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Here’s an improved version of the very popular voice recorder design we published in May 2005. It can now be set up easily to record two, four or eight different messages for random-access playback or a single message for ‘tape mode’ playback. Also, it now provides cleaner and glitch-free line-level audio output suitable for feeding an amplifier or PA system. It can be powered from any source of 9-14V DC. By JIM ROWE An enhanced 45-second Voice Recorder Module Mo dule 64  Silicon Chip siliconchip.com.au T he solid state voice recorder module published in the May 2005 issue of S ILICON C HIP proved to be very popular. It has been used in all sorts of applications where messages or sounds needed to be recorded and played back reliably under either manual or micro control. But it had limitations – one of which was that it could only be used to record and play back one long message or a number of short messages in sequential ‘tape recorder’ fashion. This was despite the fact that the recorder chip we used was capable of recording and playing back up to eight messages in ‘random access’ mode. The module needed a fair bit of ‘hacking’ to make the chip work in this mode. Another limitation was that the playback sound quality of the module was fairly noisy and each message played back was accompanied by an irritating ‘click’ at the start and finish. With the benefit of hindsight this was due to the way we had chosen to couple the output audio from only one side of the recorder chip’s pushpull output. Hindsight also revealed a third limitation: the 2005 module had been designed to operate from a 6V battery, whereas many people wanted to use it from a nominal 12V DC source. It was with these limitations in mind that we decided to develop the new and improved sound recorder module described here. It’s based on the same HK828 chip used in the 2005 module but with the rest of the circuit designed to allow more flexibility in terms of message storage and to provide much cleaner and click-free playback audio. Finally, the new circuit can run from any source of DC between 9V and 14V. The HK828 chip has the ability to store single or multiple messages with a total length of between 40 and 60 seconds, depending on the sampling rate and the voice quality you want. In this new recorder module the chip is again teamed up with a lowcost electret microphone to allow easy message recording, plus an LM358 dual op amp IC which allows the recorded messages to be played back as a line level audio signal available for feeding an external amplifier and speaker. A suitable small amplifier for use with the module would be “The Champ”, as described in the February 1994 issue of SILICON CHIP. This is available at low cost ($5.95) from Jaycar Electronics as KC-5152. We’ve given the new module a set of ‘jumper links’ so it can be easily configured to record and play back messages in any of four modes: either two, four or eight messages in random access mode or one or more messages in sequential access ‘tape mode’. Another link allows the HK-828 chip’s message start ‘beeps’ to be enabled or disabled, as you wish. All message selection, record and play functions are controlled externally, by connections to a row of screw terminals along the side of the module. All functions are enabled by switches or logic signals. This makes it easy to record or play back messages using a set of pushbuttons and a switch or under the control of a PC, microcontroller or security system if you prefer. By the way, since the HK828 voice recorder chip is only available from Jaycar Electronics in Australia and New Zealand, kits for the new recorder module will only be available from Jaycar and its dealers. How it works Because HK828 chip forms the functional heart of the recorder module, you need to have at least a rough idea of what goes on inside this chip in order to understand how the recorder works. Fig.1 shows the chip’s basic architecture. First, the chip includes a high-gain microphone preamp so that it can be driven directly by a low cost electret microphone insert. An automatic gain control (AGC) circuit follows the preamp, to ensure that good quality recordings can be made without any need for manual gain adjustment, despite input signal level variations. The output of the AGC circuit is not connected directly to the chip’s recording circuitry but is brought out to the ‘Aout’ pin instead. This is linked to the ‘Ain’ pin by the user, to record messages from the microphone. This arrangement allows the chip to be used to record from line level signals in other applications. Since the main part of the HK828 records by a process of sampling the audio signals fed into it via the Ain Fig.1: at right is the block diagram of