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bl34018 http://www.belling.com.cn - 1 - 8/15/2006 total 16 pages wrote by 2006 1. general description the bl34018 speakerphone integrated circuit inco rporates the necessary amplifiers attenuators and control functions to produce a high quality han ds free speakerphone system include 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 line-powered operation (no additional powe r 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 bl34018 applications include speakerphones for hous ehold and business use intercom systems automotive telephones and others 2. feature voice switched speakerphone circuit. all necessary level detection and attenuation contr ols for a hand-free telephone included. background noise level monitoring with long time co nstant. background sound level compensation for transmit an d receive levels as well as the background level. wide operating dynamic range through signal compres sion. on-chip supply and reference voltage regulation. power audio amplifier for typical 100mw output (int o 25 ohms) with peak limiting for speaker to minimize distortion. chip select pin for active/stand by operation. linear volume control function. 3. block diagram
bl34018 http://www.belling.com.cn - 2 - 8/15/2006 total 16 pages wrote by 2006 4. pin configurations pin name description 1 rr a resistor to ground provides a reference curr ent for the transmit and receive attenuators. 2 rtx a resistor to ground determines the nominal g ain of the transmit attenuator. the transmit channel gain is inversely proportional to the rtx r esistance. 3 txi input to the transmit attenuator. input resis tance is nominally 5.0 k ohms. 4 txo output of the transmit attenuator. the txo ou tput signal drives the input of the transmit level detector, as well as the external circuit whi ch drives the telephone line. 5 tli input of the transmit level detector. an exte rnal 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 ex ternal resistor and capacitor set the time the comparator will hold the system in the transmit mo de after speech ceases. 7 rli input of the receive level detector. an exter nal 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 ext ernal resistor and capacitor set the time the comparator will hold the system in the receive mod e after speech ceases. 9 mci microphone amplifier input. input impedance i s nominally 10 k ohms and the dc bias voltage is approximately equal to vb. 10 mco microphone amplifier output. the mic amp gai n is internally set at 34 db (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 tr ansmit detector compares the cp1 voltage with the speech signal from cp2. 12 cp2 a capacitor at this pin peak detects the spe ech signals for comparison with the background noise level held at cp1. 13 xdi input to the transmit detector system. the m icrophone amplifier output is ac coupled to the xdi pin through an external resistor. 14 skg high current ground pin for the speaker amp output stage. the skg voltage should be within 10 mv of the ground voltage at pin 22. 15 sko speaker amplifier output. the sko pin will s ource and sink up to 100 ma when ac coupled to the speaker. the speaker amp gain is int ernally set at 34 db (50v/v). 16 v+ input dc supply voltage. v+ can be powered fr om tip and ring if an ac decoupling inductor is used to prevent loading ac line signals . the required v+ voltage is 6.0 to 11 v (7.5 v nominal) at 7.0 ma. 17 agc a capacitor from this pin to vb stabilizes t he speaker amp gain control loop and
bl34018 http://www.belling.com.cn - 3 - 8/15/2006 total 16 pages wrote by 2006 additionally controls the attack and decay time of this circuit. the gain control loop limits the speaker amp input to prevent clipping at sko. t he internal resistance at the agc pin is nominally 110 k ohms. 18 cs digital chip select input. when at a logic 0 (<0. 7 v) the vcc regulator is enabled. when at a logic 1 (>1.6 v), the chip is in the st andby mode drawing 0.5 ma. an open cs pin is a logic 0. input impedance is nominally 1 40 k ohms. the input voltage should not exceed 11 v. 19 ski input to the speaker amplifier. input impeda nce is normally 20 k ohms. 20 vcc a 5.4 v regulated output which powers all ci rcuits except the speaker amplifier output stage. vcc can be used to power external circuitry such as a microprocessor( 3.0 ma max). a filter capacitor is required. the bl34018 c an be powered by a separate regulated supply by connecting v+ and vcc to a voltage betwee n 4.5 v and 6.5 v while maintaining cs at a logic 1. 21 vb an output voltage equal to approximately vcc/ 2 which serves as an analog ground for the speakerphone system. up to 1.5 ma of external l oad 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 ampli fier). 23 xdc transmit detector output. a resistor and cap acitor at this pin hold the system in the transmit mode during pauses between words and phras es. when the xdc pin voltage decays to ground the attenuators switch from the tr ansmit mode to the idle mode. the internal resistor at xdc is nominally 2.6 k ohms. 24 vlc volume control input. connecting this pin to the slider of a variable resistor provides receive mode volume control. the vlc pin voltage sh ould be less than or equal to vb. 25 acf attenuator control filter. a capacitor conne cted to this pin reduces noise transients as the attenuator control switches levels of attenuation. 26 rxo output of the receive attenuator. normally t his pin is ac coupled to the input of the speaker amplifier. 27 rxi input of the receive attenuator. input imped ance is nominally 5.0 k 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 re sistance.
