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Manufacturer's Data On The ISD2590P Single-Chip Voice Recorder IC This second-generation series of solid-state audio ICs from Information Storage Devices features extended recording/playback times as well as a new pushbutton operation mode & lower distortion. By DARREN YATES Following close on the heels of the original ISD1000 series, Information Storage Devices has released the new second-generation of solid state audio devices - the ISD2500-series. The most notable feature of the new range is that the EPROM array has jumped in size from 128,000 to 480,000 bits, which has allowed the much greater recording times. The 2500-series comes in four versions, the ISD2545, ISD2560, ISD2575 and ISD2590 which have 45, 60, 75, and 90 seconds duration respectively. The frequency bandwidth for the devices range from 4.5kHz for the 2545 down to 2.3kHz for the 2590. Fig.1 shows the basic block diagram of the internals of the IC. As with the ISD1000-series, the new 2500-series uses a patented method of storing INTERNAL CLOCK ANA IN ANA OUT MIC MIC REF AGC AMP SAMPLING CLOCK TIMING ANALOG TRANSCEIVERS ANTIALIASING FILTER DECODERS XCLK PREAMP analog signals in EPROM cells. The technique is similar to programming an ordinary EPROM except that in this case, the cell isn't blasted with a high or low voltage level but in small increments. The output of the cell is compared with the input signal and while the cell output is below the sampled input, the device continues to incrementally charge up the cell. When the two are equal, programming of that cell ceases. The size of the incremental charges is such that there are 256 possible levels which is equivalent to a conventional 8-bit system, except that that this method requires only 1/8th the amount of storage elements for the same recording time. Looking at Fig.1, input signal is applied either to the MIC preamp or SMOOTHING FILTER SP+ MUX AMP 480K CELL NONVOLATILE ANALOG STORAGE ARRAY SP- AGC POWER CONDITIONING VCCA +5V VCCD +5V ADDRESS BUFFERS A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 DEVICE CONTROL OVF PD P/R CE EOM AUX IN Fig.1: block diagram of the ISD2500 series analog voice recorder IC. The device stores the audio signal in an internal 480K EPROM that retains memory even when the power is switched off. 10  Silicon Chip directly to the main preamp via the ANA IN pin. From here, the signal undergoes automatic gain control (AGC) to produce the optimum recording level. After this, a 5-pole anti-aliasing filter removes the upper frequency signals. Transferring the signal directly to the cells is virtually the same as for the 1000-series with two rows of analog transceivers (or sample & hold circuits) which perform `parallel programming' of a given row of cells. At present, details are sketchy on the number of cells in each row but the system works with one row is receiving samples in real time while the other is programming multiple cells simaltaneously. When replaying, the stored signal passes through another 5-pole filter to remove components of the internal clock frequency, then fed through a multiplexer and out through the bridge amplifier. The internal clock does not require any external components, but it is also possible to use an external clock drive which is fed into the XCLK pin. Faster clocking The benefit of this is that by feeding the device with a higher than usual clock frequency, you can obtain some improvement in sound quality with corresponding sacrifice in recording time. However, you can't extend the recording time by decreasing the clock frequency. The reason is that if the clock frequency drops below the required level, the sampling is such that the filters can no longer remove the clock frequency component from the audio making it garbled and almost impossble to understand. Fig.2 shows a table of the various devices and their sampling rates, bandwidths and required external clock inputs. The EPROM array is divided up Table 1 Part No. Duration (secs) Input Rate Bandwidth Required XCLCK ISD2545 45 10.6kHz 4.5kHz 1365.3kHz ISD2560 60 8.0kHz 3.4kHz 1024kHz ISD2575 75 6.4kHz 2.7kHz 819.2kHz ISD2590 90 5.33kHz 2.3kHz 682.7kHz into 600 equal spaced sections, each of which can be accessed via the 10 address lines, A0 through to A9 (0 to 257 hex). For the ISD2560 60-second version, this gives a resolution of 0.1 seconds for each division, similar to the