the HK828 voice recorder chip. While the recording process relies on audio sampling, the audio is not stored digitally but using an analog sample-and-hold system. The analog samples are stored in the cells of a 256K flash EEPROM. Each analog storage cell can store any of 256 different voltage levels, making it equivalent to an 8-bit digital recording. siliconchip.com.au December 2007  65 Parts List – Enhanced Voice Recorder 1 PC board, code EC8271, 119 x 57mm 1 electret microphone insert (AM-4011) 3 3-way terminal blocks, PC board mtg 1 2-way terminal block, PC board mtg 3 2-pin sections of SIL header strip 3 jumper shunts 1 28-pin DIL IC socket, 15.24mm spacing 1 8-pin DIL IC socket, 7.62mm spacing 1 2.5mm concentric DC power plug, PC board mtg (CON1) 1 RCA audio socket, PC board mtg (CON2) Semiconductors 1 HK828 voice recorder IC (IC1) 1 LM358 dual op amp (IC2) 1 78L05 +5V regulator (REG1) 1 PN200 PNP transistor (Q1) 1 5mm green LED (LED1) 1 5mm red LED (LED2) 1 1N4004 1A diode (D1) Capacitors 1 2200mF 16V RB electrolytic 1 220mF 16V RB electrolytic 1 22mF 16V RB electrolytic 1 10mF 16V RB electrolytic 1 4.7mF 25V tag tantalum 1 220nF 100V MKT metallised polyester 5 100nF 100V MKT metallised polyester 1 100nF multilayer monolithic ceramic 1 150pF disc ceramic Resistors (0.25W 1%) 1 470kW 1 220kW 2 100kW 8 47kW 9 22kW 2 10kW 2 1kW 2 680W 1 100W 1 47W Where from, how much? This kit is only available from Jaycar Electronics, who also hold the copyright on the design and PC board. Kits will be available from Jaycar Electronics stores and resellers (Cat no KC-5454). 66  Silicon Chip pin, it needs to pass these signals through a low-pass filter before the sampling. This is done to prevent distortion caused by sampling aliases, hence the “anti aliasing” filter between the “Ain” input and the sample and hold circuit block. Now although the audio is sampled inside the HK828, this is done using an analog sample-and-hold system rather than the more common digital sampling system. It stores the samples in an array of 262,144 (256K) Flash EEPROM analog storage cells, each of which can store any of 256 different voltage levels. This gives the equivalent of 8-bit digital recording. The capacity of the storage array means that the HK828 can store a total of 256K samples. How long a recorded message this gives depends on the sampling rate that’s used. For example, if the sample rate is 8000 samples per second, 256K samples will correspond to a total message length of just over 32 seconds (262,144/8000). However if you sample at 4200 samples/second, the 256K samples will give a total message length of just over 62 seconds (262,144/4200). The recording bandwidth or ‘fidelity’ is directly proportional to the sampling rate. So if you sample at 4200 samples/second, the recording bandwidth will be just over 2kHz, whereas sampling at 8000 samples/second gives a bandwidth of just on 4kHz. Choosing the sampling rate is there- fore of a compromise: the lower the sampling, rate the longer the recording time but the lower the audio bandwidth. Conversely, the higher the sampling rate the higher the bandwidth but the shorter the recording time. The HK828 chip has an internal sampling rate clock oscillator, as well as an input for an optional external clock. Either clock signal can be fed to the sample and hold circuit via the multiplexer (MUX), to control the sampling. The frequency of the internal oscillator is set by varying the value of an external resistor connected between the ‘OscR’ pin and ground. Our circuit uses a 47kW resistor, which sets the sampling rate to about 5800 samples/ second. This gives a message recording time of about 45 seconds and a bandwidth of about 2.9kHz, for reasonable voice-quality recording. As shown in Fig.1, the recording and playback of samples in the storage array is controlled by analog write and read circuits, along with the message control and message decoding circuits. When a message is being played back the signals pass through another lowpass filter to remove sampling noise, and are then fed to the inbuilt output amplifier. The rest of the circuitry inside the HK828 chip is used for overall device control, mode switching and so on. Circuit details Fig.2 shows the complete circuit Silicon Chip Binders Issues Getting Dog-Eared? Keep Your Copies Safe With These Handy Binders REAL VALUE AT $13.95 PLUS P&P Price: Just $13.95 plus $7.00 p&p per order (buy five and get them postage free). Available only in Australia. Just fill in the handy order form in this issue; or fax (02) 9939 2648; or phone (02) 9939 3295 and quote your credit card number. Silicon Chip Publications, PO Box 139, Collaroy NSW 2097. siliconchip.com.au siliconchip.com.au December 2007  67 10k 47k AGC Ain Aout MICref MICin MSEL2 MSEL1 BE 12 VssA 