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----=• Voice Recorder Enter the world of digitized speech with this low-cost PC Voice Recorder. It can record messages up to 20 seconds long, store them on hard or floppy disc, and then replay them when you wish. By DARREN YATES If you stop flying your F19 stealth fighter and think about it for a minute, the computer (any computer, not just a PC!) is the most useful gadget ever to fall into the lap of the electronics enthusiast. Not only can you store your latest project on CAD, including the PC board, but you can also write it up and even design it on the computer using one of the various SPICE packages that are now available. And if you get stuck at some point, you can go flying your Spitfire and shoot down some ME-109s (that's known as executive stress management!). The problem is, most of these packages (except for the games) cost an arm and leg and are out of reach for most hobbyists. If you look in many of the electron- a AID MIC CONVERTER AMPLIFIER ics and computing magazines, the latest trend is towards talking objects; things like talking voltmeters and talking clocks. There's even a new IC that's just been released in the US to which you connect a microphone, a battery and a loudspeaker, and it will record and replay 20 seconds of speech! Not to be left behind in the race, we've come up with this low-cost PC Voice Recorder system. But unlike most other voice synthesiser projects, this design doesn't use any hard-toget bits. In fact, you will probably already have most of the components lying in your spare parts bin. Basically, the project comes in two parts: (1) a PC board assembly; and (2) a floppy disc with software (see panel). The PC board assembly provides the interface between the speech and your COMPUTER DIA CONVERTER AMPLIFIER Fig.1: block diagram of the PC Voice Recorder. The incoming speech signal is picked up by a microphone, amplified, turned into digital data by an analog-todigital converter (ADC), and stored in the computer. Retrieving the audio from the computer is just the reverse process. The digital data from the computer is simply turned back into an analog signal by a digital-to-analog converter (DAC), amplified and fed into the loudspeaker. 38 SILICON CHIP computer. It takes in speech, turns it into digital data which the computer can use, and then later accepts that digital data and turns it into speech again. The software is used to control this process. It stores the data on either a hard or floppy disc, retrieves it when asked, and then feeds the digital data to the PC board so that it can be turned back into speech again. Block diagram The basics of computer stored speech are shown in the block diagram ofFig.l. The incoming speech (audio) signal is first turned into an electronic signal by the microphone and then amplified. This signal is then turned into digital data by an analog-to-digital converter (ADC) and the data fed into the computer. The computer, under the control of the software, can then process these signals (known as Digital Signal Processing or DSP) and store them for later retrieval. Retrieving the audio from the computer is just the reverse process. The digital data from the computer is simply turned back into an analog signal by a digital-to-analog converter (DAC), amplified and finally fed into the loudspeaker for all to hear. Computer limitations Mind you, as versatile and useful as an XT or even an AT is, there are some very real practical limits that have to be considered before we can come up with a workable system. Most of these limits revolve around the PC itself. First, we have to con- All the parts except for the loudspeaker & the volume control pot are mounted on a small PC board. The unit plugs into the parallel printer port of the computer via a DB-25 connector. sider how we are going to get the data into the computer. Without worrying about expansion sockets and whatever, virtually all PCs come with a parallel printer socket and a serial communications socket. These sockets , at the back of the computer, provide easy access to these ports without the need for internal hardware modifications. We arbitrarily decided on the printer port. Next, we have to consider how the computer is goi:p.g to process and store the data we feed in. To keep the whole project as simple yet as practical as possible, we limited it to using GWBASIC, which everyone has via MS-DOS. This allows everyone to use this project, whether you have a 256Kb XT built out of spare bits or a 16Mb 486 machine going slightly slower than daylight. GWBASIC though is pretty slow and just isn't fast enough on its own to do the job, so we used assembly