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Build a VOX with delayed audio What's the biggest problem with VOX (voice operated relay) circuits? They chop off the first syllable of speech every time they operate. This circuit doesn't. It passes the speech signal through an electronic delay circuit so that when the relay operates, all the signal goes through. Design by DARREN YATES Ever since voice operated relay circuits were invented, they have been chopping off the first syllable of speech signals. It is inevitable. There is always a short delay in any circuit which senses a rapidly rising signal but the big problem is the finite operating time of the relay. Typically, the small relay used in VOX circuits will take 10 milli28 SILICON CHIP seconds to close, after it has been energised. This is quite a long time as far as speech is concerned and it means that the first syllable, or at least the first consonant, is missed, never to be heard of again. This applies whether the vox circuit is used to operate a cassette recorder or a transceiver. The solution to this problem has been known almost as long as vox circuits have been around: put in an acoustic delay. That way, the relay switches the delayed audio signal. Since the acoustic delay is longer than the relay closing time, the whole signal passes through. The general scheme is shown in Fig.1. There is a microphone to pick up the speech and a preamplifier to amplify the microphone signal, which is then fed to the VOX circuit and the delay. The VOX circuit drives a relay which operates a cassette recorder or sets a transceiver into the transmit mode. The acoustic delay is provided by a bucket brigade device made by Matsushita. In this circuit, it provides a time delay of around 17 milliseconds, so instead of the relay closing 10 or so milliseconds after speech has commenced, it closes about 7 milliseconds before speech AUDIO DELAY ~OUTPUT .,. MICROPHONE MICROPHONE PREAMPLIFIER I RELAY .,. Fig.1: basic scheme for a VOX with delayed audio. The output from the microphone preamplifier feeds an audio delay circuit and also triggers the VOX circuit. If the audio delay is longer than the relay closing time, the entire signal passes through. FROM PREAMPLIFIER 3RD ORDER 3kHz LOW-PASS FILTER 17mS DELAY 3RD ORDER 3kHz LOW-PASS ALTER OUTPUT CLOCK fc = 15.7kHz Fig.2: the audio delay circuit consists of two 3rd order low pass filters and a bucket brigade device which provides a 17ms delay. The bucket brigade device is clocked at 15.7kHz. appears at the output socket. The circuit The circuit diagram is shown in Fig.3. To use the unit, it is normally placed in between the microphone and the device that is to be operated by the relay; ie, a cassette recorder or transceiver. The VOX OUT socket is linked to the "remote" of the recorder or the PTT (press to talk) switch of the transceiver and the audio output is taken to the input of the recorder or transceiver. Looking at the circuit diagram, the microphone is connected to the MIG INPUT socket. This socket is wired to short the input of the preamplifier when the microphone is removed. The preamp stage of the circuit is IC1a, a FET-input op amp connected as a non inverting amplifier. Its gain is variable between unity and 100 by the tookn sensitivity control, VR1. ICta's output is fed to a filter stage comprising IC1 b and the vox section comprising IC2a and IC2b. For the vox section, the signal from the preamp is fed through a O. tµF capacitor into the inverting input of op amp IC2a, which is wired as a Schmitt trigger. The inverting input, pin 2, is biased via the 22k0 resistor while the non in- Specifications Signal Delay Clock Frequency Frequency Response Maximum Output Signal Maximum Input Sensitivity Harmonic Distortion Signal To Noise Ratio 16.4 milliseconds 15.?kHz 1 00Hz to 3kHz within ± 3dB 800mV RMS 0. 