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Build this compact VOICE DIGI This compact digital voice recorder board uses a new 60-pin surface mount LSI chip, a bank of four RAM chips and not much else. It can record up to 512 seconds of speech ir,. up to four separate messages. By LEO SIMPSON We last featured a digital voice recorder board in the December 1989 issue of SILICON CHIP and it proved to be very popular (and still is). The basic circuit of that design was a little more complex than the present design because the main voice chip did not have op amps on board. That design also featured trigger inputs and logic so that it could be part of a security system or car monitoring system. This new design reduces the circuit to its basics and provides just the recording and playback controls. Apart from the 60-pin LSI chip which is the heart of the unit, there is an LM386 audio power amplifier, four RAM chips, a 3-terminal 5V regulator, some diodes and a handful of passive components. All the circuit, including the battery and function selector switch, can be fitted onto a PC board measuring just 120 x 80mm. That is about one quarter the size of our previous design. And since there is a lot less on it, the new board will be even easier to assemble. Let's make one point clear right at the outset. This device is a not a voice synthesizer which is normally controlled by a computer or EPROM. Synthesizers are often corny or virtually unrecognisable. By contrast , the project to be described here is a complete digital audio storage system based on a new LSI (large scale integration) chip made by Samsung Electronics, of Korea. It can be used to record and play back any audio signal but it is mainly intended for voice messages. You can record voice messages in exactly the same way as you would with a telephone answering machine. When played back, the recording will not sound like a synthesizer; it will sound like you. We won't attempt to list all the uses to which this project might be put. In December 1989 when we published the previous voice recorder board, the ultimate number of applications was far beyond our range of suggestions. We expect that this project will be even more popular, because it's a lot smaller. Method of recording The Samsung voice recording chip used here, the KS5915, uses an Adaptive Delta Modulation (ADM) method (or algorithm). This is similar to the Delta Sigma Modulation method used PARTS LIST 1 PC board, 120 x 80mm, complete with surface mount KS5915 voice recorder IC 4 MCM511000 1-megabit dynamic RAM ICs (U2,U3,U4,U5) 1 LM386 power amplifier IC (U6) 1 7805 5V 3-terminal regulator (REG1) 5 1N4001 power diodes (D1-D5) 1 red light emitting diode (LED1) 1 electret microphone 24 STLTCON CHIP 1 640kHz ceramic resonator 1 8-way DIP switch (SW1) 1 4-way DIP switch (SW2) 1 pushbutton on/off switch (RECPL) 3 momentary contact pushbutton switches 1 2.1 mm DC input socket (SK1) 1 3.5mm socket (SK2) 1 2.5mm socket (SK3) 1 9V battery 1 9V battery snap connector 1 57mm loudspeaker an Capacitors 2 100µF 16VW electrolytic 2 1OµF 16VW electrolytic 4 1µF 16VW electrolytic 10 0.:1 µF monolithic 1 .01 µF monolithic 2 27pF ceram ic Resistors (0.25W, 5%) 1 5.6kQ 1 1kO 2 1.2kO 1 3300 2 4. ?kn 8-way resistor networks RECORDER in the PC Voice Recorder article described in the August 1991 issue of SILICON CHIP. Briefly, the method of recording is as follows: a sound signal from the on-board electret microphone is amplified by the on-chip op amp and then processed by the ADM block which is essentially a comparator turned on and off by the sampling rate oscillator. The output of the ADM is a 1-bit data stream. This stream is stored in a block of RAM (random access memory). When the signal is played back, it is clocked out of the RAM and fed to a 10-bit digital to analog converter (DAC). This DAC output is then fed to a low pass filter to remove hash and then fed to an amplifier and loudspeaker. The RAM used for this project is in four 1-megabit (1,048,576) chips, the maximum possible for the KS5915 voice recorder chip. Sound quality vs duration In any digital audio. storage system there is a trade-off between recording quality and recording length. If you FEBRUARY1992 25 t,,:I 'O ~ n :z: n 0 r::: Cl) c:, .,. ..... I j1 ":" ii .,. 1+ C4.,j,j R1 5.6k " Vee 1 C3 7 .,. 0.1t Cl C21 1 +■.- C7 100 '-' C6 27pf:r SC2 R 13 14 15s 11 la jg 11 Q,.c:,-c:,..-t/)NC") a:cciio.>a..a.. BUSY CASE CASE CASE CASE RA W DOU Mm OM1 ACO 1 0 CREF VREF A MP21 A A A A ~O =i= A U1 KSS91S => ~1 1 . I/ SK2 rSPEAKER CS 27pf:r 'AO IAl IA2 IA3 39 138 (37 136 (35 (34 (33 (32 131 rr rI t;~ei§!~~~~~~C~c .,...,_v.i~:e PAUSE ~ Vee ) RECPL Vee j2 j3 J41 -------------------t-----------t-----le-- DIGITAL VOICE RECORDER TBl RPANEL RESET SWITCH (OPTIONAL) FP~~u ELECTRET MIC TB2 VENTS ":" 8~~1t F 0 Vee 17 C:,: -1 I .,. RN1 8-PIN DIN a 1.,. c11• 0.1 l Vee DIN 1 2 3 ":" GN D C161 01! I 01! c11l ":' ... RAS w 01-04 4x1N4001 YM ~ 05 1N4001 01+ cm! 7 3 iDI~ a A3 I .,. 8 BATT TB2W Vee 1r-l - •• ho •• 113 =al16 A4 A7 CAS2 - V I SK1 216 BATTERY ~ I SNAP 01! c20l us 1M Vee j Vee .,. U2 1M I ~r.