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Download WAV files from your PC and play them at will! short Message RECORDER AND Player by Leon Williams Got a need for a short audio message player? Been tempted to build or buy a voice recorder? Well, here’s a voice recorder with a muchimproved recording technique. No longer limited by the shortcomings of built in microphones, you edit and enhance your messages as WAVE files on your PC and then download them direct to the Message Player. 64  Silicon Chip www.siliconchip.com.au W to-female 25-way cable. The download used in PCs and the Internet because hile there have been many software is started and the required file there is plenty of software to generate projects for voice recorder/ opened, the shunt on the PC board is and play them. They don’t employ playback boards, they all moved from PLAY to PROGRAM and complicated compression algorithms suffer from one major drawback. the file is then downloaded. It is very but instead just have a header block They can only record and playeasy and takes about 15 seconds to followed by the raw audio data. back sounds (usually voice) that are complete. recorded by a little onboard electret WAVE files come in a number of microphone. You push a button and The message player is built on a formats providing varying levels of speak into the microphone and that’s single-sided PC board, with all the sound quality from basic to CD qualabout all you can do. You can’t really components on-board except for the ity. add any exciting sound effects or enloudspeaker and the power and D To keep the Message Player inexhance your voice in any way. connectors. pensive, it has been designed to only With the Message Player all that’s All the components are standard work with 8-bit mono 8kHz files. changed. Now you can record, mix types available from most electronThis means that we don’t need a lot and edit sophisticated sound files ics shops. The exception may be the of memory to hold the files and so with your PC and download them to SRAM chip. These have been used by we can get away with a single SRAM be played when and where you want. the millions over the years, so you may chip. be able to locate one from a disposal OK, so where would you use the While the Message Player sound source. Failing this, you can get them Message Player? The answer is any output is not exactly hifi, it is entirely new from places like Farnell and RS place that you want short audible adequate for the purpose. messages to be heard. Let’s have a closer look at the WAVE What about a personfile format we are alised front door bell or a using. warm message to welcome  The start of the Stores standard WAVE (.WAV) file format shoppers, or maybe a talk-  file contains a header Battery back-up for me ing car alarm to tell you block, a block of data mory  Quick PC file downloa your lights are on? that provides inford A unique application  mation about the file Simple logic control int erfacing could be to replace your including things such Built in 250mW audio telephone ring sound with  as the sampling rate amplifier a voice message like “Hey  and the size of the file. Easy to use companion software you, answer the phone”! We won’t go into Runs from 9V plug pa All you would need to do  it in depth here but if ck at low power is detect the ring signal you are interested, a and trigger the Message search on the Internet Player to play the message. will uncover plenty of Components. The Message Player has a replay information about the While a 62256 is specified for the time of four seconds and in replay different WAVE file formats, including storage SRAM, manufacturers somemode has only two controls. These are full descriptions of the header block. times use different labelling, such as a logic level negative pulse to start the Mono obviously refers to the fact 43256. This should be fine, rememreplay (“GO”) and a similar pulse to that there is only one channel of aubering that the majority of 32K SRAM stop the replay (“STOP”). dio, hence the reason we only use one chips have the same pin outs and funcIf the message is not stopped speaker! The frequency of 8kHz refers tionality. Don’t worry about the access during the four seconds it will stop to the rate at which each byte of data is time, usually written as a couple of automatically at the end. If you want converted from digital to analog to prodigits at the end of the part number, the message to continue, it’s simply a duce sound. At 8kHz (8000 bytes per perhaps preceded with a dash. matter of holding the GO input low second), we can reproduce voice quite This is not a critical issue with the until you want it to stop. well but reproducing high frequencies Message Player, so if you are purchassuch as 15kHz is