7 OSC R 26 EXT CLK 19 20 21 18 17 25 24 11 16 VccA IC1 HK828 VssD 13 SP+ SP– BUSY CE M1 M2 M3 M4 M5 M6 M7 M8 RE STROBE 28 VccD 14 15 10 23 1 2 3 4 5 6 8 9 27 22 100nF 100nF REC OR PLAY  LED1 100 K A 680 +5V MULTI-MESSAGE VOICE RECORDER 220k 100nF 47k 100nF 100nF LK1 47k 100nF LK2 47k 220 F 10V 47 100k 100k 150pF 6 5 22k 47k IC2b 3 OUT IN OUT IN (TAPE MODE) 8 MESSAGES, RANDOM ACCESS 4 MESSAGES, RANDOM ACCESS 2 MESSAGES, RANDOM ACCESS IN 1 OUT 4 IC2a 8 +5V IN 2 10 F REC MODE  LED2 680 OUT K A C Q1 PN200 IN 220nF GND OPERATING MODE 7 B E OUT REG1 78L05 LK3 LK2 IC2: LM358 LINK1: IN = BEEP DISABLED OUT = BEEP ENABLED 47k 47k 8x 22k 47k Fig.2: Most of the circuit action takes place in the HK828 voice storage chip. IC2, the LM358 dual op amp, performs a balanced-to-unbalanced conversion of the output from the HK828 and then provides a buffered output which can be fed to an audio amplifier and loudspeaker. SC 2007 LK3 4.7 F 10V – ELECTRET + MIC 22 F 10V 1k A K 1k 1N4004 470k 47k C B E PN200 2200 F 16V K A IN D1 1N4004 9–14V DC COM A K LEDS LINE LEVEL AUDIO OUT EARTH CHIP ENABLE M1 ENABLE M2 ENABLE M3 ENABLE M4 ENABLE M5 ENABLE M6 ENABLE M7 ENABLE M8 ENABLE REC ENABLE OUT 78L05 – + details for the new Multi-Message Voice Recorder. As shown, signals from the electret microphone insert are coupled into the MicIn input of the HK828 (pin 17) via a 100nF coupling capacitor. Another 100nF capacitor is used to tie the preamp’s second ‘MicRef’ input (pin 18) to ground, to provide maximum gain. The 4.7mF capacitor and 220kW resistor connected between pin 19 and ground are used to optimise the chip’s AGC attack and decay characteristics for speech. The amplified audio from the mic preamp and AGC circuit appears at pin 21 (Aout) which is coupled directly to pin 20 (Ain) via another 100nF capacitor. As mentioned above, the internal sampling oscillator frequency is set to 5.8kHz by the 47kW resistor connected to ground from pin 7 (OscR). Setting the HK828 into record or playback modes is achieved by an external switch or logic signal connected to the RecEnable-bar terminal, which connects to the chip’s RE-bar pin (27). The terminal is pulled to ground for record mode or allowed to rise to logic high level (+5V) for playback mode. Note that when the terminal is pulled down to ground for Record mode, this also allows transistor Q1 to draw base current and turn on – allowing current to flow through LED2, the Record Mode indicator. Link LK1 is used to enable or disable the HK828’s message starting ‘beep’, by changing the logic level at pin 11 (“Beep Enable”). Similarly LK2 and LK3 are used to set the desired message recording and playback mode, as shown in the small table on the circuit diagram. To record a message in one of the random access modes, all that needs to be done is to pull down the RecEnablebar line to force the chip into recording mode, and then pull down one of the message-select lines (M1Enable-bar, M2Enable-bar etc) using an external pushbutton or a logic signal from a PC or microcontroller. The message select line must be held down for the duration of the message recording; recording ends when the line is allowed to rise high again. To play the recorded message, the RecEnable-bar line is allowed to rise high again, and the message select line for the message you want to replay 68  Silicon Chip Fig.3: the same-size component overlay matches the photograph at right – between the two there should be no construction problems. pulled down again for about 400ms. The playback audio emerges in push-pull (ie, anti-phase) fashion from pins 14 and 15 of the HK828, the SP+ and SP- pins and is connected to a 100W load resistor. The signals are fed via 100nF capacitors to a balanced-tounbalanced matching stage using IC2b, one half of an LM358 dual op amp. This effectively adds the two signals together, and cancels out the ‘common mode pedestal’ signal that appears with them on both outputs. As a result the output audio signal at pin 7 of op amp IC2b is clean and ‘glitch free’. This is then passed through op amp IC2a, connected as a voltage follower/buffer and then fed to the line-level audio output socket. All of the part of the circuit operates from +5V DC from REG1, an LM78L05 regulator. We are able to use a low power regulator because the total current drain is quite low: about 4mA in standby mode, rising to about 45mA when a message is actually being played or recorded. There’s one remaining point which should be mentioned about the circuit. You’ll note that the HK828 chip is provided with a ChipEnable-bar pin (pin 23), which in this circuit is pulled down to earth via a 47kW resistor – so the chip is enabled by default. However the ChipEnable-bar line is also brought out to a terminal, to allow you to apply a logic high (+5V) to this line