-x' c1a! 0.1J A3 A1 AO WE RAS DIN A9 a CAS1 C13 + C12 100 0.1 ! 16VWI IN '"~- .,. I GN!.,_J 18 U4 1M Vee Vee ":" GNO 18 U3 1M Vee • C15 0.1! RAS DIN w C18 + 101 +5V A1 AO RAS WE ◄ Fig.1 (left): a Samsung KS5915 voice recorder IC & four 1-megabit RAM chips form the basis of the circuit. DIP switches SWl/7 & SWl/8 set the sampling rate & thus determine the recording time & the speech quality. want longer recording times, you have to sacrifice sound quality. If you want better sound quality, you have to accept shorter recording times. In a typical digital recording system, the sound quality is primarily determined by the sampling rate and, as a consequence, by the anti-aliasing input filter. The normal practice is to set the anti-aliasing filter so that its cut-off frequency is half the sampling rate. For example, if the sampling rate is 10k/s, the input filter should roll off frequencies above 5kHz. In this circuit though, the input filter is fixed while the sampling rate can be one of four values ranging from 8k/s to 32k/s. Therefore, the built-in bandpass input and output filters are set to roll off frequencies above 2.5kHz. (By contrast, our previous digital voice board had external input and output filters and it was possible to select the filter components to increase or reduce the ban<;l.width). Table 1 shows the various combinations of sampling rate and recording time available when the circuit is fitted with 1-megabit RAMs. The sampling rate is selected by setting switches 1 and 2 of the 8-way DIP switch (SWl) on the PC board. For the maximum recording time, both switches would be set to OFF. For the minimum recording time, both switches would be set to ON. Board features As already noted, this voice board is quite compact. This is partly because the circuit has been kept to the bare essentials but also because it uses a 60-pin surface mount LSI chip. This is a great deal smaller than a conventional dual in-line chip with the same number of pins could be, if in fact there was such a thing (which there isn't). In fact, this project is a first for SILICON CHIP in that the PC board will be supplied with the surface mount chip already soldered to the copper Fig.2: here's how to install the parts on the PC board. The board comes with the surface-mount chip already soldered in. Note that the 640kHz ceramic resonator is mounted on the copper pattern side of the board. pattern. All you have to do is to mount the conventional components. So if you were a little concerned about having to solder a large surface mount chip, do not worry. It will already be done for you. The chip is mounted on the copper pattern side of the board. All the rest of the components are mounted on the topside of the board. Other features to note are the small sockets for connection of an external power supply, loudspeaker and external microphone. There is a 4-way bank of pushbuttons, one a push-on/pushoff type which is the recording on/off switch. The other three are momentary contact types for Start, Stop and Pause. Circuit details Now let's have a look at the circuit of Fig. l. On the lefthand side of the circuit is an electret microphone which is fed with DC via the 5.6k0 resistor Rl. The electret signal passes via the socket (SK3) for the external microphone. In practice, you would Table 1: Bit Rate Selection SW1/7 SW1/8 Bit Rate Time Off Off 8K bps 512 sec. Off On 22K bps 186 sec. On Off 16K bps 256 sec. Off On 32K bps 128 sec elect to build the circuit for use with either the electret or with an external microphone only. If the external microphone is to be used, the 5.6kQ resistor should be omitted (otherwise a DC current of about one milliamp will pass through the microphone) . The signal from either of the microphones is fed via a lµF capacitor (C3) to pin 49 of Ul. It is fed through two internal op amps , then into the digital processor. Pins 1 & 60 are for connection of the timebase oscillator which can use either a crystal or a ceramic resonator. In this case, it is a ceramic oscillator running at 640kHz. Pins 18-30 connect to the four RAM chips, all of which are connected in parallel except for the CAS lines, one of which goes to each RAM chip. Pins 25-30 are the A4-A9 address lines while the other seven pins are labelled as follows: WE ................................. write enable DrN .............................. ..... ...... data in CASl .......... column address strobe 1 CASZ .......... column address strobe 2 CAS3 .......... column address strobe 3 CAS4 .......... column address strobe 4 RAS .................. .. . row address strobe Pin 17 lights a LED to show that recording is in progress. When in playback mode, the filtered output signal is fed from pin 56 to the volume control 5k0 trimpot (VRl). From there, the signal goes via a lµF capacitor to op amp U6, an LM386 which drives an 80 loudFEBRU ARY 1992 27 used for recharging the onboard battery if it is a rechargeable type. If not, this RAM selection - normally on SW1/1 resistor should be omitted. Remember that if you RAM selection - normally off SW1/2 wish