language for all the really fast bits such as receiving and transmitting data. Storing the data is actually quite easy and is quick - even using GWBASIC. It's simply saved as a binary file on either hard or floppy disc. The next can of worms we have to open is how to turn the analog signal into a digital signal and back again. If you're a bit sketchy on how analog-digital-analog conversion works , then take a look at the article on this subject in next month's issue of SILICON CHIP. The powers-that-be limited GWBASIC to 64Kb of memory, which leaves us with about 59Kb of free memory by the time we get to use it. However, to allow you to expand the software to suit your own requirements, we've limited the data storage for the speech to 32Kb bytes. And that's where we strike problems. If we use standard AID techniques, an 8-bit converter will soon chew up the limited memory we have to play with. Even if we limit the frequency response to 3kHz, we would have to sample the audio at an 8kHz rate at least to overcome aliasing problems. Briefly, aliasing is the effect heard when the audio mixes with the sampling frequency to produce audible "errors" in the signal. But sampling at 8kHz gives us just 32/8 = 4 seconds of storage - and that's nowhere near enough! Not only that, but the parallel printer socket doesn't have an 8-bit input port anyway! Delta-sigma modulation The alternative sampling method used here is not widely known and is called Delta-Sigma Modulation or DSM. The name comes from the Greek letters "Delta" (the symbol used for the mathematical process called differentiation) and "Sigma" (the symbol used to denote integration). The advantage of this system is that it only has a 1-bit output stream, yet it contains enough information to reproduce speech. Fig.2 shows the circuit of an ADC based on a simple differentiator. It consists of an op amp, a resistor and a capacitor. Let's see how it works. If we assume the circuit has just been switched on, then there will be 1·BIT OUTPUT R Fig.2: basic op amp differentiator circuit. This circuit is used for analog to digital conversion in the PC Voice Recorder. Fig.3: this integrator (or low-pass filter) circuit is used for the reverse digital to analog to digital conversion. AUGUST 1991 39 ~ 0 +12V ~ I 47j n 0 I z I 10k f 1001< 0.1 56k n:r: PIN 1 -,:; ' '1 .0047! .0047! 150k I I .0047! I I 4.7k 1 .,,: I 100k sovw:r- I I I I I I .0471 , -- INPUT SECTION 1 .,,: 10k 50VW! .,. - / - - - - - - - - PARALLEL PRINTER PORT - - - - - - - - - - - - - - - - - - - - - - - - • +12V - - - J - . - - - - - - - - - - - - +12V I I I I I I PIN 140 I -vu H, • ) I ! ,.,.. l ~ 22k~ .001 "i#tnw I .0047! ... 47k 39k 7 1(".4h PIN22+ I sv+· I 10 .I: 16VW+ l10v .0047+ 18k 8 VOLUME~ ~~ 10kVRl LOG ~ · · . 10 ~ 16VW+ 10 ~ .,. ' ' --- "( ( .,. OUTPUT SECTION 16VW! VM 03 BC558 .0047! 100k 10k ' D1 1N4004 +V1N O { ~· } • 1our ,~, • +12v 1 B 12VDC PLUG-PACK + 0,1J -:- PC VOICE RECORDER '"~\ GND EOc VIEWED FROM BELOW .,. .,,: sovwI~ 1son Fig.4 (left): the complete circuit diagram for the PC Voice Recorder. The signals picked up by the microphone are amplified by ICl & fed to a 6th order low-pass Butterworth filter (IClb, IClc & ICld). This filter stage then drives an analogto-digital converter based on IC2, IC3a, IC3b & Ql. On playback, the data from the computer is fed to an integrator, amplified & filtered by IC4a-d, & then fed to audio amplifier stage Q2-Q5. no voltage across the capacitor. If we now apply an analog voltage to the non-inverting input, it will be higher than the inverting input and so the output of the op amp goes high (ie, the op amp behaves as a comparator). This high then charges capacitor C via resistor R, with a time constant of RC. This continues until the voltage across the capacitor is higher than the incoming voltage, at which point the output of the op amp goes low. This now discharges the capacitor through resistor R until the input voltage is higher than that across the capacitor. The output of the op amp then switches high again and so the cycle is repeated indefinitely. You could also call this circuit a slope detector, because it detects the slope of the input voltage; ie, if the input voltage is rising, the comparator output will be high. Conversely, if the input voltage is falling, the output will be low. Notice we have not said anything about exact voltage values. In fact, there is enough information contained in this single bit output for us to be able to store it on disc (via the parallel printer port), retrieve it, and convert it bact'k into recognisable speech. To turn the digital data back into speech, we do the reverse mathematical process, which is known as integration. The