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By GARY YATES Computers & programs that just go “beep” are old hat. This tiny digitiser records voice input through the games port & replays it on the PC’s speaker. What’s more, it can record to your hard disc for long recording & playback times. SmallTALK A tiny voice digitiser for the PC PCs are moving into the world of sound – there’s no denying it. Manufacturers are moving away from the days when the computer just beeped at you and are launching themselves into voice recog­nition and voice-annotated software packages. So much so that Compaq computers now come with a sound board as standard and there are other manufacturers about to follow suit. However, sound cards are still quite expensive and if you’re a programmer, writing programs that July 1994  17 GAMES PORT PIN 1 33  100 16VW 1k 4.7k A 100k LED1 ON 1 3 IC1a 2 LM358 4 100k MIC  K 8 1 5 47k 1k A K PIN 2 PIN 4 100k 33k 1 7 IC1b 6 .001 .0022 SMALLTALK FOR PCS Fig.1: the circuit is based on IC1, an LM358 dual op amp. IC1a functions as a microphone preamplifier stage & its output modulates a 40kHz carrier signal produced by IC1b. 18  Silicon Chip • • Sound playback is independent of PC clock speed; Uses only one IC. How it works This design uses a novel method of interfacing with the PC via the games port. Not only does this port have its own 5V supply rail, removing the need for an external power source, but it leaves the serial and parallel printer ports for their more traditional roles. Over the years, the printer and serial ports have been used for externally interfaced projects which meant you could be without the use of either your printer or mouse. By using the games port, these problems are avoided. It also has the benefit of a small con100uF 33  1k K 1uF 1 1k 100k 33k IC1 LM358 MIC .001 100k A DB-15 SOCKET 100k 4.7k LED1 47k use any sound other than a “beep” means that you’re relying on the end-user to have a compatible sound card in their machine. However, as popular as sound cards are becoming, the days when you can count on every machine having a sound card installed are still a fair way off. Those of you who built the PC Voice Recorder back in the August 1991 issue of SILICON CHIP will have been aware of its limitations – it required hardware for both recording and play­ back. GWBASIC was required to run the software and only 16Kb of storage was available which gave a maximum recording time of just 20 seconds. The SmallTALK digitiser presented here overcomes all of these problems and has to be one of the world’s smallest voice digitiser systems. It has the following features: • No additional hardware required for playback; • Either 3-minute RAM version or optional hard disc recording (an 85Mb HDD would give 13 hours recording time); • No external power supply required; • Fully self-executable software; • QuickBASIC .QLB and .LIB libraries available;. • Easily added to other programs; • Voice files can be stored on disc for replay; • Requires less than 2Kb per second storage; .0022 1uF Fig.2: install the parts on the PC board as shown in this diagram. Note that the mic insert will need a link connected to the shielding can tab so that it can be earthed via the cir­cuit. nector which results in a smaller PC board as well. Looking at the circuit diagram in Fig.1, you can see that there aren’t a great number of components involved. The circuit uses only one IC, a LM358 dual op amp. In fact there are so few components used in the circuit that it is difficult to see how it works. The first half of IC1 is connected as a non-inverting AC amplifier with a gain of 48. This is used to amplify the signal coming from the electret microphone which is biased via the 4.7kΩ resistor. That’s the straightforward part. Now comes the tricky bit. The output signal from pin 1 of IC1a is connected to pin 5 of IC1b. This second op amp has two RC filter networks providing the feedback from pin 7 to pin 6. These components have been selected so that with no signal present at the input, the output is effec­tively muted and the DC voltage at pin 2 sits at half supply; ie, around +2.5 volts DC. However, when a signal is present, IC1b rings severely at around 40kHz or so and this damped oscillation is superimposed on the amplified signal from the electret micro­phone. In effect, the audio signal from the electret modulates a 40kHz carrier and this is presented to one of the switch inputs of the game port. From here on, the signal present at the games port is sam­pled by the computer at a rate of 15kHz or, to be precise, 15 thousand samples per second. The resulting samples are stored directly as one-bit information either in RAM or on the hard disc. All of these functions are controlled by the software program which has been written to accompany this circuit. Because it’s only one bit per sample, the SmallTALK is memory efficient – it uses about 1.8Kb per second or less than 25% that required by conventional 8-bit analog-to-digital conver­sion. This method of conversion is similar to Delta-Sigma Modula­ tion and is briefly described in the accompanying panel. Storing to HDD While saving the sound data directly to RAM is relatively straight­ forward, saving the information to disc is a less simple process. What happens is that a 128Kb block of memory is allocated to storage of the sound data and this block is divided into two 64Kb regions. PARTS LIST 1 PC board, 52 x 40mm 1 DB15 PC-mount female socket 1 electret mic insert 2 male DB15 sockets 2 DB15 backshells 1 SmallTALK software disc 1 1-metre length of twin shielded audio cable (supplied with