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By Jim Rowe 45-Second Voice Recorder Module Need to record voice messages of up to 45 seconds long and then play them back at the touch of a button or under the control of a PC or microcontroller? Here’s a low-cost, easy-to-build solid state recorder module that’s ideal for this kind of application. W HEN YOU NEED to record voice messages with the highest possible reliability, there’s no substitute for a solid-state voice recorder chip. There are no moving parts at all, so there are no tapes or belts to break, no heads to clog up with gunk, no motors to burn out and no bearings to jam. Instead, there’s just a memory chip to store and replay the message electronically, any number of times. Because it’s solid-state, the message(s) can be played back instantly – there’s no tape to rewind. Small wonder that most telephone answering machines changed over to this kind of recording years ago. 28  Silicon Chip We’ve described solid-state voice recorder projects in the past and they’ve been very popular. However, the special chips they used eventually became hard to get and so these projects eventually fell by the wayside. This situation recently changed for the better again, when Jaycar Electronics managed to find a good source for a new voice recorder IC, the HK828. This made it possible for us to develop a new recorder design, incorporating not only the features that were popular in the earlier designs but a few more based on the requests we’ve received from readers over the years. The new HK828 chip can 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 our new recorder module, this chip is teamed up with a low-cost electret microphone to allow easy message recording, plus an LM386 power amplifier IC which allows the recorded messages to be played back through a small speaker. In addition, we’ve made provision for the module to be hooked up to a 600W/600W line isolation transformer for coupling into another system; eg, an alarm system or a private phone line. There’s also a simple interface so that the module’s functions can be controlled via a PC or microcontroller. The new recorder module runs from 6V DC and draws very little current, so it’s quite suitable for operating from either a battery (eg, four AA penlight cells) or from a regulated 6V plugpack supply. By the way, since the HK828 voice recorder chip is only available from Jaycar Electronics in Australia and siliconchip.com.au Fig.1: block diagram of the HK828 voice record/playback IC. The incoming signal from the microphone is amplified and fed through an AGC stage and anti-aliasing filter before being sampled and stored in a 256K EPROM array. New Zealand, kits for the new recorder will only be available from Jaycar and its dealers – see parts list. How it works Because the HK828 chip forms the functional heart of the unit, you need to have 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) stage follows this 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 instead brought out to the “Aout” pin. This is linked to the “Ain” pin by the user, to record messages from the microphone. This arrangement also allows the chip to be used to record from line level signals in other applications. Since the main part of the HK828 records by sampling the audio signals fed into it via the Ain pin, it needs to pass these signals through a low-pass filter before the sampling. This is done to prevent distortion caused by samsiliconchip.com.au pling 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. This is done because 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. However, the length of the recorded message depends on the sampling rate that’s used. For example, if the sampling 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 per second, the 256K samples will give a total message length of just over 62 seconds (262,144/4200). The recording bandwidth or “fidelity” also depends on the sampling rate – in this case, directly rather than inversely. So if you sample at 4200 samples per second, the recording bandwidth will be just over 2kHz, whereas 8000 samples per second gives a bandwidth of just on 4kHz. Choosing the sampling rate is therefore something 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 internal oscillator is particularly easy to use, because its frequency is set simply by varying the value of an external resistor connected between the “OscR” pin and ground. In this recorder, we have selected 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 May 2005  29 Fig.2: the complete circuit of the voice recorder uses just two ICs – the HK828 voice record/playback IC (IC1) and an LM386N audio amplifier. Power can come from a regulated 6V plugpack supply or from batteries (4 x 1.5V cells). and are then fed to the inbuilt output amplifier. The rest of the circuitry inside the HK828 chip is used for overall device control and mode switching, etc. Circuit details Fig.2 shows the complete circuit details for the Solid-State Voice Recorder. As shown, signals from the electret mic