bl34018 http://www.belling.com.cn - 4 - 8/15/2006 total 16 pages wrote by 2006 5. electrical characteristics (refer to figure 1) absolute maximum ratings (voltages referred to pin 22) (ta = 25 c) parameter value units v+ terminal voltage (pin 16) +12, -1.0 v cs (pin 18) +12, -1.0 v speaker amp ground (pin 14) +3.0, -1.0 v vlc (pin 24) vcc, -1.0 v storage temperature -65 to +150 c maximum ratingsare those values beyong which the safety of the device cannot be guaranteed, they are not meant to imply that the de vices should be operated at these limits. the electrical characteristics tables pro vide conditions for actual device operation. recommended operating conditions parameter value units v+ terminal voltage (pin 16) +6.0 to +11 v cs (pin 18) 0 to +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 amp ground (pin 14) -10 to +10 mvdc ambient temperature -20 to +60 c electrical characteristics parameter symbol pin min typ max units supply voltages v+ supply current v+ = 11v, pin 18 = 0.7v v+ = 11v, pin 18 = 1.6v iv+ 16 -- -- -- -- 9.0 800 ma ua vcc voltage ( v+ = 7.5v) line regulation (6.5v< v+ <11v) vcc d vcc ln 20 4.9 -- 5.4 65 5.9 150 vdc mv
bl34018 http://www.belling.com.cn - 5 - 8/15/2006 total 16 pages wrote by 2006 output resistance (icc = 3.0 ma) dropout voltage ( v+ = 5.0 v) rovcc vccsat -- -- 6.0 80 20 300 ohm mv vb voltage (v+ = 7.5v) output resistance ( ib = 1.7 ma) vb rovb 21 2.5 -- 2.9 250 3.3 -- vdc ohm attenuators receive attenuator gain (@1.0 khz) rx mode, pin 24 = vb; pin 27 = 250 mvrms range ( rx to tx modes) idle mode, pin 27 = 250 mvrms rxo voltage ( rx mode) delta rxo voltage (switch from rx to tx mode) rxo sink current (rx mode) rxo source current (rx mode) rxi input impedance volume control range ( rx attenuator gain, rx mode, 0.6vb < pin 24 < vb) grx d grx grxi vrxo d vrxo irxol irxoh rrxi vcr 26, 27 2.0 40 -20 1.8 -- 75 1.0 3.5 24.5 6.0 44 -16 2.3 -- -- -- 5.0 -- 10 48 -12 3.2 100 -- 3.0 8.0 32.5 db db db vdc mv ua ma k w db transmit attenuator gain (@1.0 khz) tx mode, pin 3 = 250 mvrms range ( tx to rx mode) idle mode, pin 3 = 250 mvrms txo voltage ( tx mode) delta txo voltage (switch from tx to rx mode) txo sink current (tx mode) txo source current (tx mode) txi input impedance gtx d gtx gtxi vtxo d vtxo itxol itxoh rtxi 3, 4 4.0 40 -16.5 1.8 -- 75 1.0 3.5 6.0 44 -13 2.3 -- -- -- 5.0 8.0 48 -8.5 3.2 100 -- 3.0 8.0 db db db vdc mv ua ma k w acf voltage (vcc - pin 25 voltage) rx mode tx mode idle mode d vacf 20, 25 -- -- -- 150 6.0 75 -- -- -- mv mv mv speaker amplifier speaker amp gain (pin19 = 20mvrms) ski input impedance sko voltage ( pin 19 = cap coupled to gnd) sko high voltage (pin 19 = 0.1v, -100 ma load at pin 15) gspk rski vsko vskoh 15, 9 33 15 2.4 5.5 34 22 3.0 -- 35 37 3.6 -- db k w vdc vdc
bl34018 http://www.belling.com.cn - 6 - 8/15/2006 total 16 pages wrote by 2006 sko low voltage ( pin 19 = -0.1v, +100 ma load at pin 15) vskol -- -- 600 mv microphone amplifier mike amp gain (pin 9 = 10mvrms , 1.0 khz) mike amp input resistance gmci rmci 9, 10 32.5 6.5 34 10 35 16 db k w logamps rlo leakage current (pin 8 = vb + 1.0v) tlo leakage current (pin 6 = vb + 1.0v) transmit -receive switching threshold ( ratio of itli to irli - at 20 ua - to switch tx-rx comparator) ilkrlo ilktlo ith 8 6 5,7 25 -- -- 0.8 -- -- -- 2.0 2.0 1.2 ua ua transmit detector xdc voltage -- idle mode -- tx mode cp2 current source vxdc icp2 23 12 -- -- 5.0 0 4.0 10 -- -- 13 vdc vdc ua distortion rx mode C rxi to sko (pin 27 = 10mvrms ,1.0khz) tx mode C mci to txo (pin 9 = 5.0mvrms ,1.0khz) rxd txd 27, 15 4,9 -- -- 1.5 2.0 -- -- % % notes: 1. v+ = 7.5v, cs = 0.7v except where noted. 2. rx mode : pin 7 = -100 ua, pin 5 = +100 ua, exce pt where noted. tx mode : pin 5,13 = -100 ua, pin 7 = +100 ua, pin 11 = 0v. idle mode: pin 5 = -100 ua, pin 7, 13 = +100 ua. 3. current into a pin designated as +; curr ent out of a pin designated as - . 4. voltages referred to pin 22. ta = +25 c. temperature characteristics (-20 to 60 c) parameter pin typical change units v+ supply current (v+ = 11v, pin 18 = 0.7v) 16 -0.2 % / c v+ supply current (v+ = 11v, pin 18 = 1.6v) 16 -0.4 % / c vcc voltage ( v+ = 7.5v) 20 +0.1 % / c attenuator gain ( max and min settings) 0.003 db/ c
