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Action Replay

Sound Sampler for Sinclair Spectrum

If you've got access to a Spectrum, sound sampling can be yours for under £200. Mike Drane reports on a revolutionary British development.

It's not an Emulator for £200, but if you've already got a Spectrum, Ricol Electronics' new hardware and software should get you something close.

As an add-on for the 48K Sinclair Spectrum, the Action Replay consists of the hardware necessary to digitise an input waveform, perform some digital signal processing under the control of the associated software, and finally convert the digital signal back into analogue so that you and I can hear the result.

Any such device must consist of an input filter to prevent aliasing, an analogue-to-digital converter (ADC), some memory store (the size of which determines the amount of sound you can store for a given bandwidth), and a digital-to-analogue converter (DAC) with a low-pass filter to smooth the quantisation of the input signal. The Action Replay uses 32K of Spectrum memory for storage, and a maximum sampling rate of 32kHz gives a maximum storage time (or delay time) of one second.

The amount of noise present in a digital system depends on the number of bits being used to quantise the signal, and given that the Spectrum is essentially an eight-bit system, the dynamic range ought to be something in the region of 48dB. However, Ricol Electronics data sheet quotes 72dB, so all is not what it seems to be: clearly, some form of companding must be going on in order to achieve that extra 24dB of (rather useful) dynamic range.

Meanwhile, input and output filters are both 36dB/Octave, while the cutoff frequency is under software control to match whatever sampling rate is selected. There's also a provision for turning off the input filter, which I suppose might come in handy if you're interested in hearing what aliasing sounds like...

The Action Replay comes in a smartly unobtrusive ABS plastic box (190 x 110 x 60mm) with all control designations neatly printed on the lid. The connecting cable supplied is only 8cm long, which does limit the user's positioning options somewhat, but on the credit side, the edge connector itself is of high quality and fits snugly onto the Spectrum, so it's unlikely to work its way loose. Internal construction is compact and professional, all components being mounted on two large PCBs, but input and output connections are 3.5mm mini-jacks, so you'll need some converters if you're only equipped with more common phonos or quarter-inch jacks.

In Use

On loading up the software supplied, the user is presented with a menu of options and a graphic representation of a music keyboard which relates notes to keys on the Spectrum's QWERTY keyboard. You start off in Mode 7, which just samples the input waveform and outputs it again without processing to enable you to set the correct levels. In practice - and especially if you've never done any sampling before - this process is initially a bit hit-and-miss, though you'll get there in the end, and when you do, the results are well worthwhile.

The software's available options are Record, Play, Reverse, Echo, Harmonisation and Set-Up. The last-mentioned mode allows you to select the sample rate and the amount of memory to be used, this being selected in pages from 1 to 121. Taking Echo first, sound quality was surprisingly high (though whatever you do, don't expect it to match that of studio quality machines) on both synthetic and vocal signals, and using maximum bandwidth (ie. fastest sampling time) and varying the number of pages of memory being used makes a whole range of related effects possible, from a decent ADT to vast, cavernous echo. One minus point is that the unit's Feedback control is far too sensitive, which wouldn't in itself be so much of a problem if it wasn't for the fact that if you go just a little bit too far, you end up with an unpleasant attack of gross digital howling.

Of the remaining options, Play and Reverse are probably the most useful. Again, you're required to specify sample rate and number of memory pages, but this time, the sample is stored in memory permanently until you ask for another one. The sound in memory can be replayed via the Spectrum's QWERTY keyboard over a four-octave range, though only 17 semitones (C-to-E) are available at any one time. The Play option outputs the signal right way round, while Reverse (surprise, surprise) replays it backwards.

The Harmonisation feature is less usable in practice, since it suffers from excess noise and sounds a shade flat - to these ears, at least. The supplied instructions suggest winding up the Feedback control at this point, but although this undoubtedly increases the harmonic content of the output waveform, the signal rapidly breaks up into 'Cosmic Space Battle' noises: not really musically viable outside a Tomita concert.

Fourier Analysis

The only other software module available at present is a waveform plot and Fourier analysis package, and this program operates in two parts. The first section behaves very much in the manner of a digital storage oscilloscope, and requires you to specify sample rate (variable from 100Hz to 70kHz), display resolution and trigger level. The user manual suggests that the sample rate be 20 times the input frequency, but I found 40 times gave a rather better display. Once you've produced a good clean display of your waveform, you can then proceed to the second section of the program, which computes and displays the waveform's Fourier transform. For those not in the know, a Fourier transform is a means of converting a periodic time-domain signal into its frequency spectrum. This is of tremendous use to engineers and musicians alike, and I was fascinated to see a time waveform being broken down into its harmonics. Using nothing in the way of special hardware (actually, the VCO output from a Korg MS10 monosynth, with no filtering whatsoever) and taking a bit of care over ensuring reasonable sample quality, it proved astonishingly simple to produce spectra almost identical to classic text-book examples of triangle sawtooth, and square waves.


You'll doubtless have gathered by now that the Action Replay system as it stands at the moment is far from being fully comprehensive, but several future additions are at an advanced stage of development. These include a keyboard interface that plugs into the back of the unit and enables samples to be played from a conventional one-volt-per-octave keyboard instead of the Spectrum's typewriter one, and at least two further software packages. These are a Fourier synthesiser (basically, the same as the Fourier analysis routine except that it works the other way round, ie. it enables you to create a waveform by selecting the amplitudes of up to 16 harmonics), and a further wave-shaper that'll allow waveforms to be drawn onscreen using a light-pen: provision will be made for two user-specified waveforms to be programmed to fade into one another at a predetermined rate, thereby providing a form of dynamic timbral control.

When you add the Action Replay's RRP to that of a Spectrum, the total cost is similar to that of budget dedicated digital delay lines, and if that was the only function the unit could perform, its value for money would be questionable. However, the additional software already available and the potential open-endedness of the design put the system in an entirely different league from dedicated units. And if, as I do, you believe in the Spectrum as a micro of the future as well as the present, the possibilities of connecting the Action Replay to other systems based on the same computer (eg. E&MM's own OMDAC and MIDI interface boards) are clearly immense.

The Action Replay hardware unit and sound storage and replay software retail at £178 all in, inclusive of VAT and p&p. Further software modules cost £10.95 each. Further information - and an audio demonstration tape - from the manufacturers, Ricol Electronics, (Contact Details).

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Casio CT6000

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Yamaha D1500

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Nov 1984

Review by Mike Drane

Previous article in this issue:

> Casio CT6000

Next article in this issue:

> Yamaha D1500

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