first series. The other addition is the new OVFbar (overflow) output. When in either record or playback mode, this line pulls low when the device has reach the end of its memory, or is as full as a boot. The benefit is that it is easy to cascade devices together and using this pin to control the next device in the chain. With regards to cascading devices, its possible to extend the recording time without limit. Using the EOM (end-of-message) and OVF lines, the first device is connected as the master and a number of other devices connected as `slaves' or memory modules. However, the cost of such a system is likely to be prohibitive. Pushbutton mode One of the more interesting features is the addition of a push-button mode. This allows the device to triggered by the rising or falling edge of signal rather than having to tie the corresponding input high or low. The makes design of peripheral circuitry much easier. The mode is entered into by pulling the two most significant address lines high as well as the M6 mode pin. The chip enable (CE) pin now becomes a toggle START/PAUSE control while the power down (PD) line is now a STOP/RESET control. The pause feature is a very useful one as it allows you to stop recording or playback of a message, and then to continue on from that spot, just as you would with a normal tape deck. When recording, pressing the PAUSE key inserts an EOM (endof-message) marker at the present memory location. When replaying, each time, the IC comes across the EOM marker, it pauses at that memory location. Pressing the START/PAUSE key will cause the IC to begin playing the next message starting at the next memory location. This is ideal for example if you have five commands which explain how a piece of machinery should be used. At the end of each command, the user has to press the START/PAUSE key to hear the next command. The pause prevents the user from hearing all five commands at once and possibly making errors. Message looping There are many applications where you would record a message into the device and then have it continuously loop, playing the message continuously. Examples of this would be answering machines, in-store advertising, etc. By pulling the M3/A3 address line high, the device enters the Message Looping mode. It is activated when the Chip Enable (CE) line is pulled low. This continous looping continues until the CE line is pulled low again at which time the current mode and address lines are looked at and the corresponding mode executed. The ISD2500-series are still fabricated in the same 28-pin DIL package but are also available in SOIC, TSOP and bare die formats. In addition, theses devices are also avilable in a low-voltage range (3.6-4.0V). The total harmonic distortion for all devices is quoted as 1% <at> 1kHz and the output power amplifier can supply 50mW into 16W. If using an 8W speaker, a 10W 0.25W resistor should be placed in series. The output stage is a bridge amplifier with both anti-phase signals appearing at pins 14 and 15. To connect the device to an external amplifier, a series capacitor and 10kW resistor or pot should be connected to one of the outputs while the other is left floating. Connecting either output to ground will more than likely destroy the output stage. See the project based on the ISD2590P on page 16 for an SC example of this. VCR ALIGNMENT TOOL KIT • 7 Assorted head & guide aligners • Hex key set • Retaining ring remover • 3 Reversible screwdrivers – SML – Flat – Philips • Spring hook • Fitted vinyl • Micro screwdriver • Zippered • VCR head puller Our Low Price $99.95 WOMBAT COMPONENTS WOMBAT COMMUNICATIONS 83 - 85 Railway Ave Werribee, Vic 3030 Phone: (03) 742 7330 Fax: (03) 741 6834 AUDIOPHILES! Now high audiophile quality components & kits are available in Australia. Buy direct & save. *Kimber, Wonder, Solen & MIT Capacitors *Alps Pots *Holco resistors *High Volt. Cap *Gold Terminals & RCA *WBT Connectors *Kimber Cables *Interconnect Cables *Output Transformers (standard or customised) *Power Transformers *Semiconductors *Audio Valves & Sockets *Wonder Solder *Wetborne Labs Accessories Valve & Solid State Pre-Power Amplifier Kits *Contain Stereo 80 Valve Power Amp (As per Elect. Aust. Sept. & Oct. ‘92) *Welborne Labs Hybrid Preamp. & Solid State Power Amplifier Send $1.00 for Product Catalog. PHONE & FAX: (03) 807 1263 CONTAN AUDIO 37 WADHAM PARADE MT. WAVERLEY, VICTORIA 3149. February 1994  11