if you want to disable the chip for any reason. You might want to do this if you have a microcontroller or PC controlling a number of the modules, in which case it will need to be able to select between them using their ChipEnablebar lines. Construction All of the components used in the Voice Recorder module on a compact Resistor Colour Codes No. Value   o   1 470kW   o   1 220kW   o   2 100kW   o   8 47kW   o   9 22kW   o   2 10kW   o   2 1kW   o   2 680W o   1 100W   o   1 47W 4-Band Code (1%) yellow purple yellow brown red red yellow brown brown black yellow brown yellow purple orange brown red red orange brown brown black orange brown brown black red brown blue grey brown brown brown black brown brown yellow purple black brown 5-Band Code (1%) yellow purple black orange brown red red black orange brown brown black black orange brown yellow purple black red brown red red black red brown brown black black red brown brown black black brown brown blue grey black black brown   brown black black black brown yellow purple black gold brown siliconchip.com.au JOIN THE TECHNOLOGY AGE NOW with PICAXE Team this little module with a small audio amplifier, to provide a great range of sound effects for a model railway layout, for example. It has the ability to store up to eight different “sound grabs” which could be switched to different parts of the layout as trains pass through stations. PC board. This is coded EC8271, and measures 107 x 57mm. It can be mounted inside a standard UB3 size jiffy box. As all of the terminals and connectors are along one side the board, they will all be accessible via a slot or series of holes along that side of the box. Only three holes will be needed in the box lid: two 5mm holes for LED1 and LED2, and a larger hole to allow sound to reach the electret mic insert. The location and orientation of all components on the board can be seen in the overlay diagram of Fig.3, and also in the matching photo of the module. Start board assembly by fitting the four screw terminal blocks, then the DC input and audio output sockets. Follow these with the two IC sockets, the three 2-pin headers for LK1-LK3 and the short wire link which fits just near the end of the 28-pin IC socket. After this you can fit the resistors and smaller non-polarised capacitors. Next come the 4.7mF tantalum and the electrolytic capacitors, which are all polarised, so make sure you fit them with their orientation as shown in the diagram. Now you’ll be ready to fit the semiconductor parts. These are again all polarised, so make sure you follow Capacitor Codes Value 220nF 100nF 150pF mF Code IEC Code EIA Code 0.22mF 220n 224 0.1mF 100n 104 n/a 150p 151 siliconchip.com.au the diagram carefully as a guide to their orientation. Fit diode D1 first, then transistor Q1 and the two LEDs, followed by regulator REG1. Then fit the electret mic insert. This has only two wire leads, but it is polarised, so make sure you check the back of the insert to make sure which lead connects to the metal body of the insert. This is the negative lead, which must be connected to the earthy outer pad under the board. The other lead is the positive lead. Finally, plug the LM358 op amp IC2 into its 8-pin socket and the larger HK828 chip IC1 into its 28-pin socket. Make sure they’re both orientated as shown in Fig.3. Your Multi-Message Voice Recorder should now be complete and ready to go. Trying it out To check that your recorder is working correctly, first decide on which message mode you want to use it in, and then place jumper shunts on link headers LK1, LK2 and LK3 to set the module for that mode of operation. (Use the table in Fig.2 as a guide.) Then connect a small toggle switch and one pushbutton switch for each message you want to select to the appropriate screw terminals of the module, as shown in Fig.4. For the present switch the toggle switch off, which corresponds to message playback mode. The audio output of the module can now be connected to the line input of any suitable audio amplifier. Then you can connect its DC power input to a source of 9-14V DC. Developed as a teaching tool, the PICAXE is a low-cost “brain” for almost any project Easy to use and understand, professionals & hobbyists can be productive within minutes. Free software development system and low-cost in-circuit programming. Variety of hardware, project boards and kits to suit your application. Digital, analog, RS232, 1-Wire™, SPI and I2C. PC connectivity. Applications include: Datalogging Robotics Measurement & instruments Motor & lighting control Farming & agriculture Internet server Wireless links Colour sensing Fun games Distributed in Australia by Microzed Computers Pty Limited Phone 1300 735 420 Fax 1300 735 421 www.microzed.com.au December 2007  69 At this stage neither of the