7mV RMS (to actuate relay) (0 .5% at 250mV and 1kHz ( 1 .5% at 800mV and 1kHz - 66dB unweighted with respect to 500mV RMS at the output verting input, pin 3, is biased from a voltage divider consisting of the 120k0 resistor from pin 1 and the tkn resistor to OV . When the output signal of ICta exceeds about 200mV peak to peak, the output of the Schmitt trigger is toggled between the supply rails, producing a square wave of about 20 volts peak to peak. This square wave signal has the same frequency as the input signal from the microphone. The square wave output is AC coupled to a voltage doubler involving diodes D4 and D5. The DC voltage developed is stored in a O. tµF capacitor connected to pin 5 of IC2b which is connected to work as a non-inverting comparator. When the voltage at pin 5 is low, the ouput of IC2b is low. When the voltage at pin 5 is high (ie, above the + 3.75V threshold set by the 33k0 and 15kn resistors at pin 6), the output of IC2b is high and this turns on transistor Qt which drives the relay. The "attack" and "release" time of the vox circuit is set by the components at pin 5 of IC2b. The attack time is a function of the O.lµF capacitor and the associated charging resistance made up of diodes D4 and D5 and the output impedance of IC2a. Since this total impedance is quite low, the attack time is very fast (less than a millisecond). Since the Schmitt trigger signal will cease as soon as the person pauses between words, a defined "release" time is needed to prevent the relay from dropping out during these short breaks. This is provided by the 560k0 resistor at pin 5 of IC2b. This sets the release time at around 200-300 milliseconds. This stops the relay from chattering rapidly on and off during normal speech. As well as driving the relay, Qt drives LED 1 via a 2.2k0 resistor so you can see when the relay is operating. Diode D6 protects Qt against spikes from the relay coil when it is de-energised. Diode D5 protects the base of the transistor from being pulled below 0. 7 volts by the output of comparator IC2b. Now we'll look at the acoustic delay section of the circuit. The block diagram of Fig.2 will help in APRIL 1990 29 This scope photo shows an input signal at 820Hz (top) and the output signal from the bucket brigade device before the clock signal is filtered out (pin 13). The input frequency was chosen to be an exact sub-multiple of the 15. 7kHz clock frequency so that both traces would be stationary. understanding how it works. The heart of the circuit is the MN3004 512-stage bucket brigade device (BBD). This can be thought of as a series of 512 switches and capacitors. The input signal to the BBD is chopped into small samples at a rate determined by the clock To keep hum to a minimum, a ground plane is installed beneath the PC board and connected to circuit earth. The PC board is stood off the groundplane using 6mm spacers. 30 SILICON CHIP A feature of the MN3004 bucket brigade device is the facility to cancel out the clock signal. This is made possible by two in-phase outputs with out-of-phase clock signals. This scope photo shows the outputs at pins 13 & 14 when no audio signal is present. Note that the 15.7kHz clock signals are exactly out of phase. frequency. These small voltage samples are then shuffled through the 512 stages until they appear at the other end, to be reconstituted as a delayed version of the input signal. Just how much delay there is depends on the number of stages, in this case 512, and half the period of the clock signal. The lower the frequency of the clock signal. the longer will be the delay. There is a practical limit and that is set by the desired frequency response of the circuit. This must be limited to less than half the clock frequency otherwise an audibly unpleasant effect called "aliasing" will occur. In this circuit, we wanted to maximise the delay but could put up with a fairly limited frequency response since it is intended for speech. Therefore, we used a clock frequency of 15.7kHz which gives a delay of 16.4 milliseconds. In theory, a clock frequency (or sampling frequency) of 15.7kHz should result in an audio frequency tit response to about 7kHz, just as the compact disc sampling frequency of 44. lkHz allows an audio frequency response to 20kHz. However, to achieve that result, you need complex "brick wall" filters which give very savage filtering above the cut off frequency. Our