insert are coupled into the MicIn input (pin 17) of the HK828 via a 100nF capacitor. Another 100nF capacitor is used to tie the preamp’s “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) and is then fed to pin 20 (Ain) via another 100nF capacitor. As mentioned previously, the inter30  Silicon Chip nal sampling oscillator frequency is set to 5.8kHz by the 47kW resistor connected to ground from pin 7 (OscR). Manual selection of the HK828’s operating mode (record or playback) is achieved by switch S2, which connects the chip’s RE-bar pin (27) to ground for record mode. When S2 is in the Play position, the RE-bar pin is pulled up to +6V via the 22kW resistor and the base-emitter junction of transistor Q1, which forces the HK828 to operate in playback mode. Note that when S2 is in the Record position, this not only grounds pin 27 of the HK828 but also turns Q1 on. This in turn drives LED2 – the Record Mode indicator – via a 680W currentlimiting resistor. When the unit is switched to Record mode, recording the message you want to save in the chip is very simple – press pushbutton switch S1 and hold it down while you record the message. During recording, LED1 flashes to indicate that the HK828 is operating, while LED2 is turned on continuously. At the end of the recording, you simply release S1. However, if your message is too long for the chip’s memory, it will automatically stop recording when the memory is full. To replay the recorded message, S2 is first switched back to the Play position. Then the manual “rewind” button (S3) is pressed briefly to take the chip’s CE-bar pin (23) to +5.4V, which makes sure the HK828 is reset to the start of its memory. If you then briefly press pushbutton switch S1, the HK828 will replay the message. The replayed audio emerges from pins 14 & 15 of IC1 (ie, from SP+ and SP-). In this circuit, the signal from pin 14 is fed though a 10kW resistor and 10mF coupling capacitor to trimpot VR1, which is used to set the playback volume. The signals are then fed to an LM386N audio amplifier (IC2) which drives the external loudspeaker. siliconchip.com.au This is what the assembled PC board should look like. Note this is a photo of an early prototype before we fitted the power switch. In addition, both output pins of the HK828 are brought out to terminal pins L1 and L2. These can be used to connect the recorder module to the primary winding of a 600W/600W isolating transformer (such as the Jaycar MA-1510 or MA-1512), so that the audio can be fed to other equipment (eg, an alarm system). The remaining part of the circuit provides a simple interface which allows the voice recorder module to be controlled by a PC or a microcontroller. This involves bringing the HK828 control lines out to 10-pin IDC header CON1, so they can be manipulated by an external microcontroller programmed to duplicate the actions of switch S2 and pushbutton switches S1 & S3. Note that when this interface is connected to a microcontroller, switch S2 must be left in the Play position. This is necessary to allow the microcontroller to control the logic level on IC1’s RE-bar pin (pin 27). There’s one final point to note about the HK828 recorder chip and the way we’re using it here. The HK828 can actually be programmed to record in a number of different “message mode” formats, by manipulating the logic levels on pins 9 (M8option), 24 (MSEL1) and 25 (MSEL2). In this circuit, we operate the chip in “tape mode” format, where it can record either a single continuous message or a number of shorter messages in sequence. However, the module’s PC board has been designed to allow you to change the format if you wish, by cutting short tracks and/or fitting pull-up resistors or links. It’s not hard to set the HK828 to siliconchip.com.au record two, four or eight short fixed-length messages, which can be replay-ed in random-access fashion (more on this later). Construction All of the components used in the Voice Recorder module except the speaker and battery mount directly on a PC board coded 01105051. This board measures 107 x 57mm, which Fig.3: follow this parts layout to assemble means that it can be mountthe Voice Recorder. Make sure that all polarised components are correctly ed inside a standard UB3installed and be sure to install S1 & S3 as size utility box. shown – ie, with the “flats” on the switch Fig.3 shows the parts laybodies facing LED1 & LED2. out on the PC board. Start the assembly by fitting the eight PC-board terminal pins, then install the three wire links. to fit them as shown in Fig.3. In parThese links are all relatively long and ticular, note that the 1000mF electrolytic should be run using insulated hookup capacitor mounts on its side, with its wire. leads bent down by 90°. Next, fit the 28-pin DIL socket for Now for the semiconductors. Once IC1, making sure you orientate it with again, these parts are all polarised, so its notched end towards the right. That follow Fig.3 carefully when installing done, fit the 10-pin IDC header, with them. Fit diode D1 first, then transistor its slot side towards the left