bl34018 http://www.belling.com.cn - 7 - 8/15/2006 total 16 pages wrote by 2006 delta rxo, txo voltages 4,26 0.24 % / c speaker amp gain 15,19 0.003 db/ c microphone amp gain 9,10 0.001 db/ c microphone amp input resistance 9 +0.4 % / c tx - rx switching threshold (@ 20 ua) 5,7 0.2 na/ c 6. design guidelines(ref to fig. 1) attenuators the transmit and receive attenuators are complement ary in function, i.e., when one is at maximum gain the other is at maximun attenuation, a nd 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-duplex operation required of a speakerphone. the attenuato rs are controlled safely by the voltage at the acf pin. the acf voltage is provided by the attenua tor control block, which receives 3 inputs:a) the tx - rx comparator, b) the transmit detector co mparator, and c) the volume control. the response of the attenuators is based on the differe nce of the acf voltage from vcc (referred to as d vacf). if d vacf is approximately 6mv the transmit attenuator i s fully on and the receive attenuator is fully off( transmit mode). if d vacf is approximately 150mv the circuit is in the r eceive mode. if d vacf is approximately 75mv, the circuit is in the idle mode, and the two attenuators are at gain settings approximately half way (in db) between the ir fully on and fully off positions. the maximum gain and attenuation values are determi ned by the three external resistors rr, rtx, and rrx. rr affects both attenuators according to its value relative to rtx and rrx. rtx affects the gain and attenuation of only the tr ansmit attenuator, while rrx affects the gain and attenuation of only the receive attenuator. a value of 30 k is recommended for rr as a starting point, and then rtx and rrx selected to suit the particula r design goals. the input impedance of the attenuators (at txi and rxi) is typically 5.0 k w , and the maximum input signal which will not cause output di stortion is 250 mvrms(707 mvp-p). the 4300 ohm resistor and 0.01 uf capacitor at rxo (in figure 1) filters out high frequency components in the receive path. this helps minimize high frequency acoustic feedback problems. the filters insertion loss is 1.5 db at 1.0 khz. t he 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 ou put is summarized in the following truth table: tx C rx comp transmit det comp volume control d vacf mode transmit transmit no effect 6.0 mv transmit transmit idle no effect 75 mv idle receive transmit affects d vacf 50 C 150 mv receive receive idle affects d vacf 50 C 150 mv receive
bl34018 http://www.belling.com.cn - 8 - 8/15/2006 total 16 pages wrote by 2006 as can be seen from the truth table, the tx-rx comp arator dominates. the transmit detector comparator is effective only in the transmit mode, and the volume control is effective only in the receive mode. the tx-rx comparator is in the transmit position wh en there is sufficient transmit signal present over and above any receive signal. the tran smit detector comparator then determines whether the transmit signal is a result of backgrou nd noise ( a relatively stable signal) or speech which consists of bursts. if the signal is due to b ackground noise, the attenuators will be put into the idle mode ( d vacf = 75 mv). if the signal consists of speech, th e attenuators will be switched to the transmit mode ( d vacf = 6.0 mv.) a further explanation of this function will be foun d in the section on the transmit detector circuit. the tx-rx comparator is in the receive position whe n there is sufficient receive signal to overcome the background noise and any speech signal s. the d vacf will now be 150 mv 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, and the attenuation of the transmit attenuator, will vary in a complementary manner. at the minimum recommended op erating level (vlc = 0.55vb) the gain of the tranmit attenuator is actually greater than that of the receive atttenuator, although it is at receive