circuit has easy to make third order filters so we have had to settle for a frequency response to about 3kHz which is adequate for speech signals. Also essential to the operation of the circuit is the 2-phase clock generator, the MN3101. As well as providing the clock signals, it also provides bias signals to the MN3004. An important aspect of the MN3004 is its clock cancelling feature. It has two outputs, pins 13 & 14, both of which produce an inphase audio signal but which have out-of-phase residual clock signal components. When these two outputs are mixed together, the audio signals are added while the clock signal components are largely cancelled out. One of the photos accompanying this article shows the two outputs of the MN3004, with no signal present and with the out-of-phase clock components. ' ~ + I• ► "'[ ~Q ....::, ~ "' 0 z "' ....::, ~ ;! ~ "'!fE !a I > J;-il· ,-..=" ~ ~~ + ,. > N + f-1•· ... ~ + f-t•· ~ ,-.. ~ ..; ,n "'"' + ;,: ,-.. > + I• ~ ,-..=" 0 ~~ H•· <Q ,n ,-..=" J-1•· = ~~ H•· = ~ 0 ;! ~ ... ~ ... ,. - ><t ..J ... '--'"' -z ..,g 0 =?~ w "' •· :a :E H•· 0 :c I- ;= ~ = 0 N ~ ~ ~ 0 0 J. ;! ,n ~ N N >< ~ ~ ; -~ 0 0 Delay circuit details The output from the preamp, ICla, is fed to IClb which is connected as a third order, unity gain low pass filter to attenuate frequencies above 3kHz at the rate of 18dB/octave. Its output signal is fed to the input of the bucket brigade device, IC3. The delayed outputs at pins 13 & 14 are mixed via 4.7k0 resistors and the associated lOkO trimpot, VRZ. The trimpot is there to adjust the clock signal components to a minimum. Signals from the wiper of VR 1 are fed to IClc, which is a third order filter identical to the input filter (IClb), except that it has a =-g 2!i - ~ .... O>- I· ;!: 0 > I I• I ~ = I-.!•· "'~~"' N N ~ + + ~ I ~ J. I -~ o=" o> + H•· o=" o> -~ .... -~ ::, 0 a,O >;! 0 ;!: HI• 'a 15 .,__;,:H•· ...· > ~ g "' N 0 NO "'"";: g I· Fig.3 (right): ICla is the microphone preamp and this feeds filter circuit IClb and a VOX section consisting of IC2a, IC2b & Ql. IC3 is the bucket brigade device, IC4 the 2-phase clock & IClc the output filter. lf I• .... ~ N 0 "';i: -o ;;; c~ c.. ! ~~ 5:5 1ljc.. 0 >< ~~: - ~ff§ ~ APRIL 1990 31 small amount of gain to make up the signal loss occuring in the BBD. The result is a clean audio signal that has been delayed by 16.4ms. The 2-phase clock, IC4, has its frequency determined by the components on pins 5, 6 and 7. The two clock phases appear at pins 2 and 4, and are fed into pins 2 and 12 of IC3. Power supply The rear panel carries the power socket, power on/off switch & the delayed audio output socket (VOX OUT). Power for the unit is derived from a 12V AC plugpack supply. PARTS LIST 1 PCB, code SC061 04901 , 140 x 122mm 1 front panel label, 143 x 54mm 1 plastic case, 150 x 160 x 65mm, Jaycar Cat. HB-5913 1 knob 2 6.35mm phono sockets 1 6 .35mm phono socket with shorting contacts 2 5mm LED bezels 1 SPOT mini PCB relay (Jaycar Cat. SY-4060) 1 2. 1mm DC power socket 1 SPST switch 4 PC pins 1 1 2V AC plugpack 4 6mm spacers, non threaded 1 single sided blank PCB, 140 x 122mm (for ground plane) 4 16mm x 4G self tapping screws 1 14-pin IC socket (optional) 1 8-pin IC socket (optional) Semiconductors 1 LF34 7 quad FET input op amp (IC1) 1 TL072 dual FET input op amp (IC2) 1 MN3004 bucket brigade delay (IC3) 1 MN3101 BBD clock IC (IC4) 1 7812 positive 12V regulator 1 7912 negative 12V regulator 6 1 N4002 silicon diodes (D1-D6) 32 SILICON CHIP The power supply is derived from a 12 volt AC plugpack, which feeds two half-wave rectifiers, D1 & D2. The rectifier outputs are then filtered by the lOOOµF and 470µF electrolytic capacitors. This results in smoothed DC supplies of about ± 17V which are then regulated to ± 12V by 7815 and 7915 3-terminal regulators. Their outputs are further bypassed by lOOµF capacitors. Power indication is provided by LED 1 which is mounted on the front panel. 1 BC338 NPN transistor (01) 2 5mm red LEDs (LED1 , LED2) Construction Capacitors 1 1 OOOµF 25VW PC electrolytic 1 4 70µF 25VW PC electrolytic 4 1OOµF 16VW PC electrolytic 4 4 7 µF 25VW PC electrolytic 2 4. 