to maintain a recordSW1/3 6-phrase selection - normally on ing in memory, then power must always be present. This Bit rate selection (see Table 1) SW1/7 can be supplied either by Bit rate selection (see Table 1) SW1/8 the battery or the external supply. On for voice trigger mode SW2/1 There are two DIP SW2/2 On for beep mode switches on the board and these select all the operatOn for variable phrase mode SW2/3 ing functions of the voice (normally off) recorder chip. We have alSW2/4 Off for mute (pseudo ALC) ready mentioned how the sampling rate and recording time can be selected. The speaker via a lO0µF capacitor. other switch functions are shown in Power for the circuit comes from a Table 2. Each DIP switch is associated 9V battery which can be mounted on with a resistor network shown on the the PC board. It feeds a 5V 3-terminal circuit as RN-1 and RN-2. These are regulator via diode D5. The 5V output both 4.7kQ networks (ie, eight 4.7kQ from the regulator is bypassed with a resistors with a common return, pin lOµF capacitor and with seven 0. lµF 1). monolithic capacitors dotted around Assembling the PC board the PC board. As an alternative, the board can be Putting this board together is little powered from an AC or DC plugpack different from putting any other PC of up 12 volts. This is fed via bridge board together except that the main rectifier diodes D1-D4 to the input of chip is already soldered into place. the 3-terminal regulator. There should also be much less chance Note that there is a lkQ resistor in of solder bridges between pads or parallel with diode D5. This can be tracks since the board has a green Table 2: DIP Switches Where to buy the kit A complete kit for this project is available for $135. This kit comes with all parts, including a screen printed and solder masked PC board with the voice chip already soldered in place. An optional case with screen printed front and back panels is also available for an additional $19.99. An approved 12VAC plugpack can be supplied fot a further $17.95. Add $10 for postage and packing where applicable. The kit is available from the following suppliers: • PC Marketplace Pty Ltd, PO Box 1100, Lane Cove, NSW 2066. Phone (02) 418 6711. Fax (02) 418 6713. • Geoff Wood Electronics Pty Ltd, 229 Burns Bay Road, Lane Cove West 2066. Phone (02) 428 4111. Fax (02) 428 5198. • Bitware Pty Ltd, 14/260 Wickham Road, Moorabbin, Vic 3189. Phone (03) 532 2274. Fax (03) 532 2279. • Colourview Electronics, 5 Commerce Street, Salisbury, Qld 4107. Phone (07) 275 3188. Fax (07) 275 3238. • C. L. Burton Associates Pty Ltd, 20a Maple Avenue, Forestville, SA5035. Phone (08) 293 8955. Fax (08) 293 8903. Note 1: data sheets on the KS5915 voice recorder chip and MCM511000 RAMs will be included with the kit, as will a listing of all DIP switch settings. Note 2: the copyright for the PC board associated with this project is owned by Comma Corporation. 28 SILICON CHIP solder mask over the pattern side. The component layout is shown in Fig.2 and as you will see when you assemble the board, it is identical to that screen-printed on top of the board. One additional component is soldered onto the copper pattern side and that is the blue 640kHz ceramic resonator. It goes right next to the main chip and should be glued to the PC board as well because its leads tend to be on the fragile side. Use super glue for this job. Install all the tinned copper wire links first, followed by the resistors, diodes and monolithic and ceramic capacitors. This done, install the electrolytic capacitors, making sure that each is polarised correctly, then install the switches, multi-turn trimpot, the LED and the three sockets. Finally, install the ICs and the electret microphone. Check your work carefully, then set the DIP switches to the settings you want. If you haven't worked those out yet, use the following settings: DIP switch SWl - Sl On, S2-S8 Off; DIP switch SW2 - Sl & S2 On, S3 & S4 Off. Now connect a 9Vbattery and measure the voltage at the output of the regulator (LK13). It should be close to +5V. The same voltage should be present at pin 9 of the RAM chips and at pin 6 of the LM386. Now depress the Ree/Play button and press the Start button momentarily. This starts the recording cycle. The LED should flash while this is occurring. If you need to pause during a recording, press the Pause button once, then again to resume recording. At the end of your recording, press the Stop button. The speaker will then beep. Alternatively, if you run out ofrecording time, the speaker will also beep. To play back a recording, unlatch the Ree/Play button (ie, it should be in the out position) and then press Start briefly. The recording should then play back. You can control the volume with trimpot VR1. The speaker will again beep at the end of the recording: Ifno recording is present, as occurs at first switch-on, pressing the Start button will cause the speaker to beep. The beep can be disabled if you set switch SW2/2 to off. Now you can play around with the other recording modes by changing the switch settings - see Table 2. SC