Computer Music Chronicles: Audio on 1982's ZX Spectrum

You know, some musical devices defy categorisation; they're boxes that do not like to be put in a box. This is never more apparent than in the world of computer music, where the lines between "soundchip" & "synthesizer" are blurry.

Generally speaking, in the very early 80's a sound chip (aka programmable sound generator) might output:

• Square waves (nearly all of them do this)
• Triangle waves (the NES has a legendary one)
• Digitally clocked noise (think Atari explosions!)
• Digital single-cycle waveforms
• PCM samples (by playing short pulses in fast succession)

Due to this variety, a number of computers were released that had an individual sonic stamp. It's funny to think of now, but the ZX Spectrum was released without a sound chip! Later 128K models fitted the AY-3-8912 as standard of course - but the 16 & 48K "rubber key" & "plus" variants had to make do with a small piezoelectric buzzer.

Beep Me Beep Me

The Speccy was most often programmed in BASIC (with optional machine code elements) and included a BEEP command that tooted the simplest of square wave tones (what, no PWM? Nick). However, with coding trickery, you could manipulate and arpeggiate these squares into an illusion of polyphony:

Alongside colour clash and loading from tape, the super crunchy 48K sound contributes greatly to my Speccy nostalgia, but in order to fully assess the musical legacy of the micro, we'll need to expand our horizons.

Say AY

I mentioned before that the 3 channel + noise AY-3-8912 came as standard on 128K Spectrums, but it's use in Sinclair systems actually predates the Spectrum altogether. Gladstone Electronics developed a dystopian-looking tower system for the ZX81, the sound chip being an optional expansion.

AY expanders were made for our 48K friend too, with modern variants still on sale. The informative resource, zxspectrum48.i-demo.pl details history of their development, with many musical examples.

After 1986 there appeared AY interfaces fully compatible with Spectrum 128K standard. They were popular mostly in Central Europe, and one of their base was Czech'o'Slovakian MELODIK, or Polish "Stavi" AY-interface (also sometimes called "Stawi").

Spectrum 48K can play all chiptunes written on Spectrum 128K and some digital songs (Amiga MOD-like music) that are...up to about 38Kb and don't frolic on RAM areas higher than 48Kb. However, ZX48K - in contrast to 128K - is STEREO in most cases because AY interfaces became popular some time after releasing 128K series, so the stereo output was becoming more and more common.

I could do a whole article on AY. 3 square wave channels, a variable digital noise generator and a single audio-rate looping envelope offer more variety than you might think!

Speak to Me

We're just getting started on ZX sound expansions! Speech synthesis was catered for by products like the Currah Microspeech (which also amplified the internal beeper tones through your TV!) and Cheetah Sweet Talker. Sure, by rapidly writing a pattern 0's and 1's to the beeper, a series of clicks could become synthesized speech or even polyphonic music - but such techniques eat up CPU cycles, so are unsuitable for adding speech to fast-paced action games.

The Microspeech & Sweet Talker instead make use of a dedicated chip, the General Instrument SP0256. This chip uses a 12-pole digital filter to implement a Vocal Tract Model, generating speech with a 10 kHz sample rate. Variants of the GI SP0256 powered speech synthesizers for the Atari 8-bit, Intellivision and TRS-80, also.

Little Drummer Boy

The SpecDrum turned a few heads when released in 1985! For a modest £29.99 it promised to turn your Spectrum into a fully fledged drum machine, with features to rival expensive standalone units. You know what? It pretty much delivered on that promise! The default kit had a distinctly Linn-ish feel, but other kits were available (including a sampled-909 "Afro kit") on cassette tape.

The 8-part drum kit could be played back with 3 voice polyphony (it's workable!) and synchronization (crucial to music production) was handled in an ingenious way: Sync pulses are sent & received via the Speccy's EAR & MIC sockets. Having listened to these accidentally, I think they may be 48PPQ - a high degree of accuracy is possible. With a bunch of tinkering, it's even possible to add your own 8-bit 2023Hz(!) samples.

https://www.mjharrison.co.uk/loading-your-own-samples-into-the-cheetah-specdrum/

Sample Culture

The SpecDrum was far from the only sample-playback device to grace our rubber-keyed wonder. Both Cheetah & Datel released sampling add-ons for the system. These allowed you to sample a sound, loop it and then play it back via an on screen keyboard. Real-time effects like Echo, Fuzzbox and Bubbleizer(?) add to the fun. You can hear the Cheetah unit in action here:

Of course, we should mention the RAM Music Machine here. I've written about this device before (https://sonicstate.com/news/2023/11/20/the-poor-mans-50-fairlight) but briefly, it's a flexible box that offers sampling, real-time echo (no bubbles), 3 channels of drum sounds or 2 channels of chromatic samples, plus MIDI in, out and thru.

MIDI Marvels

MIDI is close to our final destination. The RAM Music Machine can either:

1. Become a real-time echo unit

2. Trigger internal drums whilst sending MIDI to a synth*

3. Trigger internal chromatic samples whilst sending MIDI to a drum machine

*I always imagine the synth being a CZ-101 for some reason; a 12 year-old kid poring over the Spectrum, writing the best material of their life (so the cliche goes)!

Actually, the Spectrum was supported by a number of MIDI options. From 1986 onward, the 128k range featured RS232 as standard, making the job of wiring a MIDI out port fairly trivial. Other expansions include the, presumably rare, ZX-MIDI (there's been one on eBay for £399 for a year or so!) and the Cheetah MIDI Interface. Whilst I'd like to try them all at some point, I currently own the Currah Microspeech, SpecDrum & RAM Music Machine so that's a good start at least. :)

Odds and Ends

I'm very glad that the Spectrum was my first computer - it taught me so much about graphics & sound - two creative avenues I've continued to explore ever since. Hardware & Software possibilities for the platform seem endless, making retrospective articles (like this one) quite a feat! Odds and ends-wise there's the Muzix 81 ZX81 CV/Gate interface (I felt that this was outside the scope of this article) plus, some modern products that I've recently become aware of, but will need to do some more research on - perhaps for a future article?

These products include a Commodore SID sound chip expander (haha!), a "MIDI" expander (which doesn't offer MIDI connectivity - it's a software-controllable General MIDI sound module!) and a Covox DAC (for playing back sample-based tracker music).

I should be able to test the Covox, as it's apparently built into my Sizif 512 - a modern Spectrum clone with 512k ram, 14Mhz turbo mode and extra colours via the magic of "ULAplus"! My last paragraph has nothing to do with music, but might entertain 80's/90's kids like myself - using the ULAplus feature, I managed to hack the Speccy's colour palette to make Michelangelo (The Teenage Mutant Hero Turtle) his proper colour! I hope you enjoyed this nostalgic look at Speccy sound, and I'll see you next time!

Posted by MagicalSynthAdventure an expert in synthesis technology from last Century and Amiga enthusiast.

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