not possible. At this These pulses can be derived from ing a new one, select the slowest to rate we can get four seconds of replay simple pushbutton switches or other save some money. from a 32Kbyte file. more elaborate interfacing circuits. The Message Player is housed in a Each byte of audio data is held as If you think that the Message Player plastic case but there is no reason why eight bits, which gives us a maximum memory is not big enough, just time it couldn’t go into an existing piece of 256 different bit combinations. The yourself speaking for four seconds and of equipment if you have the space. reproduced audio waveform is comyou will see that you can get a lot said The normal power supply would be prised of 255 equal voltage steps and in that time. The Message Player is a 9V plugpack but any regulated DC will be at minimum amplitude when only intended for short message applisource between 8V and 12V will also all the bits are zero (00 Hex) and at cations and anyway, who needs a 60 be suitable. maximum amplitude when all the second message every time a wanted bits are a one (FF Hex). A feature of event happens? Wave files the WAVE format is that when there To download the sound files into WAVE (“.WAV”) files are probably is silence (no sound), the amplitude the player, you connect it up to your the most common audio file format rests at midway (7F Hex). PC printer port with a standard male- MESSAGE PLAYER F EATURES www.siliconchip.com.au November 2001  65 66  Silicon Chip www.siliconchip.com.au Fig.1: it might look complicated but in reality there’s not much to the circuit. Its operation is described in the text. Note that the two jumpers were brought out to a DPDT slide switch in our final prototype. Perhaps a more correct way of looking at a WAVE file is that it swings negative and positive about a central resting point. Circuit description The circuit is shown in Fig.1. The audio file is stored in IC1 which is a 32K (32768) by 8-bit wide SRAM (Static Random Access Memory) chip. As this is the heart of the design, let’s have a close look at how it operates. An SRAM chip looks like any other logic chip with a black plastic body and pins. The difference is that it can hold lots and lots of bytes (eight bits) of data, which can be entered (written), stored and read back very quickly. Each location (memory cell) within the SRAM is selected by the bit pattern on the address pins. When all the address lines are low we select the first memory cell. As they increment in binary they select the next byte and so on, until they are all high when the last memory cell is selected. The SRAM chip we are using is a 62256, where the sequence 256 refers to the fact that it can store 32K x 8 bits (32 x 8 = 256). A 6264 would store 8K x 8 bits, etc. Our chip has 15 address lines to address a maximum of 32,768 locations, and eight data lines. Those who have access to a 32K SRAM data sheet will notice that the address and data line labelling is different to that shown in Fig.1. This was changed in this instance to simplify the PC board layout, however changing the notations does not cause a problem. The term random in SRAM refers to the fact that any memory cell within the chip can be accessed (written or read) in any sequence. The data lines can also be interchanged, as long as we read and write the data with the same bit pattern. That is, if we write a bit to a line we label D7, then we must also read back the bit as D7. Because we do not need to access the memory cells in any particular sequence and because all the data bits can be treated equally, we can label the address and data lines as we wish. As well as the address and data lines, the SRAM has three control lines that must be used correctly to write and read the SRAM. Pin 27 is the Write (WR) pin and is normally high. It is taken low when writing www.siliconchip.com.au This shot of the inside of the message recorder was taken before we decided to add the two pushbuttons switches (“GO” and “STOP”) on the end panel, as well as bringing the “PROGRAM” and “PLAY” headers out to a slider switch on the front panel. These switches make the unit much easier to use: you don’t have to whip the front panel off every time you want to change the message! eight bits of data into a cell selected by the address lines. To read data back from the chip, the write line must be high. Pin 22 is the Output Enable (OE) pin and controls the output buffers. When high the output is disabled and placed in a high impedance state, while taking it low enables the output buffers. Pin 20 is the Chip Select (CS) input and when it is high the chip is de-selected. In this state, read and write requests are ignored and the chip is placed in standby mode. When the chip select pin is low, the chip