as shown Q1 and the two LEDs. Finally, fit the in Fig.3. LM386N amplifier (IC2). Don’t plug Once these hardware items are in the HK828 chip into its socket just yet place, fit trimpot VR1 and the resistors. though – that step comes later. Table 1 shows the resistor colour codes Switches S1-S3 can go in next – it’s but it is also a good idea to check them just a matter of pushing them all the using a multimeter as the colours can way down onto the PC board and solsometimes be difficult to read. dering their pins. Note that the “flats” Next come the low value MKT ca- on the bodies of S1 & S3 must face pacitors (100nF and 47nF). These are towards LED1 & LED2. unpolarised so you can fit them either The electret mic is fitted by solway around. dering its two leads to the PC board The tantalum and electrolytic capaci- terminal pins just behind switch S2. tors can go in next. Unlike the MKT Note that the mic is polarised – the types, these are all polarised, so be sure lead which is connected to its metal May 2005  31 VR1 to set the replay volume to an acceptable level. This will depend on the sensitivity of your speaker. If your recorded test message plays back as it should, your Solid State Voice Recorder is working correctly and should now be ready for use. One further point – you’ve possibly noticed the link on the circuit diagram labelled “Beep Mute” and indicated on the PC board overlay diagram as “BM” (just to the right of VR1). This link may be fitted if you don’t like hearing the small “beeps” which the HK828 chip sends out to the speaker to acknowledge the control signals fed to it from switches S1-S3. Fitting the “BM” link grounds pin 11 of IC1 and disables this “beep” function. The completed PC board from the opposite angle. Note the mounting method for the 1000mF electrolytic capacitor (top left). case must be connected to the lefthand terminal pin on the board. Next, solder the battery snap leads and the speaker leads to their respective terminal pins. It doesn’t matter which way around you connect the speaker but take care with the battery leads (ie, connect the red battery lead to the “+” terminal and the black lead to the “-” terminal). Finally, complete the assembly by plugging the HK828 chip into its socket. Make sure that all pins go into the socket and check that the notched end of the IC is to the right. Your Solid State Voice Recorder should now be ready to go. Trying it out To check that your recorder is working correctly, first set trimpot VR1 to mid-position and connect a 6V battery or regulated 6V power supply to the battery lead. That done, press the Rewind button (S3), to make sure that the HK828 has reset its storage array address correctly. Changing message length Next, set switch S2 to the Record position and check that the Record LED (LED2) starts glowing. If it does, press S1 (the Run/Start button) and hold it down while you talk into the electret mic to record your message. As you speak, you’ll notice that the green Run LED (LED1) is flashing. Keep talking until you reach the end of your message, or until LED1 stops flashing (indicating that the recording has stopped, because you reached the limit of the HK828’s memory). Finally, release S1 and that’s it – your message has been recorded. To replay the message, first set S2 over to the Play position and briefly press pushbutton S3 to reset the HK828’s memory address (ie, to “rewind” the unit). Now press pushbutton S1 again but this time only briefly because in Play mode, S1 only triggers the replay operation (ie, it only has to be held down during recording). Your recorded message should now be replayed through the speaker, although you may need to adjust trimpot As mentioned earlier, the message length stored in the HK828 chip’s memory is determined by the sampling rate and this is set by the resistor connected from pin 7 (OscR) to ground. A value of 47kW – as shown in the circuit and overlay diagram – gives a sampling rate of 5800 samples per second, resulting in a message length of 45s and an audio bandwidth of about 2.9kHz. We picked this as a reasonable compromise between message length and recording quality but you are free to experiment with the value of this resistor to try longer/shorter recording times and narrower/wider audio bandwidths. For example, a value of 82kW, will lower the sampling rate to about 4200 samples per second and increase the recording time to 60s. At the same time, the audio bandwidth will drop to about 2kHz, so the replayed message(s) will sound rather muffled. On the other hand, a value of 24kW will increase the sampling rate to about Table 1: Resistor Colour Codes o o o o o o o o o No.   1   2   6   2   2   2   1   1 32  Silicon Chip Value 220kW 47kW 22kW 10kW 1kW 680W 47W 10W 4-Band Code (1%) red red yellow brown yellow violet orange brown red red orange brown brown black orange brown brown black red brown blue grey brown brown yellow violet black brown brown black black brown 5-Band Code (1%) red red black orange brown yellow violet black red brown red red black red brown brown black black