7 µF 16VW PC electrolytic 1 1µF 50VW PC electrolytic 8 0 .1µF metallised polyester (greencap) 2 .0056µF metallised polyester 2 .0033µF metallised polyester 2 470pF ceramic 2 1 OOpF ceramic vox are mounted on a PC board Potentiometers 1 1 OOkO log potentiometer 1 1 OkO miniature vertical trimpot Resistors (0.25W, 1 560k0 1 1 50k0 1 120k0 6 100k0 1 43k0 1 % 1 39k0 1 33k0 1 27k0 1 24k0 1 % 5%) 2 22k0 3 1 5k0 1 12k0 1 9 .1 kO 1 % 1 8 .2k0 2 4 . 7k0 2 2.2k0 2 1 kO 1 2200 Miscellaneous Hookup wire, shielded audio cable, solder, nuts, washers . Most of the components for the measuring 141 x 12 2mm (code SC06104901}. This is housed in a standard instrument case measuring 150mm wide, 160mm deep and 70mm high. Before commencing assembly, carefully check the PCB pattern for shorts or breaks in the copper tracks, which should be corrected at this stage. Fig.4 shows the wiring details. Start by installing the PC stakes on the PC board. Once this has been done, you can install the wire links and the resistors. We suggest you use a digital multimeter to check each resistor value as it is installed. Be sure that the polarised components are correctly oriented on the PCB. These parts include the electrolytic capacitors, diodes, the transistor and the ICs. Mount the ICs on the board last of all. We used IC sockets for IC3 and IC4 but they are optional. We have provided for two different relay pinouts on the board so no matter which one you use, there will be some holes vacant. The relay we used is a Jaycar model, Cat SY-4060. Equivalents are available from other suppliers. Fig.4: watch component orientation when wiring up the PCB & check that the microphone socket has shorting contacts. o7 ,.... C a, ,q- ,.... 0 (0 C (.) en 0 Fig.5: here is an actual size artwork for the PC board. APRIL 1990 33 Vox with delayed audio - ctd normal speech causes the transmit LED to turn on and stay on during the brief pauses that occur between words - in normal speech. To test the audio section, feed the output into an amplifier and speak through the microphone. You should hear your voice coming through loud and clear. Don't expect to hear the actual delay between the time you speak and the time you hear it from the loudspeaker. Rather, your speech will have a slight echo to it. And turning up the gain will not produce acoustic howl. If you have an oscilloscope, adjust VR2 so that the signal at its wiper has minimum clock signal. This will result in the best signal to noise ratio. ~ , r1-- :::::, • ll.. z () !::: • :E :E en z <I: > a: 1-- > The PCB & groundplane assembly is secured using four self-tapping screws which go into integral plastic pillars in the bottom of the case. Use shielded cable to wire the microphone socket to the PC board. Once the board assembly has been completed, check it for correct installation of all the components. You can now connect the 12V AC plugpack to the circuit. Check the DC voltages around the circuit with respect to one of the PC stakes which is at 0V. You should find + 12V present at pin 4 of ICl, pin 8 of IC2, pin 1 of IC3, pin 1 of IC4 and the collector of Ql. For the negative rail, - 12V should be present at pin 11 of IC1 and pin 4 of IC2. The PC board can now be installed in the case. To keep hum and noise to a minimum, a ground plane needs to be installed underneath the PC board. This can be made from sheet steel, aluminium or from 34 SILICON CHIP PC board copper laminate which is what we used. Whatever material is used, it must be electrically connected to the earth track of the PC board. With copper laminate, this is easy - just solder a wire to it. With this done, the two boards can be mounted in the case. Use 4-gauge 16mm-long self tapping screws and 6mm spacers. The screws go into the integral pillars in the bottom of the case. When all the wiring is complete, you can switch on and check the voltages again. Testing Now plug in a microphone and adjust the sensitivity control so that 1-- • > j:: en z w en a: • <C ...J w C J: - ~ == >< > 0 w 3: 1-- 0 :::::, ll.. ll.. • 1-:::::, 0 0 c :::::, <I: L Fig.6: this artwork can be used as a template for drilling the front panel.