operates normally and data can be read and written. In contrast to memory devices such as EPROMs and EEPROMs, IC1 is a volatile memory. This means that it will only hold the data in its memory cells while power is applied. Luckily, the memory can be maintained when the main power is removed, through a secondary battery backup, as long as pin 20 is held high. The backup supply can be as low as 2V, needing a current of only a few microamps. This can be easily supplied by a couple of AA cells which under these conditions should last a long time, probably as long as they would left on the shelf. Addressing and control IC2, a 4040 12-stage binary counter, is used to address the first eight lines of the SRAM. The clock input is on pin 10 of IC2, and a high on pin 11 resets all outputs to zero. IC3, a 4024 7-stage binary counter, addresses the remaining seven lines of the SRAM and is clocked when pin 13 of IC2 goes low. The reset line of IC3 is connected to the reset line of IC2, so that both are reset simultaneously. IC4d is configured as a Schmitt trigger clock oscillator with a frequency of 8kHz, set by the .022µF capacitor and VR1. With the clock operating at 8kHz, we address 8000 memory locations per second. The memory size is 32,768 bytes, so the time taken to address all the memory, and hence the replay time is 32,768/8000 = 4.096 seconds. The clock is enabled when pin 13 November 2001  67 Fig.2: almost all the components mount on one PC board, as shown here. The two “jumpers” (for programming and playing) can be moved to a front panel DPDT switch to save opening the case every time you want to change the message. Likewise, the “GO” and “STOP” PC stakes can be brought outside the case. is taken high and stops when pin 13 is low. Pin 8 of IC4c is normally held high by a 10kΩ resistor, so clock pulses can pass through via pin 9 and onto the address counters. When the clock is stopped, IC4d pin 11 is forced high, which allows the write pulses through IC4c in program mode. More on this later. Starting and stopping of the clock and hence the replay is controlled by IC4a and IC4b. This crossover configuration is called a set-reset flipflop, and toggles between two states. Pin 1 of IC4a and pin 6 of IC4b are normally held high by 100kΩ resistors. Assuming pin 3 of IC4a is low, pin 4 of IC4b and hence pin 2 of IC4a will be high. When the GO input is pulled low, pin 1 of IC4a is pulled low, forcing pin 3 to go high. This enables the clock and forces pin 4 of IC4b to go low. Even when the GO input is taken high again, the flipflop stays in Compare this picture with the component overlay above when assembling the PC board and you shouldn’t have any problems. 68  Silicon Chip www.siliconchip.com.au Parts List – Message Player 1 PC board, code 01111011 1 plastic case, 197mm x 113mm x 63mm (Jaycar HB6012 or equivalent) 19 PC board stakes 2 2-pin headers with shunts, OR 1 DPDT mini slider switch 1 2-pin header shunt 1 25-pin male D connector with mounting hardware 1 76mm 8Ω speaker 1 DC panel-mount socket to match plug pack 1 28-pin IC socket 1 Twin AA battery holder 4 12mm x 3mm screws and nuts 4 Self adhesive feet Light duty hook up wire, tinned copper wire,10-way ribbon cable Double-sided tape pads (for securing battery holder) Fig.3: here’s how to wire the 25-pin “D” socket which connects to your PC via a standard parallel cable. Pins 18-25 would normally be soldered together with a straight length of tinned copper wire. the same state. The flipflop will only change state when pin 6 of IC4b is pulled low. This can occur in one of two ways: (1) by pulling the STOP input low; pin 3 of IC4a then goes low, disabling the clock and forcing pin 4 of IC4b high. (2) when pin 6 is pulled momentarily low by the negative pulse generated after the last memory cell has been addressed. This is accomplished by differentiating the negative edge of IC3 pin 3 with a .01µF capacitor and a 100kΩ resistor. In a similar way, the rising edge of IC4b pin 4 is differentiated by a .01µF capacitor and a 100kΩ resistor, creating a high-going pulse to reset the address counters. Diode D5 is employed to limit negative spikes which could damage the ICs when IC4b switches from high to low. Because the GO and STOP inputs may be controlled from external control circuits, diodes D1-D4 and the 10kΩ resistors are included to protect the inputs from excessive current and voltages. In summary, the start/stop operation works like this. Pulsing the GO line low starts the clock and the replay. www.siliconchip.com.au Semiconductors 1 32K x 8 SRAM 62256 or equivalent (IC1) 1 4040 12-stage binary counter (IC2) 1 4024 7-stage binary counter (IC3) 1 4093 quad NAND gate (IC4) 1 LM358 dual opamp (IC5) 1 LM386 audio power amp (IC6) 1 BC547 NPN transistor (Q1) 5 