kit) Semiconductors 1 LM358 dual op amp IC 1 5mm red LED The board is connected to the games card inside the computer via a DB15-DB15 cable. Because there are only three connections, you can easily make up your own cable using two male DB15 sockets & some twin shielded audio cable. When sound recording begins, data is stored in the first region, which for ease of understanding we’ll call the “lower” region. Once the data fills the lower region, the computer switches over and begins to fill the “upper” region. While it is recording to this upper region, it stores the contents of the lower region to the hard drive. When the upper region has been filled, the program loops the data address counter back down to the beginning of the lower region and begins to fill this region up again, over-writing the data in the RAM which has been saved to the hard drive. Similarly, while it’s recording in the lower region, the contents of the upper region are stored to disc and this cycle continues until the user ends the recording by pressing a key. In effect, what happens is that while recording is continu­ing into one memory region, the other memory region is being saved to disc. This way, we can store huge amounts of sound data whilst only using 128Kb of memory, which is great for systems that only have 640Kb of RAM. Creative Lab’s Sound Blaster and other sound cards use a similar process to achieve the same result. Capacitors 1 100µF 16VW electrolytic 2 1µF 63VW electrolytics 1 0.0022µF 63VW MKT polyester 1 0.001µF 63VW MKT polyester Resistors (0.25W, 5%) 3 100kΩ 1 4.7kΩ 1 47kΩ 2 1kΩ 1 33kΩ 1 33Ω System requirements In order for SmallTALK to work, your system must have the following: • One floppy drive; • One hard disc drive (with at least 500Kb free); • One joystick port; • DOS 3.0 or later (DOS 5 or later preferred); • 512Kb of RAM minimum; • 80286 processor or higher. Sound recording on a PC is by its nature a very CPU-hungry process and unfortunately the 8086/8088 processor just isn’t fast enough to do the job. However, any sound file recorded on a 286 can be replayed at exactly the same pitch on any other ma­chine and you don’t need to set any special parameters. This is made possible by the program’s use of what can be termed “interrupt-driven sampling” or IDS. This relies on reprogramming the computer’s internal clock circuitry to take approximately 14,900 samples per second, re­gardless of the machine architecture. It also does this in the “background”, which means that provided you have the Quick­BASIC libraries (which we’ll get onto shortly), it’s possible to do other things such as print to the screen or get keyboard input while all this is happening. Software The software is available in two versions – RECORD and PLAY.EXE for the 3-minute version and HDRECORD and HDPLAY.EXE for the HDD option. In both cases, to record a file, you simply plug in the SmallTALK board, type in the program name and then a filename on the same line; eg, RECORD SOUND.VOC would start a 3-minute maximum RESISTOR COLOUR CODES ❏ No. ❏   3 ❏   1 ❏   1 ❏   1 ❏   2 ❏   1 Value 100kΩ 47kΩ 33kΩ 4.7kΩ 1kΩ 33Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown orange orange orange brown yellow violet red brown brown black red brown orange orange black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown orange orange black red brown yellow violet black brown brown brown black black brown brown orange orange black gold brown July 1994  19 Delta-Sigma Modulation (DSM) Delta-Sigma ModulaHIGH FREQUENCY tion (DSM) is a form of OSCILLATOR analog-digital converter (ADC) which transforms COMPARATOR ANALOG analog signals into a series CLK DIGITAL IN D Q of high and low digital OUT FLIP voltage levels. Fig.3 shows FLOP the basic elements of a DSM ADC. R The analog input signal is connected to the C LOW-PASS non-inverting input of a FILTER/ INTEGRATOR comparator. The output of this comparator is then Fig.3: the basic elements of a DSM ADC. fed to the D-input of a D flipflop, which is clocked at a very is higher that the returning signal, high frequency by an oscillator. the comparator produces a high The digital output of the D output; otherwise it is low. flipflop then passes through a The D flipflop and the associatlow-pass filter which is then routed ed clock circuit ensure that the back to the inverting input of the samples produced by the circuit comparator. The low-pass filter are at equal intervals. reconstructs the original signal so The D flipflop and oscillator that the comparator can com- circuitry is not necessary for the pare the slope of the incoming SmallTALK as the sample rate signal against that of the recon- produced by the op amp itself structed signal. If the input signal is sufficient for our application. recording with the data stored in the file SOUND.VOC in the current drive and directory. The sound files are compatible on both systems provided that files recorded on the HDD system are less than 320Kb. Longer files can only be replayed using the HDD system. The 3-minute version must load the complete file into memory before playback begins whereas the HDD system needs to only load in 64Kb before playback will begin, regardless of the size of the file. In both cases, only 128Kb of memory are used for data storage. If you are conscious about using up too much disc space for your sound files, a byte counter displays the current number of kilobytes used on screen and the exact number of bytes used when recording is completed. If you find that a sound file is too long, you can simply re-record the file and check it against the byte counter. Uses The PLAY.EXE