Looking a lot like an Airfix model of a synthesiser, the CSO1 squeezes a two-and-a-half octave keyboard into a 19in long grey plastic case.

Yamaha have designed it with up-front solo playing in mind. The light body has sockets at each end to take strap buttons so you can dangle the CS01 round your neck. And similarly the keys are half their proper size and effortlessly light so the pinkies can skip up and down the scales.

But the electronics themselves are completely serious. Yamaha have made a few short cuts on controls like filter resonance — just a switch for high or low — but in no way is the CS01 a toy.

It's a single oscillator machine with footages from 4 to 32 and waveforms from triangular, through brassy sawtooth, to two sets of square wave. The first is an evenly balanced square sounding rich and rounded, the second is a spikier version more like the tone of a clavinet.

There's also pulse width modulation and a speed control for it that swings between fat and thin square waves for a chorusy sound — useful down on the lower footages which sound weedy on their own.

All the controls are on short sliders across the top of the F-to-C keyboard, and the top right-hand square of the panel is taken up by a built-in amplifier and speaker. The CS01 has an ADSR envelope generator and two sliders that determine how strong an effect it has on the VCA (to decide how the volume of the note changes) and the VCF (to work on the tone via the filter).

The filter is the CS01's weakest point. You can't get any powerful wangy sounds and at times it's no stronger than a tone control. Getting those rumbling, decaying notes is impossible and it's really only good for fixing how much "fizz" is in the sound.

The ADSR is better as it's quite short and punchy, ideal for fast fingered solo stuff. The general quality of the CS01 is clean, clinical and electronic. It favours tight, plinky sounds with fast decays and the square wave is flutey and innocent sounding. The triangular wave is much smoother and romantic, but the poor filter cheats on the brass sounds, even though there's a bright sawtooth waveform included on the controls.

The CS01 is based around digital electronics, and synths using this system are frequently excellent at the pure, crystal tone percussive settings but let you down when it comes to dirty or warm sounds.

The CS01 also offers an LFO with a speed control from very slow to "Sooty" type trills, glissando and fine tuning. The two modulation wheels are in the top left-hand corner of the panel and both are about half an inch wide with ridges across their surface for a sure grip.

The pitch bend wheel is strongly sprung so it will always snap back to zero, and the modulation wheel is switchable between the oscillator and the filter, though not both at once.

Though they work fine when the synth is down flat, the reason they're close to the top of the panel is so when you've got the CS01 on a strap, you can creep your fingers around the back of the synth and up over the edge to get at the wheels that way.

The pitch bend is great, very smooth, really responsive and perfect for soloing. But there are other performance controls of a more peculiar nature. The CS01 has a Breath Controller — a mouthpiece connected to a socket at the side of the synth by a thin wire which will boost the VCA or the VCF depending on how hard you blow.

Now I have heard demonstrators use the Breath Controller and get some superb effects. The synth really seems to take on a dynamic life of its own. Unfortunately, to get the best out of it, you need to have good "embrochure" which is the blowing and tonguing technique that sax players and the like spend years perfecting.

Pick up the CS01's controller for the first time and all you're likely to do is go blue in th face and make the note a fraction louder! It's a smart idea, but unless you already have some brass ability it could make the CS01 harder to master than it need be. It's also a shame that it doesn't appear possible to mix in white noise with the Breath Controller as this would add authenticity to brass and woodwind sounds.

The half-size keys do take some adjusting too, especially on long runs when I found my fingers tripping over themselves, but it's helped by the CS01's very clean triggering system that produces a new note at the lightest of touches, even when one finger is still resting on another, lower key.

It's incredibly light, weighing not much more than a telephone directory and so is easy to carry around. Four rubber feet stop it sliding too far and because of the overall size — 19in long, 6½in wide and only an inch deep — it can be slipped into a corner of the stage or squeezed between a couple of other keyboards in your set-up.

The CS01 runs off six penlight batteries and has a mini jack socket for a 9-12 volt mains adapter. There are sockets for 'phones and line out — it sounds much stronger through an external amp but still with that thin, slightly cold tone to it.

£189