1N4148 signal diodes (D1-D5) 3 1N4004 power diodes (D6-D8) 1 1N5819 Schottky diode (D9) 1 7805 positive 5V regulator (REG1) Capacitors 2 470µF 25VW PC electrolytic 1 470µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 5 0.1µF MKT polyester 1 .047µF MKT polyester 1 .022µF MKT polyester 4 .010µF MKT polyester 1 .0022µF MKT polyester Resistors (0.5W, 1%) 1 4.7Ω 1 10Ω 9 1kΩ 1 5.1kΩ   1 6.2kΩ 1 12kΩ 7 15kΩ 10 30kΩ 3 100kΩ 2 20kΩ horizontal trimpots (VR1, VR2) Pulsing the STOP input low during a replay will stop the clock and the replay. Replay will also stop automatically when the last address line goes from high to low. It is also possible to replay continuously by holding the GO input permanently low. In this case, there will be a small gap in the replay as the address counters go from maximum count to zero at the end of the message but it is hardly noticeable. Sound generation To read data from the SRAM, the 6 10kΩ shunt must be in the Play position. As each memory cell is addressed, the respective data will appear at the data pins. The array of 15kΩ and 30kΩ resistors connected to the data pins forms what is referred to as an R/2R digital-to-analog converter. This type was chosen because it is much cheaper than a dedicated D-to-A converter IC, and in any case does an excellent job in this circuit. The voltage at the D-to-A output is buffered and appears at pin 1 of IC5a. It has a resolution of 256 equal steps November 2001  69 ranging from around 0V to 5V. 0V represents the minimum level of the audio waveform and 5V the maximum of the audio waveform, while 2.5V is the rest or silence level. IC5a is wired as a non-inverting buffer and has a very high input impedance. This is necessary to stop the low impedance of the following circuits loading the D-to-A converter and reducing its accuracy. Due to the low sampling rate used (8kHz), the audio waveform needs to be low-pass filtered to remove high frequency components and improve the listening quality. IC5b is configured as a 2-pole lowpass filter with a cut-off frequency of 4kHz. The output level is quite high at this point, so a 10kΩ resistor is included between the output of IC5b and the volume control to avoid overloading the audio power amp stage. The signal from the volume control is capacitively coupled to the audio power amp IC6. This is a well-proven circuit using an LM386 in its basic form, driving an 8Ω speaker. A 4.7Ω resistor and a 470µF capacitor provide supply decoupling, while the .047µF capacitor and 10Ω resistor connected to pin 5 help to prevent instability in the output stage. Power supply The power supply is a standard 7805 3-terminal voltage regulator fed from a 9V plugpack. The circuit draws minimal current, so one rated at say, 150-300mA, will be ample. Avoid using a plugpack with a higher voltage rating, because the LM386 is not designed to withstand a supply voltage much greater than 12V. Of course, you can use a 9V regulated supply if you prefer. Diode D6 is used to prevent damage to the circuit from supplies connected with reverse polarity and a 470µF capacitor smoothes the usually unregulated plugpack output. The regulator is ‘jacked up’ with a diode (D7) in the ground lead, giving an output voltage of 5.6V. A 0.1µF capacitor is included at the output of REG1 to help prevent instability. The output voltage is reduced back to 5V by diode D8 which feeds the main circuit. The audio sections are powered from the unregulated supply and consequently do not operate when the main DC power is removed. When the main power is disconnected and the output of the regulator goes below 4V, Q1 turns off and CS is pulled high. When CS is high the SRAM is placed in standby mode and consumes very little power. The CS lead must be high before the battery backup supply switches in otherwise the data retention will not work. The keen-eyed will notice that the battery backup supply is also connected to the remainder of the logic ICs. These are CMOS chips with minimal current drain and so do not significantly degrade the expected battery life. Battery backup Programming Diode D9 is used to connect the battery backup supply when the main supply is removed. With the main supply connected, D9 is reverse-biased because the cathode is more positive than the anode, so no current flows from the batteries. However, when the main supply is missing, D9 conducts and the batteries supply power to the SRAM. D9 is a Schottky diode, which has a much lower forward voltage drop (about 0.3V) than a normal diode and is used to maximise the SRAM data retention time as the battery ages. A typical SRAM will hold its memory with a supply as low as 2V. With 3V or more from a pair of new AA cells, minus the 0.3V drop in the Schottky diode (D9), around 2.7V is available for the