Last December, I decided that it would be nice to have some sound for my ZX81. Of the three sound synthesisers that were on the market, the ZONX was the cheapest, just, and it was a self-contained unit, the others required an amplifier and a speaker although they did have two I/O ports.

My decision made, I sent off and prepared for a Sinclair-style wait. I need not have worried. Although it was less than a month till Christmas the ZONX arrived in less than one week! I quickly tried to fit the ZONX to my ZX81, only to discover that my full size keyboard got in the way. The problem was that there is a small volume control on the front of the ZONX and it was this that prevented the ZONX from fitting. My solution was to fit a new control into the top of the ZONX.

I opened the ZONX to find a speaker and several chips, all soldered very neatly I might add, and the AY-3-8912. The volume control turned out to be a preset pot, which is not really designed for a lot of use, and I would suggest that it should be moved as little as possible to reduce wear. I replaced it with a standard 'volume control' which I mounted through the top of the ZONX.

The manual for the ZONX is small (5.5" by 4") and only twenty pages long, but it is fairly clear and it explains what each of the AY-3-8912's thirteen registers are for and how they can used. The ZONX resides in the I/O area of the Z-80's memory and so a short (9 bytes) machine code routine is needed to access it. The routine is held in a REM statement and can be typed in directly from the keyboard. The manual also contains eight short programs including one which turns the top row of keys (1-0) into an electronic organ.

The 16K RAM pack connects to the ZX81 through the ZONX, the edge connector of the ZX81 being duplicated on the ZONX, which is just as well since a simple explosion takes up twenty-five lines of BASIC.

The main disadvantage of the ZONX, or any sound synthesiser, is the time needed to develop a given sound - in most cases done by trial and error. Since neither Bi-Pak nor any other software company that I know of, markets any software for the ZONX, you will have to work on them yourself. The results can be worth it though. The other problem is with speed, most sounds are built up from BASIC loops and so that must either be run in fast mode or they sound far too slow. To use sound in games, therefore, will require some simple machine code programming.

I have had a lot of fun with my ZONX and I would recommend it to anyone, provided that they are prepared to spend a lot of time with it. If not, then they would be wasting their money. The ZONX units cost £25.95 for the ZX81 and Spectrum. (An extension board is required for the Spectrum at £6.80.)