basic circuit is shown in Fig.3. If you think that it looks remarkably like a low-pass filter, you are absolutely correct. A lovy-pass filter works perfectly as an integrator because the charging and discharging action on the capacitor replaces ~he slopes that were removed in the original conversion. So this simple low-pass filter is all we need to perform digital to analog conversion. In fact, if you look back at the ADC section (Fig.2), you'll see that we had a similar low-pass filter in the negative feedback loop of the comparator. The circuit Refer now to Fig.4 which shows the complete circuit diagram of the PC Voice Recorder. It can be split into two sections: the input section which converts the speech into the 1-bit data stream; and the output section which converts this bit stream back into speech. Starting at the input, the speech is converted into electrical signals by an electret microphone and then amplified approximately 34 times by noninverting amplifier ICla (which is part of a TL074 quad op amp). ICla's output is then fed into a 6th order lowpass Butterworth filter with a 3dB cutoff frequency of 3kHz (IClb, IClc & ICld). The idea here is to block the upper frequencies which can cause aliasing problems at the 18kHz clock frequency. The 6th order low-pass filter actually consists of three cascaded 2nd order stages formed by IClb, IClc & ICld. The non-inverting inputs of these stages are all biased to half supply (1/2Vcc) via a voltage divider consisting of two lOkQ resistors, which is also used to bias ICla. With the exception of IClb, this bias voltage is fed to each op amp via a lOOkQ resistor. The output of the 6th order filter appears at pin 14 ofICld and is fed to pin 3 of op amp IC2, a TL071. This, along with NAND gates IC3a and IC3b, forms the delta ADC; ie, it behaves as a differentiator. IC3a & IC3b form a CMOS buffer which ensures that the output of the ADC swings to both ends of the supply rail. Note that because of its tracking nature, the delta ADC also oscillates at about 18kHz. This 18kHz oscillation is later filtered out by a 6th order filter section in the output stage. The output of the converter appears at pin 4 of IC3b and is fed to NPN pull-down transistor Ql, which is connected to pin 1 of the parallel printer port. This is the -STROBE Jine. This simple transistor interface can be used because those pins designated as port C on the printer port have 4. 7kQ pull-up resistors. Thus, we can input data without worrying about PARTS LIST 1 PC board, code SC07107911, 132 x 82mm 1 floppy disc containing PCVOICE software (see panel) 1 0B25 male printer port plug 1 8Q mini loudspeaker 1 12V DC 300mA plugpack 1 electret microphone insert 1 10kn log potentiometer (VR1) Semiconductors 2 TL074 quad op amps (IC1 ,IC4) 1 TL071 op amp (IC2) 1 4011 quad 2-input NANO gate (IC3) 3 BC548 NPN transistors (Q1,Q2,Q4) 2 BC558 PNP transistors (Q3,Q5) 2 1N4004 rectifier diodes (D1 ,D2) Capacitors 2 100µF 25VW electrolytics 3 10µF 16VW electrolytics 5 1µF 50VW electrolytics 5 0.1 µF 63VW 5mm-pitch polyester 1 .047µF 63VW 5mm-pitch polyester 6 .0047µF 63VW 5mm-pitch polyester 2 .0015µF 63VW 5mm-pitch polyester 3 .001 µF 63VW 5mm-pitch polyester 2 82pF 5mm-pitch ceramic Resistors (5%, 0.25W) 1 270kn 1 18kQ 2 1S0kQ 4 1Skn 9 100kQ 4 10kQ 1 82kQ 1 5.6kQ 4 56kQ 1 4. 7kQ 1 47kn 1 1kQ 3 39kQ 2 150Q 2 27kQ 1 18Q 4 22kn 1 10n Miscellaneous Hookup wire, solder, zippy box (optiorial), ribbon cable etc. keeping to the "5V rule" of the port. As mentioned earlier, the reverse process is used to turn the digital data back into speech. The output from the computer is taken from pin 14 of the printer port; ie, from the -AUTO FEED AUGUST 1991 41 Fig.5: follow this wiring diagram carefully when installing the parts on the PC board & take care with component orientation. The circuit diagram (Fig.4) shows the pinout details for the transistors & 3-terminal regulators. line. One of the normal data out lines could have been used but this would have necessitated switching the port addresses. The digital bitstream is then fed into an 82kQ resistor and two 0.lµF capacitors, which together form the integrator. The signal is then amplified by a bandpass filter stage IC4a which has a gain of 3.6 and a frequency response of 1Hz-3.3kHz, as set by the associated l0µF and .00lµF feedback capacitors. The output from IC4a at pin 1 is then fed into another 6th order lowpass Butterworth filter (IC4b-IC4d), which is an exact copy of the circuit used in the input stage. As before, the filter is DC-coupled throughout and the non-inverting inputs are all biased to 1/2Vcc. The output appears at pin 14 of IC4d and is coupled to an audio amplifier via a