SRAM. Transistor Q1 is normally biased on due to the base resistor connected to the output of REG1. The collector of Q1 is connected to the Chip Select line of IC1, s o when main power is connected, CS is pulled low, enabling normal chip operation. Programming is done by connecting to the 25-pin parallel or printer port of a PC operating under DOS or a DOS box in Windows 95 or Windows 98 (Windows NT, Me and 2000 use the parallel port differently and may not work properly). The data to be programmed into the SRAM is output in parallel on pins 2-9, and the negative going programming clock pulses are output on pin 1. The ground connection is made through paralleled pins 18-25. To place the board into program mode, the shunt must be moved from the Play header onto the Program header pins. This causes pin 22 (Output Enable) to go high, disabling the output buffers, and connects the PC clock signal to IC4c and pin 27 (Write) of IC1. Before programming starts, the replay must be stopped so that the address counters are reset and addressing the first memory location. The 1kΩ resistors couple the data leads from the PC into the data pins of IC1 and also protect the inputs from damage from surge currents. The resistors associated with the D-to-A converter are much higher in value than 1kΩ and so do not interfere with the programming process. A 1kΩ resistor and a .01µF capacitor Table 1: RESISTOR COLOUR CODES              No. 3 10 7 1 6 1 1 9 1 1 Value 100kΩ 30kΩ 15kΩ 12kΩ 10kΩ 6.2kΩ 5.1kΩ 1kΩ 10Ω 4.7Ω 70  Silicon Chip 4-Band Code (1%) brown black yellow brown orange black orange brown brown green orange brown brown red orange brown brown black orange brown blue red red brown green brown red brown brown black red brown brown black black brown yellow violet gold brown 5-Band Code (1%) brown black black orange brown orange black black red brown brown green black red brown brown red black red brown brown black black red brown blue red black brown brown green brown black brown brown brown black black brown brown brown black black gold brown yellow violet black silver brown Table 2: CAPACITOR CODES Value IEC code EIA code 0.1µF   100n   104 .047µF  47n  473 .022µF  22n  223 .01µF  10n  103 .0022µF  2n2  222 www.siliconchip.com.au Fig.4: the full-size PC board pattern, ready for you to make your own or to check commercial boards for any defects. This pattern can also be downloaded from the SILICON CHIP website. filter the programming clock input, to eliminate unwanted noise from providing false write pulses. Before programming starts, pin 9 of IC4c will be high and as the programming pulse is also high, pin 10 of IC4c will be low. Programming starts with the PC outputting eight bits of data onto the data leads. The programming clock line is pulsed low, pulling the Write pin of IC1 low and writing the data into the addressed memory cell. When the programming pulse goes high again, pin 10 of IC4c goes low, clocking the address counters onto the next location. It is important when programming memory that the address and data lines are steady while the Write line is pulsed low and returns high. At first glance, the circuit may seem at odds to this requirement. However, the address counters will not change state until well after the Write line has been taken high due to the propagation delay in IC4c, IC2 and IC3. The software repeats this process until all the memory locations have been programmed. The rate that the programming pulses are generated and hence the total programming time could have been much faster but it has been purposely slowed down. This has been done to avoid any problems that might arise with long cable lengths and different PC printer ports. In any case, the whole process only takes about 15 seconds on an average PC. www.siliconchip.com.au At the conclusion of programming the shunt is moved back to the Play position. A 10kΩ resistor holds the Write pin high when the shunt is removed, avoiding unwanted writes to the SRAM. Construction Start construction by assembling the PC board. There are seven wire links to be installed, so do these first. Ensure they are straight and lay flat on the PC board. Follow this with the smaller components, such as the PC stakes, IC socket, trimpots, resistors and diodes. Next, install the capacitors, ensuring that the electrolytics are installed with correct polarity. Follow this with the transistor and ICs but leave the SRAM chip until later. Note that not all the ICs face the same way, so check the component overlay diagram before soldering them in. Take care with the CMOS chips, by trying to avoid touching the pins, earthing yourself before holding them and soldering the power supply pins first. The 5V regulator (REG1) is installed with its metal tab facing into the PC