mode. the effect of varying vlc is to vary d vacf, with a resulting variation in the gains of the attenuators. the capacitor at acf (pin 25) smooths the transitio n 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 r educed from the typical 45 db by adding a resistor between pins 20(vcc ) and 25(acf). the eff ect is a reduction of the maximum d vacf voltage in the receive mode, while not affecting d vacf in the transmit mode. logamps (transmit and receive level detectors) the log amps monitor the levels of the transmit and receive signals, so as to tell the tx-rx comparator which mode should be in effect. the inpu t 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 respec tive amplifiers, and has an effect on the switching times between tranmit and receive modes. the feedback elements for the amplifiers are back-to-back diodes which provide a logarithmic gai n curve. the outputs of the amplifiers are rectified, having a quick rise and a slow decay tim e. the rise time is determined primarily by the external capacitor and an internal 500 ohm resistor , and is on the order of a fraction of 1 ms. the decay time is determined by the external resistor a nd capacitor, and is on the order of a fraction of 1 s. the switching time is not fixed, but depend s on the relative values of the transmit and receive signals, as well as these external componen ts. the tx-rx comparator responds to the voltages at tl o and rlo, which in turn are functions of the currents sourced out of tli and rli, respect ively. 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 b y 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
bl34018 http://www.belling.com.cn - 9 - 8/15/2006 total 16 pages wrote by 2006 amplifier to be biased slightly higher than normal. this amplifier then becomes the preferred one in the system operation. resistor values from 500 k to 10 m ohms are recommended for this purpose. speaker amplifier the speaker amplifier has a fixed gain of 34 db (50 v/v), and is noninverting. the input impedance is nominally 22 k w as long as the output signal is below that require d to activate the peak limiter. since the output current capability i s 100 ma, the output swing is limited to 5.0vpp while load is 25 ohms. the output impedance depends on the output signal level and is relatively low as long as the signal level is not near the max imum limits. at 3 vpp, it is < 0.5 ohms, and at 4.5 vpp, it is < 3 ohms. the output is short circui t protected at approximately 300 ma. when the amplifier is overdriven, 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 impeda nce (at ski) to a value not less than 2.0 k w . the capacitor at pin 17 (agc) determines the respon se time of the peak limiter circuit. when a large input signal is applied to ski, the vo ltage at agc will drop quickly as a current source is applied to the external capacitor. when t he large input signal is reduced, the current source is turned off, and an internal 110 k w resister discharges the capacitor so the voltage a t agc can return to its normal value (1.9 vdc). the c apacitor additionally stabilizes the peak limiting feedback loop. if there is a need to mute the speaker amplifier wi thout disabling the rest of the circuit, this may be accomplished by connecting a resistor from t he agc pin to ground. a 100 k w resister will reduce the gain by 34 db (0 db from ski to sko ), and a 10 k w resister will reduce the gain by almost 50 db. transmit detector circuit the transmit detector circuit, also known as the ba ckground noise monitor, distinguishes speech (which consists of bursts) from the backgrou nd 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 pi n have a time constant of approximately 5 seconds(in figure 1). the voltage at pin 11 is appl ied to the inverting input of the transmit detector comparator. in the absence of speech signa ls, the noninverting input receives the same voltage level minus an offset of 36 mv. in this con dition, the output of the comparator will be low, the output transistor turned off, and the voltage a t xdc (pin 23) will be at