increase. background noise monitors for testing or circuit analysis purposes, the trans mit or receive attenuators can be set to the on p osition, by disabling the background noise monitors, and applyi ng a signal so as to activate the level detectors. grounding the cpr pin will disable the receive background noise m onitor, thereby indicating the presence of speech to the attenuator control block. grounding cpt does the sa me for the transmit path. additionally, the receive background noise monitor is automatically disabled by the dial tone detector whenever the receive signal exceeds the detectors threshold. transmit / receive detection priority although the ts34118 was designed to have an idle m ode such that the attenuators are halfway between t heir full on and full off positions, the idle mode can be bia sed towards the transmit or the receive side. with this done, gaining control of the circuit from idle will be ea sier for that side towards which it is biased since that path will have less attenuation at idle. by connecting a resistor from ct (pin 14) to ground , the circuit will be biased towards the transmit s ide. the resistor value is calculated from: r = ?? ? ?? ? - 1 ? v vb r t where r is the added resistor, rt is the resistor n ormally between pins 14 and 15 (typically 120 ? ), and ? v is the difference between vb and the voltage at c7 at idle c refer to (figure 10) by connecting a resistor from ct (pin 14) to vcc, t he circuit will be biased towards the transmit side . the resistor value is calculated from: r = ?? ? ?? ? - - 1 ? v v v r b cc t r, rt, and ? v are the same as above. switching time will be som ewhat affected in each case due to the different voltage excursions required to get to transmit and receive from idle. for practical considerations, th e ? v shift should not exceed 100mv. ts34118 voice switched speakerphone circuit 19/21 version: a13 application information - continue volume control if a potentiometer with a standard linear taper is used for the volume control, in situations where th is may be objectionable, a potentiometer with an audio taper (commonly used in radio volume controls) will provi de a more linear relationship as indicated in figure 12. the slight non-linearity at each end of the graph is du e to the physical construction of the potentiometer, and will vary am ong different manufactures. figure 12. receive attenuator gain versus potentiom eter position using audio taper rfi interference potential radio frequency interference problems sho uld be addressed early in the electrical and mechan ical design of the speakerphone. rfi may enter the circuit thro ugh tip and ring, through the microphone wiring to the microphone amplifier, or through any of the pc boar d trances. the most sensitive pins on the ts34118 a re the inputs to the level detectors (rli1, rli2, tli1, tl i2) since, when there is no speech present, the inp uts are high impedance and these op amps are in a near open loop condition. the board traces to these pins should b e kept short, and the resistor and capacitor for each of t hese pins should be physically close to the pins. a ny other high impedance input pin (mci, hti, fi, vlc) should be c onsidered sensitive to rfi signals. the final analysis proper operation of a speakerphone is a combination of proper mechanical (acoustic) design as well as proper electronic design. the acoustics of the enclosure m ust be considered early in the design of a speakerp hone. in general, electronics cannot compensate for poor aco ustics, low speaker quality, or any combination of the two. proper acoustic separation of the speaker and micro phone, as described in the design equations, is ess ential. the physical location of the microphone, along with the characteristics of the selected microphone, will p lay a large role in the quality of the transmitted sound. the microp hone and speaker vendors can usually provide additi onal information on the use of their products. in the fi nal analysis, the circuits shown in this datasheet will have to be fine tuned to match the acoustics of the enclosur e, the specific hybrid, and the specific microphone and speaker selected. the component values shown in this datash eet should be considered as starting points only. t he gains of the transmit and receive paths are easily adjusted at the microphone and speaker amplifiers, respectiv ely, the switching response can then be fine turned by varyi ng (in small steps) the components at the level det ector inputs until satisfactory operation is obtained for both l ong and short lines ts34118 voice switched speakerphone circuit 20/21 version: a13 sop-28 mechanical drawing marking diagram y = year code m = month code ( a =jan, b =feb, c =mar, d =apl, e =may, f =jun, g =jul, h =aug, i =sep, j =oct, k =nov, l =dec) l = lot code sop-28 dimension dim millimeters inches min max min max a 17.70 18.00 0.697 0.709 b 7.41 7.59 0.292 0.299 c 10.15 10.55 0.400 0.415 d 2.37 2.63 0.093 0.104 e 1.27bsc 0.05bsc f 0.40ref 0.016ref g 0.10 0.30 0.004 0.012 h 0.60 1.00 0.024 0.040 i 0.25bsc 0.010bsc j 0.254typ typ k 0.5 0.020 ts34118 voice switched speakerphone circuit 21/21 version: a13 notice specifications of the products displayed herein are subject to change without notice. tsc or anyone on its behalf, assumes no responsibility or liability for any erro rs or inaccuracies. information contained herein is intended to provide a product description only. no license, express or implied, to any intellectual property rights is granted by this document. except as provided in tscs terms and co nditions of sale for such products, tsc assumes no liability wh atsoever, and disclaims any express or implied warr anty, relating to sale and/or use of tsc products includi ng liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, cop yright, or other intellectual property right. the products shown herein are not designed for use in medical, life-saving, or life-sustaining applica tions. customers using or selling these products for use i n such applications do so at their own risk and agr ee to fully indemnify tsc for any damages resulting from such i mproper use or sale. |
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