board. It runs cool and won’t need a heatsink. Once the PC board is loaded you can prepare the case which needs to have a number of holes made in it. See the photographs as a guide. Start with a hole to mount the DC socket at the righthand end of the case. The D connector is mounted on the side of the case near the programming PC stakes. The rectangular cutout for the D connector is easily made by drilling a number of large holes and finishing to shape with a small file. You will also need to drill two holes on either side of the cutout to secure the connector with the mounting hardware. Place the PC board on the bottom of the case, locating it so that there is enough room at the lefthand end to sit the battery holder. Mark the positions of the holes, remove the PC board and drill with a 3mm drill. Drill a pattern of holes in the middle of the lid to allow sound to escape from the speaker. If you use the specified case you’ll find dimples on the underside of the lid which make drilling neat, evenly-spaced holes relatively easy. Once the case has been prepared, install the DC socket and D connector and mount the PC board in the case with 3mm screws and nuts. An extra nut is placed on each screw between the case and the PC board to act as a spacer. Mount the speaker on the inside of the lid with a bead of silicone adhesive placed around the edge and leave to cure. The DC socket and the speaker are wired to the PC board stakes with hookup wire. Ensure that the speaker wires are long enough to allow the lid to be removed and placed alongside the case. The D connector is wired to the PC board using a short length November 2001  71 MESSAGE RECORDER/PLAYER 9V DC PLAY SILICON CHIP GO www.siliconchip.com.au PROGRAM STOP PC PARALLEL PORT Fig. 5: here’s the full-size artwork for the front panel. You can photocopy this or if you want it in colour, download it from www.siliconchip.com.au. As you can see, this panel incorporates a switch for the Play/Program function and also brings the “GO” and “STOP” pins outside the case. Exactly how you do this is up to you! of multi-coloured ribbon cable, with pins 18-25 of the D connector soldered together with a piece of bare tinned copper wire. The battery holder is placed at the end of the case and soldered to the battery PC stakes using the wires that come with the holder. Apply four self-adhesive feet to the bottom of the case when finished. Initial testing Once construction is complete, check your component placement and soldering carefully. Remember that the SRAM chip should not be in place yet. If all appears OK, connect the plug pack to the DC socket and measure the voltage at the power supply socket with a multimeter. This should be somewhere around 9-12V, depending on the plugpack used. Next, measure the voltage at REG1’s input, which should be around 0.6V less. Then check the voltage at the junction of D8 and D9, which should be close to 5V. If not, disconnect power quickly and look for errors, especially with the power wiring 72  Silicon Chip and the installation of the polarised components. When you are satisfied that the power supply is working, remove the supply and adjust the two trimpots to mid-position. Now plug the SRAM chip into its socket (with pin 1 closest to IC2) and place the shunt in the Play position. Apply power again, and briefly ground the GO input. A raucous noise should come from the speaker for about four seconds as the SRAM outputs its random data. Ground the GO input again and then momentarily ground the STOP input to check that the replay stops before the 4-second period elapses. Now permanently ground the GO input and measure the frequency of the clock at pin 11 of IC4d. Adjust VR1 until it is as close as you can get to 8kHz. In the unlikely event that no sound comes out, check firstly that VR2 is turned from the ground position. If this appears OK then you will have to check out the other parts of the circuit, such as the oscillator and the audio stages. Fault finding will be a lot easier if you have access to an oscilloscope, however with the foregoing description of the circuit you should be able to find most problems with just a multimeter and monitor amplifier. The software To download files into your Message Player, you will need to load the companion software file called MSGPLAY.EXE. This is available from the SILICON CHIP website and comes as a zipped up file. Once you have downloaded the file, unzip it using WinZip and copy it to a new folder on your PC. Use this folder to store all your generated sound files, as it makes them easier to find if they are all together. MSGPLAY is a DOS program, and is obviously designed to work with a PC running DOS. However, it runs quite happily in a DOS window under Win95 or Win98. As previously mentioned, Win