ground. if the tx-rx comparator is in the transmit position, the attenua tors will be in the idle mode ( d vacf = 75 mv). when speech is presented to the microphone, the sig nal burst appearing at xdi reaches the noninverting input of the transmit detector compara tor before the voltage at the inverting input can change, causing the output to switch high, driv ing the voltage at xdc up to approximately 4 v. this high level causes the attenuator control bl ock to switch the attenuators from the idle mode to the transmit mode (assuming the tx-rx comparator is in the transmit mode). the series resistor and capacitor at xdi (pin 13) d etermine the sensitivity of the transmit detector circuit. increasing the resistor, or lower ing the capacitor, will reduce the response at cp2 and cp1.
bl34018 http://www.belling.com.cn - 10 - 8/15/2006 total 16 pages wrote by 2006 the response at cp2, cp1, xdc to a varying signal a t the microphone is as follows: 1. cp2 ( pin 12) follows the peaks of the speech si gnals, and decays at a rate determined by the 10 ua current source and the capacitor at cp2. 2. cp1 (pin 11) increases at a rate determined by t he 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 36 mv. this voltage, compa red to cp1, determines the output of the comparator. 4. xdc (pin 23) will rise quickly to 4 vdc in respo nse to a positive transition at cp2, but will decay at a rate determined by the rc at this pin. w hen xdc is above 3.25 vdc, the circuit will be in the transmit mode. as it decays towards groun d, the attenuators are taken to idle mode. microphone amplifier the microphone amplifier is noninverting, has an in ternal gain of 34 db(50 v/v), and a nominal input impedance of 10 k w . the output impedance is typically < 15 ohms. the maximum p-p voltage swing available at the output is approx imately 2.0 v less than vcc, which is substantially more than what is required in most ap plications. the input at mci(pin 9) should be ac coupled to the microphone so as to not upset the bias voltage. generally, microphone sensitivity may be adjusted by varying the 2 k microphone bias resistor, rather than by attempting to varying the gain of the amplifier. power supply the voltage supply at v+ (pin 16) should be in the range of 6.0 to 11 v, although the circuit will operate down to 4.0 v. the voltage can be supp lied either from tip and ring, or from a separate supply. it is imperative that the v+ supply be a good ac gr ound for stability reasons. if it is not well filtered (by a 1000 uf capacitor at the ic), any va riation at v+ caused by the required speaker current flowing through this pin can cause a low fr equency oscillation. the result is usually that the circuit will cut the speaker signal on and off at the rate of a few hertz. vcc vcc (pin 22) is a regulated output voltage of 5.4 0.5 v. regulation will be maintained as long as v+ is (typically) 80 mv greater than the re gulated value of vcc. up to 3 ma can be sourced from this supply for external use. the outp ut impedance is < 20 ohms. the 47 uf capacitor connecting to this pin is essential for s tabili ty reasons. the capacitor must be located adjacent to the ic.if th e circuit is deselected, the vcc will go to 0 v. vb vb is a regulated output voltage with a nominal val ue of 2.9 0.4 v. it is derived from vcc and tracks it, holding a value of approximately 54 % of vcc. 1.5 ma can be sourced from this supply at a typically output impedance of 250 ohms. the 47 uf capacitor connecting to this pin is essential for stability reasons. the ca pacitor must be ocated adjacent to the ic. if the circuit is deselected, t he vb will go to 0 v. chip select the cs pin (pin 18) allows the chip to be powered down an ytime its functions are not
bl34018 http://www.belling.com.cn - 11 - 8/15/2006 total 16 pages wrote by 2006 required. a logic 1 level in the range of 1.6 v t o 11 v deselects the chip. the input resistance at pin 18 is > 75 k w . the vcc and vb regulated voltages go to 0.0 when the chip is deselected. leaving pin 18 open is equivalent to a logic 0 (c hip enabled).
