This is an unfortunate case of one synthesiser trying to do too much — and for rather a large sum of money. The old, discontinued CS80 was virtually the byword in "human element" polyphonic keyboards since it was decked out in effects that let you control modulation, dynamics, filter, etc, from pressure or speed of attack on the keyboard itself.

The CS70M attempts to preserve those features and marry them with the programmable style of Prophets, Rolands and Oberheims. Sadly both sides suffer. The second touch facility is much reduced from the CS80 which was able to treat notes individually — press one out of a chord and only that would receive the second touch modulation. The CS70M is more like a switch. Extra pressure anywhere affects all the notes being held down.

And the memories — though they can be overlaid — are limited to 30 positions compared to 64 on the Roland JP8 and 120 on the latest Prophets.

On the plus side, the CS70M has a built-in polyphonic sequencer split into four banks. It's designed for live performance — you record into one of the banks while you're playing, say, a chord sequence, then let the synth memory take over repeating the arrangement while you do something else.

The memory size is quite limited, not enough for a complete song of any complexity and the sequencer will forget everything once the power is turned off — very annoying and something that should have been possible to overcome with battery backup.

Yamaha are famous for going out on their own as far as gate and cv circuits are concerned. This time they've spurned everybody else's idea of cassette stored libraries of programs, and gone for slim magnetic cards that fit into a slot on the front of the synth at the far right. It does add to the CS70M's self containment, you don't have to cart a cassette player around with you, but it must add to the expense.

Physically the CS70M is about 44in long, weighs 29kg, has a five octave C-to-C keyboard and is capable of playing six notes at once. The majority of controls are on sliders, colour coded yellow if their values can be programmed into the memory. Switches are in the form of square tabs with integral LEDs, and are grey if they can be memorised, or white if they can't.

The memory buttons are arranged in two rows of 15, one under the other. You can have two on at once to overlay different programs (each half can play six notes, one oscillator each) or keep to the same sound to thicken its overall texture. The real advantage here is you can set up, say, two string textures with subtle differences in tone between them to make the final mixture more realistic.

But even here there are weaknesses. There's only one modulation LFO in the programmer, so individual layers can't have slightly different vibrato speeds which would help enormously on strings.

The CS70M has three filter options — low, high pass and band pass, all with their own sharp or muted characteristics. The variety is admirable, but I felt it was there to make up for the filter itself being weak. It was peaky and resonant, but didn't reach down into the grumbling bowels of a sound.

The two levels of the CS70M can be detuned for a natural chorus effect, though there is an ensemble section as well that has two speeds of tremolo for Leslie cabinet specialities.

The waveforms are limited; just sawtooth and square wave with a slider to vary the pulse width from 50 to 90 per cent. The VCA's envelope generator also contains a slider for an additional sine wave that seems to have been tacked on as an afterthought.

The keyboard can be split for four notes on one side, two on the other and vice versa and then memories one to eight operate on the left-hand side, nine to 15 on the right and the synth is still capable of layering settings on either side.

It does do silly things such as immediately reverting to memory one when you switch the CS70M to unison for soloing. You have to hit the program button again to get back to the sound you want.

Editing is possible, but because of the two-tier system you have to punch the memory location, then move the controls before they know an edit is required.

Polyphonic portamento and glissando are included and Yamaha have got it right in providing separate sustain and brilliance sliders to override whatever is in the programs. These are the factors you're most likely to want to change on stage.

The second touch facility can slip in sine, sawtooth or square wave modulation on the oscillators, filter or VCA. The harder you press, the stronger the effect. There's is a sensitivity control, but I found even with that full off, it needed a very delicate touch just to introduce a subtle degree of vibrato. A shade too much and you'd just converted your oboe to a police siren.

In short, I found the CS70M unusually difficult to get on with. The controls were often unpredictable or sluggish in their function, nothing happened for a while then they went crazy. It's a cold and unfeeling synthesiser despite the second touch option, and I think Yamaha would have done better concentrating on the sound circuits rather than spending R and D time investigating the sequencer which has only a restricted use, and the card reader.

One of the toughest tests of a good poly synth is its string sound and the CS70M's was lost without its chorus unit, whereas as other poly's for considerably less ackkers do the job on the warmth of their oscillators alone.

Where the CS70M did shine was in sparkling, digital, spikey sounds full of high, resonating filters and edged with a crystal clarity. Clavinets came out well, pianos did okay, but brass sounded rather chill, and £3,800 should give you far greater versatility.

£3800