NT, Me and 2000 operate differently and will not work properly with MSGPLAY. (Everything appears to be working but the WAV file doesn’t download). When you run MSGPLAY you will www.siliconchip.com.au be greeted with an opening menu screen. On this menu you have four options, which you highlight with the Up and Down arrows keys and then choose by pressing Enter. The program is easy to drive and provides lots of on-screen messages to help you through. The Open file menu allows you to enter a file name for opening, the Download menu steps you through the download process and choosing the final option exits you from the program. The Setup menu is the third option and allows you to select the printer port that you have the board connected to (usually LPT1) and also provides two test files for downloading. These files can be used if you think you have troubles with the hardware, or you simply can’t wait to hear something other than noise come out the speaker. The first file is four seconds of 1000Hz sinewave, which you can use to test that the whole board and the PC connection is working. The second test file is a 31Hz sawtooth wave with 256 steps. If you download this file and run it, you will be able to check the linearity of the D-to-A converter with an oscilloscope connected to pin 1 of IC5a. Making files and downloading Producing your own WAVE files is quite easy. Every PC that has a sound card and a sound recorder program can make WAVE files. To make advanced ones, through editing, mixing and adding special effects, you really need a special sound file editor. Don’t think that you have to rush out and spend a fortune on software, because you can download shareware or freeware from the Internet. They may not give you all the features of a professional sound file editing program but you’ll still be able to produce exciting results. Just get on the Internet, use your favourite search engine and start searching. The file that you create must be mono, 8kHz and 8-bit. If you have created it under another format, you need to convert it first. Just to make sure, the download software inspects the file when it is opened and informs you if it is not suitable. If your final file is large, you will only be able to store the first 32KB. If your file is smaller than 32KB, the download software will fill the remainder of the SRAM with silence (7F Hex). So let’s download and play a file. Firstly, run the download program, select the Setup option and choose the connected printer port. Then select the File open option and enter the file name. Select the Download option and follow the instructions. Place the shunt in the Program position and connect the cable. Start the download and you will see the counter showing you the progressive count as the bytes are written to the SRAM. You will also notice, as the download is progressing, that you can hear in slow time the file that you have created. This is quite normal as the data being downloaded to the SRAM is also fed to the D-to-A converter and audio amplifier but at a much slower rate than normal. Once the downloading is finished, move the shunt to the Play position and disconnect the cable. Pulse the GO input low (eg, short it to 0V) and you will hear your creation being played. To change the playback volume, adjust volume trimpot VR2 with a small screwdriver. Now is a good time to check the battery backup operation. Insert two new AA cells into the battery holder and disconnect the plugpack. Wait a few minutes and then re-connect the plugpack. Ground the GO input and if the file is still in the SRAM, you will hear it being played. If not, it may be that the batteries are in the wrong way or that diode D9 was installed with the incorrect polarity. Or the batteries could be flat! Interfacing The Message Player requires inputs to start and stop the replay. We’ll leave it up to you to work out an interface for your particular application. The robust GO and STOP input circuits allow a wide range of control possibilities. The simplest way is to use two pushbutton switches wired between the inputs and ground for manual control. However, if you have external inputs they can be either 5V negative-going pulses from logic circuits or a set of normally-open relay contacts or even open-collector transistor switches. Remember that the GO input must be normally high and grounded momentarily to start replay but you only need a similar STOP pulse if you don’t want the full 4-second replay. If you build a small interface board, you’ll find plenty of space within the case to mount it and you’ll probably be able to power it from the existing plugpack supply as well. SC K&W HEATSINK EXTRUSION. SEE OUR WEBSITE FOR THE COMPLETE OFF THE SHELF RANGE. www.siliconchip.com.au November 2001  73