lµF capacitor and volume control VRl. Transistors Q2-Q5 form a fairly standard class AB audio amplifier circuit. DC bias for Q2 is provided by the 150kQ and 270kQ resistors on its base, while the 5.6kQ and lkQ feedback resistors (at QZ's emitter) set the overall gain of the amplifier to 5.6. The lµF capacitor rolls off the response below 150Hz. Q2 is wired as a common emitter amplifier and also provides much of the voltage gain. Its collector output drives the base of Q3 which functions as a driver stage for complementary output pair Q4 & Q5. These transistors in turn provide the necessary current gain to drive the loudsp eaker. Note that the bottom end of Q5's 300Q (2 x 150Q) base bias resistor has been connected to the output rather than to ground. Because Q5 functions as an emitter follower, its voltage gain is almost unity and so there is almost no AC signal voltage across the two 150Q resistors. This means that very little signal current flows in these two The voice recorder board is plugged into the parallel printer port of the PC via a 3-wire cable & a standard DB-25 connector. Try to keep the cable length to less than two metres. 42 SILICON CHIP resistors and so the impedance of the bias network appears to be much higher than it really is. This technique is known as "bootstrapping" and results in greater signal output with lower distortion. D2 and its associated 18Q resistor provide some forward bias to the output pair under no-signal conditions to minimise crossover distortion. Finally, a Zobel network consisting of a lOQ resistor and 0. lµF capacitor has been connected across the output to ensure amplifier stability. Power for the circuit is derived from a 12V DC plugpack supply. This delivers about 17V when lightly loaded and is fed to a 7812 3-terminal regulator via reverse polarity protection diode Dl. The resulting +12V rail from the 7812 is then used to power the various circuit stages. Software Once you have obtained your copy of the software, you will need to load your copy ofGWBASIC onto the disc. If you have a hard disc, simply get into your DOS directory and type: COPY GWBASIC.EXE A:<enter> When the file has been copied, type A:<enter> to go back to your floppy disc drive, then type GO<enter>. This will automatically load in and run PCVOICE. Once it is running, you will see a menu on-screen asking you to designate the drive in which you wish to load and store your PCVOICE files. You have the choice of either the A, B or C drive. If you select either the A or B drives, PCVOICE will automatically store them in the main directory. However, if you choose the C drive, CAPACITOR CODES D D D D D D D This photograph shows the menu screen of the PCVOICE software package. You can select different options by pressing the spacebar & then pressing the enter key when the option you want has been highlighted. Value IEC Code 0.1µF .047µF .0047µF .0015µF .001µF 82pF 100n 47n 4n7 1n5 1n 82p EIA Code 104 473 472 152 102 82 will show up on the screen over the first option but you can select any of the other options simply by further pressing the spacebar. When the bar is over the option you wish to select, press <enter>. The software copyright is retained by the author but for those who wish to experiment further, remarks are included on the operation of the assembly language subroutines. Construction PCVOICE will store the files in a subdirectory called C: \PCVOICE. Once you have made your choice by typing in the letter and pressing <enter>, you will see a new menu which asks you to choose one of six options: (1) record a message; (2) load & replay a message from the specified disc drive; (3) replay the message just recorded; (4) save the message in memory to the specified disc drive; (5) erase an existing PCVOICE file from the specified disc drive; and (6) quit PCVOICE. To select one of the options, first press the spacebar. A solid white bar Virtually all the parts for the PC Voice are mounted on a single PC board. This board carril:ls the code number SC07107911 and measures 132 x 82mm. Fig.5 shows the wiring details. Start construction by soldering in PC stakes at the external wiring points, followed by the seven wire links. This RESISTOR COLOUR CODES D D D D D D D D D D D D D D D D D D D No. 1 2 9 1 4 1 3 2 4 4 4 1 1 2 Value 270kQ 150kQ 100kQ 82kQ 56kQ 47kQ 39kQ 27kQ 22kQ 18kQ 15kQ 10kQ 5.6kQ 4.7kQ 1kQ 150Q 18Q · 10Q 4-Band Code (5%) 5-Band Code (1%) red violet yellow gold brown green yellow gold brown black yellow gold grey red orange gold green blue orange gold yellow violet orange gold orange white orange gold red violet orange gold red red orange gold brown grey orange gold brown green orange gold brown black orange