bl34018 http://www.belling.com.cn - 12 - 8/15/2006 total 16 pages wrote by 2006
bl34018 http://www.belling.com.cn - 13 - 8/15/2006 total 16 pages wrote by 2006
bl34018 http://www.belling.com.cn - 14 - 8/15/2006 total 16 pages wrote by 2006 switching time the switching times of the speakerphone circuit dep ends not only on the various external components, but also on the operating conditions of the circuit at the time a change is to take effect. for example, the switching time from idle t o transmit is generally quicker than the switching time from receive to transmit ( or transm it to receive). the components which most significantly affect the timing between the transmit and receive modes are those at pins 5 (transmit turn-on), 6 (tr ansmit turn-off), 7 (receive turn-on), and 8 (receive turn-off). these four timing functions are not independent, but interactive since the tx-rx comparator operates on a relative tx-rx comparison, rather than on absolute values. the components at pins 11, 12, 13 and 23 affect the tim ing from the transmit mode to the idle mode. timing from the idle mode to transmit mode is relat ively quick ( due to the quick charging of the various capacitors), and is not greatly affected by the component values. pins 5-8 do not affect the idle-to-transmit timing since the tx-rx comparator must already be in the transmit mode for this to occur. the following table provides a summary of the effec t on the switching time of the various components, including the volume control: components tx to rx rx to tx tx to idle rc @ pin 5 moderate significant no effect rc @ pin 6 significant moderate no effect rc @ pin 7 significant moderate no effect rc @ pin 8 moderate significant no effect rc @ pin 11 no effect slight moderate c @ pin 12 no effect slight significant rc @ pin 13 no effect slight slight rc @ pin 23 no effect slight significant v@ pin 24 no effect moderate no effect c@ pin 25 moderate moderate slight
bl34018 http://www.belling.com.cn - 15 - 8/15/2006 total 16 pages wrote by 2006 additionally, the following should be noted: 1) the rcs at pins 5 and 7 have a dual function in that they affect the sensitivity of the respective log amplifiers, or in other words, how loud the speech must be in order to get control of the speakerphon e circuit. 2) the rc at pin 13 also has a dual function in tha t it determines the sensitivity of the transmit de tector circuit. 3) the volume control affects the switching speed, and the relative response to transmit signals, in t he following manner: when the circuit is in the recei ve mode, reducing the volume control setting increases the signal at txo, and consequently the signal to the tli pin. therefore a given signal at txi will switch the circuit into the transmit mo de quicker at low volume settings. applications information the bl34018 speakerphone ic is designed to provide the function additionally required when a speakerphone is added to a standard telephon e. the ic provides the necessary relative level detection and comparison of the speech signal s provided by the talkers at the speakerphone(near end speaker) and at the distant t elephone(far end speaker). the bl34018 is designed for use with an electorate type microphone a 25 ohms speaker, and has an output power capability of 100mw.all externa l components surrounding this device are passive, however, this ic does require additional c ircuitry to interface the tip and ring telephone lines. two suggested circuits are shown in the data sheet. fig 16 depicts a configuration which does not inclu de a handset, dialer, or ringer. the only controls are s1(to make the connection to the line) ,s2(a "privacy" switch),and the volume control. it is meant to be used in parallel with a normal te lephone which has dialing and ringing functions. fig 17 depicts a means of providing logic level sig nal that indicate which mode of operation the bl34018 is in. comparator a indicates whether t he circuit is in receive or transmit mode and comparator b indicates(when in transmit/idle mode) whether the circuit is in the transmit or idle mode. the lm393 dual comparator is chosen because o f its low current requirement (<1.0ma), low voltage requirement (as low as 2.0volts), and l ow cost .
bl34018 http://www.belling.com.cn - 16 - 8/15/2006 total 16 pages wrote by 2006

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