LEDs should light but you may hear a small turn-on ‘plop’ from the speaker connected to the external amplifier. If you wish you can use a digital multimeter to confirm that the supply voltage at pin 8 of IC2 is very close to +5V, relative to the module’s earth terminal. Now switch the external toggle switch on, pulling the RecEnable-bar line down to earth potential. This should switch the module into Record mode, so LED2 should begin glowing. (If it doesn’t begin glowing, you either have the DC power polarity reversed, or LED2 fitted to the board the wrong way around.) Next, press one of the message select pushbuttons – say MSG1 in Fig.4. Holding it down, begin talking into the electret mic to record your test message. As you speak, you’ll notice that the green Strobe LED (LED1) is flashing. Keep talking until you reach the end of your message or until LED1 stops flashing (which indicates that recording has stopped automatically, because you have reached the end of that segment of the HK828’s memory). Then release the pushbutton. To replay the recorded message, turn the toggle switch off to swing the module into Play mode and briefly press the message pushbutton again, but this time only briefly because in Play mode, the message buttons only trigger the replay operation. Your recorded message should then be replayed through the external amplifier and speaker. If it does, your Multi-Message Voice Recorder is working correctly and should now be ready for use. Changing message length As mentioned earlier, the total message length stored in the HK828 chip’s memory is determined by the sampling rate which is set by the resistor connected from pin 7 of the chip (OscR) to ground. The 47kW value shown for this resistor in the circuit and overlay diagram gives a sampling rate of 5800 samples/ second, resulting in a total message length of 45 seconds and an audio bandwidth of about 2.9kHz. We picked this as a reasonable compromise between message length and recording quality but you can experiment with the value of this resistor to try longer/shorter recording times and 70  Silicon Chip Fig.4: Staying with the model rail theme, you could use reed relays or other switches to play back the sound grabs when the train triggers them or they are switched by the operator. If the sound grabs played in different locations you will need additional speakers and relays to switch them to the amplifier. narrower/wider audio bandwidth. For example, if you change the resistor value to 82kW, this will lower the sampling rate to about 4200 samples/second and give a total recording time of just on 60 seconds. However the audio bandwidth will also drop to around 2kHz, so the played-back message(s) will sound rather ‘muffled’ – a bit like talking through a wet sock! On the other hand, if you lower the resistor value to 24kW, this will increase the sampling rate to about 8000 samples/second and drop the recording time to just on 32 seconds. But the recording quality will improve, as the audio bandwidth will increase to about 4kHz. So experiment by all means, and settle on the resistor value you decide gives the best combination of total message length and acceptable audio quality for your application. Changing message mode As noted earlier, header links LK2 and LK3 on the board can be used to change the module’s message access mode. For example with a jumper shunt fitted to LK2 but removed from LK3, the module will be able to record and play two messages (each using half the HK828’s memory space). You’ll only need two external pushbuttons to select one of these messages: MSG1 and MSG2, along with the Record/Play toggle switch. If you want to record and play four messages, remove the jumper shunt from LK2 and place one on LK3 instead. You’ll now need four external pushbuttons as well as the Record/Play toggle switch: MSG1, MSG2, MSG3 and MSG4. Note that in this case each message will be able to use one quarter of the HK828’s memory. Leave the jumper shunts off both LK2 and LK3 if you want to record and play any of eight short messages (each using one eighth of the HK828’s memory). You’ll now need all eight external pushbuttons MSG1 - MSG8, along with the Record/Play toggle switch. The last option is to fit jumper shunts to both LK2 and LK3, which sets the module for “tape mode” operation. In this mode you normally only need one external pushbutton (MSG1), because the HK828 records and plays back either one message or a sequence of messages, using all of its memory space. That’s it then – an easy-to-build solid state Multi-Message Voice Recorder module that can be used for all kinds of applications, especially those involving sending pre-recorded messages over an amplifier or PA system under the control of a PC or microcontroller. SC siliconchip.com.au