red brown brown black black brown brown blue grey black black brown yellow violet black gold brown brown black black gold brown siliconchip.com.au Table 2: follow this table to change the message recording mode – see text. 8000 samples per second and reduce the recording time to 32s. However, 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 recording mode If you wire up the module exactly as described, it will operate in “Tape Mode” . We decided to make this the default mode because we believe it’s the most suitable for a majority of applications. However, if you need the module to work in one of the HK828 chip’s alternative “random access fixed length messages” mode, this can be done fairly easily. All you have to do is cut one or both of the short narrow tracks which currently link pins 24 & 25 of the HK828 to ground and fit one or two extra 22kW resistors to pull these pins up to +6V instead. Table 2 shows how to program the HK828 for Random Access mode with either two, four or eight fixed duration messages, as well as the default Tape Mode. As you can see, it’s relatively straightforward. But remember that if you set it for a relatively large number of fixed-length messages, they will each have a relatively short length. So with the sampling rate left at 5800 samples per second, giving a total recording time of 45s, you’ll get two fixed length messages of 22.5s each, or four messages of 11s each, or eight messages of only 5.5s each. Remember too that in any of the Random Access modes, pin 1 (M1-bar) of the HK828 chip no longer becomes the record/replay trigger line for all messages. Instead, S1 becomes the Record/Start Play button only for the first randomly accessed message. You’ll need to connect additional pushbuttons for recording and playing back the remaining messages. siliconchip.com.au For example, if you set pins 24 and 25 for recording two fixed length messages, you’ll need to connect an extra pushbutton switch between pin 2 (M2bar) of the HK828 and ground, to allow the second message to be recorded and played back. This switch can be connected between pins 9 and 2 of the 10-pin IDC header, by the way. If you set pins 24 and 25 for recording four or eight fixed length messages, things get more complicated because you then need an extra pushbutton and 22kW pull-up resistor for each of the additional messages. These extra pushbuttons and pullup resistors need to be connected to pins 3 & 4 of the HK828 for four messages and to pins 3, 4, 5, 6, 8 & 9 for eight messages – although pin 9 won’t need a pull-up resistor, because it already has one. The reason for this extra complexity is that in its random-access modes, the HK828 has a separate record/ replay trigger line for each message. That’s why pin 2 is labelled M2-bar, because it becomes the record/replay trigger input for message 2. When you program the chip for four messages, pin 3 becomes M3-bar (the trigger input for message 3) and pin 4 becomes M4-bar (the trigger input for message 4). And if you program the chip for eight messages, pins 5, 6, 8 & 9 become M5-bar, M6-bar, M7-bar and M8-bar respectively. Expansion possibilities As mentioned earlier, this unit could be used as a voice module for an alarm system. For this and other applications of the Voice Recorder, you’ll probably want to connect it to a PC or microcontroller so that it can be controlled automatically. As shown in Fig.2, all the logic lines needed for controlling the recorder chip are available via the 10-pin IDC header CON1 (pins 6-10), along with a couple of lines which can be monitored to check the HK828’s status (pins 3 & 5). Interfacing the unit is really Par t s Lis t 1 PC board, code 01105051, 57 x 107mm 1 electret mic insert 1 10-pin (5 x 2) IDC header 2 SPST PC-mount pushbutton switches (S1,S3) 2 SPDT mini toggle switch (S2, S4) 1 28-pin 0.6-inch IC socket 8 PC board terminal pins 1 battery snap lead 1 battery holder (4 x AA cells) 1 20kW horizontal trimpot (VR1) Semiconductors 1 HK828 voice record/playback IC (IC1) 1 LM386N power amplifier (IC2) 1 PN200 PNP transistor (Q1) 1 5mm green LED (LED1) 1 5mm red LED (LED2) 1 1N4004 diode (D1) Capacitors 1 1000mF 10V RB electrolytic 1 470mF 10V RB electrolytic 2 220mF 10V RB electrolytic 1 22mF 16V RB electrolytic 1 10mF 16V RB electrolytic 1 4.7mF 35V TAG tantalum 5 100nF MKT polyester (code 104 or 100n) 1 47nF MKT polyester (code 473 or 47n) Resistors (0.25W 1%) 1 220kW 2 1kW 2 47kW 2 680W 6 22kW 1 47W 2 10kW 1 10W Miscellaneous Hook-up wire for PC board links, speaker leads, etc. Where To Buy A Kit This project has been sponsored by Jaycar Electronics and they own the design copyright. A full kit of parts will be available from Jaycar Cat. KC-5412. quite straightforward – just connect these pins to the printer port of a PC (or to the I/O pins of a microcontroller) and then arrange for the software to control the recorder by duplicating the actions of switches S1-S3. Finally, remember to set S2 in the Play position, so that the PC or microcontroller will be able to toggle the SC HK828’s RE-bar line. May 2005  33