program has also been designed to be incorpo­rated into your own programs – it plays the file 20  Silicon Chip without writing any information to the screen. You can simply use the SHELL command in either Quick­ BASIC or DOS’s QBasic to play sound files within your own programs. For example, you may wish to have the computer say “Press a key to continue”. You could record this into a file called, say, PRESS.SND and use the shell command at the appropriate time to replay the file: SHELL “PLAY PRESS.SND” The only condition is that both PLAY.EXE and the sound file, PRESS. SND, must be in the same directory that is currently in use. If your program is in a different directory or even a different drive, you can type: SHELL “E:\JUNK\PLAY D:\SOUND\ PRESS.SND” The only concern that this method raises is that you can’t do anything else while the SHELLed program is running. You can add this PLAY program to games, process control programs, word processors, database management programs, file utilities – just about anything where the computer needs to warn or indicate to the user that some process is occurring or needs the user’s attention. You could even use it as a message recorder, talking clock, talking voltmeter etc – the list is basically as long as your arm. QuickBASIC libraries Now if you’re sitting down and thinking “Wait a minute! BASIC’s not fast enough to do that!” then you’re quite right. The crucial routines which sample and play back the audio have been written in assembler and linked into QuickBASIC libraries, SMAL­ TALK.LIB/QLB and HDTALK.LIB/QLB which are also being made avail­able. The beauty of these libraries is twofold. Firstly, you can create your own programs using only QuickBASIC and not have to know anything about assembler. Secondly, you can combine all of the routines into one program name and do away with the SHELL command. These libraries contain easily-accessible routines which carry out the initialisation and the setting up of the record and replay clock reprogramming parameters, and a status routine which returns the total number of bytes either played or recorded so far. This feature is handy for when you need to keep an eye on file size or wish to stop at a certain point in the file. The libraries allow you to access any part of the sound file and initiate playback from one point to another. One example of where this idea would be useful is in speech pathology where speech analysis of a particular word spoken is necessary. Construction This is quite simple and can be done in about an hour or so – less if you’re more experienced. Before you begin any solder­ing, check the board thoroughly for any shorts or breaks in the copper tracks. These should be repaired with a small artwork knife or a touch of the soldering iron where appropriate. When you’re sure that the board is OK, you can start con­struction by installing the resistors, capacitors, the LED and the IC. These last two components and the two electrolytic capacitors are polarised so make sure that you follow the overlay wiring diagram and install them correctly. The final two components are the electret mic insert and the DB15 female connector. In both cases, each component should just drop into place. The mic insert will need a link connected to the shielding can tab so that it can be earthed via the cir­cuit. Simply use one of the clipped off leads from a resistor to do this. Wiring the cable Rather than buy a complete DB15 male-male cable which costs about $32, you can make your own (all of the required parts will be included in the kit). Twin shielded microphone cable is used to make the connections. Use the two inner conductors to make the pin 1 and pin 2 connections to each socket and the shield to make the pin 4 connections. Testing To test the unit, connect the cable and the PC board to your computer and measure the voltage drop across the 33Ω resistor. This should be around 120mV (0.12V). Any more than 200mV and you should disconnect the board and check for errors. If this measures correctly, you should also see the LED light up. Other voltages to check are the +5V rail Where to get the kit SmallTALK is available in two versions: the 3-minute ver­sion including software and kit for $34.95; and the HDD version including software and kit for $39.95. Additional QuickBASIC .QLB and .LIB libraries of the record and playback routines for either versions are available for $7 each. Please add $3.05 to all orders for postage and packaging and allow two weeks for delivery. You can send your cheque or money order to: R.A.T. Elec­tronics, PO Box 641, Penrith, NSW 2750. Note: Copyright © 1994. All software, circuits and PC art remain the property of R.A.T. Electronics. from pin 1 which should also appear at pin 8 of IC1 (it should be around 4.9V). Pins 3, 5 and 7 of IC1 should be 2.45V as well. Now depending upon the software you request, run the in­stall program to load it onto your hard drive. Once in the direc­tory SMALTALK type: RECORD TEST.VOC and press enter. You’ll be asked to press enter again to initiate recording. At this point, say a few words and then press the space bar. You should get a file byte count of around 8-10Kb depending on how long you speak. Now type in the same directory: PLAY TEST.VOC and you should hear the file being replayed through your PC’s speaker. If you purchased the HDD system, then you would substi­tute the names HDRECORD and HDPLAY for these tests. If all goes well, you can now include the PLAY program and your own sound files into your own programs whether they are written in PASCAL, C or “plain SC old” BASIC. July 1994  21