HOW do you add 65 to 64 and come up with 110? The answer's all tied up with them little micro-chip thingies, as you might have guessed. It works like this: Commodore 64 + £65 - DDM110.

Or, to put it another way, start with a Commodore 64 and disc drive and invest £65 or so on a Syntron module and you'll end up with something very similar to Korg's budget DDM110 digital drum machine.

Syntron's sampled-sound module is the essence of simplicity. It's a little black box with an edge connector which slots into the 64's User Port on one end and a single jack and phono socket on the other end. Inside is a small circuit board with just a dozen discrete components and two chips, each meticulously scrubbed to hide their identities. Must be moonlighting from a Cruise Missile or something.

The module is in fact just a little Digital-Analogue convertor, and its operation depends entirely on software. This comes in the form of a single floppy disc (which took a good minute to load, a long time even by the Commodore 1541 disc-drive's snail-like standards). Must be lots of good things on this one, I thought.

What comes up first is a menu with five options: Program Rhythm (not rythm!) Line; Rhythm Track Composition; Set Tempo; Load Rhythms and Rhythm Track; or Save Rhythms and Rhythm Track. Let's look at these in turn.

Program Rhythm Line (P) calls up an exotic-looking display of a single pattern up to 38 beats long. The Rhythm Number is labelled on the top left, the Tempo on the top right, and the seven available instruments down the right hand edge. These are C (Crash Cymbal), O (Open Hi Hat), H (Closed Hi Hat), D (Drum — small tom), S (Snare) and G (Grand Tom). Leaving aside the slightly unusual notation, it's easy enough to program a pattern. Just use the computer's cursor keys to manoeuvre a pointer to the relevant beat, tap the keys corresponding to whichever instruments you want on that beat (taking note of all the wacky display symbols used for the instruments) and tap the computer's Function 1 key to mark the end of the bar.

When you've completed a pattern you can play it by hitting F7, but you can't make it loop unless you write it into a Track, nor can you make the individual sounds play "live" unless you've written them into a rhythm.

You can compose up to ten rhythms, and then go on to the Rhythm Track Composition section, Page R. This simply consists of five columns into which you enter the repeats you want (for instance, Pattern 2 x4, Pattern 0 x2, Pattern 7 x1), and as soon as you're happy you can press F7 to play the track once or F5 to loop it. There's a slight glitch between track repeats when looping, though — you may be able to program this out, say with one 31-beat rhythm — but often you won't need to repeat a track. The capacity is around 115 32-beat patterns, which is plenty long enough for most songs.

The "T" function is dead easy — it just allows you to type in a new tempo, from 1 (terribly, awfully slow) to 64 (dreadfully, horribly fast). Think I'll stick to good old 42. "L" allows you to load rhythms from disc along with a complete rhythm track, and there's a very impressive demo track included under the title "DEMO*".

In fact, you can use the Load function to make up a track consisting of more than ten different patterns, but you have to break out of the Compose mode, load the new patterns, and return to Compose to do it. So when you're playing a Track you appear to have to break out of the program and run it again if you just want to stop in the middle, which seems odd.

Access to the disc is reasonably fast, as it is in the S (Save) mode which allows you to store your current ten rhythms and single rhythm track under any title you like. Doesn't seem to be any way to get a disc directory, though — you'll have to label your discs efficiently or use the Commodore BASIC directory commands outside the Syntron program.

The beauty of something like Syntron is that it is software-driven, so lots of changes can be made. If you type an extra 8 while loading the program you'll get a low-pitched snare instead of a high-pitched snare, and the company are making a disc of seven additional sounds (such as "Clap" and "Syndrum") available as an option — although it's not clear yet how to mix them with the existing sounds. On the hardware module itself there's the phono plug, and this puts out a one-pulse-per-beat clock signal which should allow you to synchronise the Syntron with sequencers and other equipment. Not much chance of adding individual audio outputs for the sounds, however.

And as for the sounds, well, they're every bit as good as the cheaper sampled drum machines. The bass drum is very thumpy indeed, the snare is good, the hi-hats sizzle nicely, the crash cymbal is a little curtailed but has plenty of tone, and the toms are horrible (they're too short and they bend too much — roll on the "Syndrum" sounds).

The Syntron's six-page instructional leaflet needed several thorough readings, but that's being anglicised (Mr Syntron himself coming from sunny Holland). The spelling in the software displays is being tidied up, too. There's a slight amount of background noise in evidence while the drums are playing, but nothing the average recordist couldn't minimise, and certainly the Syntron is well up to home studio recording quality. Marketing of the module hasn't been finalised at the time of writing — it may go to retailers or through mail order — but the address included will get you further information.

Existing Commodore 64 owners should get on the phone immediately and thank Mr Syntron for giving them a wonderful new toy, and if you haven't got a computer of any sort yet, then here's one more factor to help make up your mind.

SYNTRON C64 drum module: £65

CONTACT: Vince Hill Associates, (Contact Details).