gold green blue red gold yellow violet red gold brown black red gold brown green brown gold brown grey black gold brown black black gold red violet black orange brown brown green black orange brown brown black black orange brown grey red black red brown green blue black red brown yellow violet black red brown orange white black red brown red violet black red brown red red black red brown brown grey black red brown brown green black red brown brown black black red brown green blue black brown brown yellow violet black brown brown brown black black brown brown brown green black black brown brown grey black gold brown brown black black gold brown AUGUST 1991 43 in front of the loudspeaker to allow the sound to escape (but don't do this with the loudspeaker in position). Where To Buy The Software The software for this project is only available direct from the author, Darren Yates. The software for the PCVOICE recorder, PCVOICE .BAS, is priced at $25 plus $3 p&p, and includes runnin~ instructions and ~II assembly language routines. The TIME.BAS talking clock software 1s priced at $15 plus $3 p&p. All software comes on a single 360Kb 5 1/4-inch floppy disc and will run on any PC with 256Kb of RAM and a printer port. Payment should be made by cheque or postal money order to: Darren Yates, PO Box 134, French's Forest, NSW 2086 . Note: Copyright of the software is retained by the author. done, install the resistors. Most of these lay flat on the board, except for those around the output amplifier area which are mounted end-on to save space. Check each resistor on your DMM before installing it on the board, to make sure you have the correct value. Next, solder in the 5mm fixed-pitch capacitors. Check their values carefully against the wiring diagram as they all look the same and are easy to mix up . The electrolytics can now be installed but make sure that they all go in the right way around. Now for the semiconductors. Once again, you must make sure that they are all correctly oriented. In particular, take care with the orientation of the transistors in the audio amplifier stage. Fig.4 shows their pinout details. Mount the 3-terminal regulator with its metal tab away from D1. Once all the parts have been installed, connect the volume pot (VRl), microphone insert and 8Q loudspeaker to the board using suitable lengths of hookup wire. Finally, connect a 3-wire cable between the DATA IN, DATA OUT & GND pins on the PC board and pins 1, 14 & 22 respectively of a DB-25 printer plug. It's a good idea to use different coloured leads for these connections if you intend making up a long cable run, as this makes the leads easier to sort out. We used a 2-metre long cable with the prototype, with no obvious effect on circuit operation. We left our board in the "barebones " state but you can mount your version in a plastic zippy case, or some other suitable case. The volume control can be mounted on the front panel along with the loudspeaker. Don 't forget to drill a number of holes Testing Once you have finished construction, check the PC board carefully for solder splashes and missed solder joints. Check also that the correct part has been installed at each location and that all parts are correctly oriented. When you're satisfied that everything is OK apply power but don't hook the unit up to your computer just yet. If you have a CRO handy, take a look at the output (pin 4) of IC3b. You should get a squarewave of 12V amplitude at about 18kHz or so. If you don't have a CRO, use a frequency meter to check that pin 4 of IC3b is oscillating at about 18kHz. This frequency is not overly critical however, and can be anywhere in the region of 16-25kHz. If you don 't have a frequency meter eith er, use your DMM to check that the output of the regulator is at +12V and that pin 4 of IC3b sits at an average voltage of about 6V under no-signal conditions. Applications So what can the unit be used for? Well, if you write your own software, you can incorporate this project to give your programs the added glory of speech. For example, we wrote a program called TIME.BAS (see panel), which turns the PCVOICE board into a talking clock. It 's similar to the talking clock serv ice provided by ....--.......... SC0? 107911~ Telecom, although the voice on our unit is nowhere near as refined. Other possible applications are to use the unit as a household message machine, as a sales gimmick or just for experiment. It's limited only by your imagination. SC o,,ci,-o.,u ,.....0~o-o-o-od11h 1111111 :SI ~\.-=--__,--,--- s 1 44 SIUCO,\J CHIP o-: Fig.6: use this full-size pattern to make your own PC board or to check that a commercially made board has been correctly etched.