"I have always wanted to know, "yes..." a bit more, "yes, yes..." about, "YES, YES, YES..." the Roland SVC-350 Vocoder." So David Henham of Aldershot, you are a secret vocodist. Here, then, is a quick How About concerning the £680 SVC-350 of which you spake. But the rest of you can read it as well, if you want.

Vocoders are like electronic gossips – they listen very closely to what you're saying then pass the information on to a synthesiser and get it to repeat the same thing.

In the seventies many keyboard companies were producing them. There were sophisticated examples from Moog and EMS, and simpler all-in-one-mike-and-mono-keyboard versions from Korg.

The principle was the same for all of them. Paraphrasing Roland's SVC-350 manual a vocoder needs two inputs, the carrier and the program. The program is, in the majority of cases, your voice supplied to the machine via a microphone. The carrier is normally a synth either monophonic or polyphonic.

One half of the vocoder analyses the program, breaking it down into its constituent frequencies at any given instant. The second half takes the 'recipe' of frequencies and reconstructs the original but using the carrier sound as its source.

This analysis is operated by a series of filters which determine the harmonic content of the voice as it changes. A matching set of filters then control the synth being fed to the carrier input. If the first set detects several high frequencies, it communicates those measurements to its partners which then allow through a corresponding mix. The synthesiser appears to pronounce the words you've given it.

Vocoders enjoyed a brief splurge of success in the seventies when they were relatively new, then passed from popularity until Herbie Hancock revitalized them with a series of hits, most notably, 'I Thought It Was You'.

Vocoders will only pass on expression – the pitch has to be selected from the keyboard of the carrier synth. Part of Hancock's appeal and success was that he developed a fluid right hand playing style closely reproducing the slides, slurs and vibrato a REAL voice would exhibit.

The birth of the vocoder can be traced back a long way. Roland argue that the initial principles were contained in a band width compression device produced by H Dudlay in 1939 for the telecommunications industry. Harold Bode, a compatriot of Dr Robert Moog's, is purported to be the first man to develop the system in a musical sense.

The SVC-350 is a rack mounted unit with several additions which place it above the early alternatives... natural refinements you'd expect from several years' experience.

There are jack and XLR sockets for the microphone, a jack socket for the synth input and a second input socket for a guitar, with its own distortion section. Guitars on their own don't vocode too well as there are few harmonics for the filter to latch on to. An overdriven sound has many more, so accepts the signal more convincingly.

You can determine the output balance between the direct mike signal and the vocoded signal. (Bearing in mind that the pitch is determined by the carrier instrument NOT your voice, you could easily sing harmonies.)

The centre of the panel is taken up by 11 centre notched white sliders which correspond to the filter bands. This way you have a degree of control over how the vocoder reacts – taking out sibilants, emphasising certain formants (the oohs and aahs of speech that give it character) and shaping the final sound to your desires.

Note the word 'final' there, because Roland have been sneaky enough to include a chorus unit as the last link in the chain. It augments the bubbling, choral effect which most people associate with vocoders. You can turn it off with a footswitch. Incidentally a second footswitch lets you freeze the harmonic analysis at one point. The pitch can change, but the tone remains.

There are stereo and mixed outputs for the direct and vocoded signal, and a third outlet for the guitar. The remaining controls are a final volume, output level switch, headphone volume and on/off button. All the sockets are at the front as is correct and commendable for a well thought out item of rack mounted gear.

In use, the SVC-350 turns out to be fast and crisp, rapidly responding to your vocal inflections and transferring them accurately to the carrier. Be bold with the mike, don't mumble, or your message will never reach the machine in the first place. Useful red and green indicator LED's help you get the level right.

Just because vocoders are associated with voices, don't let your thinking stop there. A drum machine can provide an excellent program signal – the bass drum, snare and hi-hat each pulling out different frequencies from the carrier synth.

Alternatively, try modulating a recording of your own voice, with your own voice, or perhaps the entire band. You need to be careful with the balance control, but the results can be weird or disturbingly subtle.

Vocoded guitars work far better with the harmonic distortion turned up, especially on single string lead lines. In fact the most important matter to bear in mind is your choice of a carrier signal, particularly involving the right setting on your synth. Roland recommend sawtooth or pulse width waves with a short duty cycle, and suggest setting the filter cut-off high, then giving the sound the fullest possible sustain, no decay. Avoid sounds with long attack times – they're not necessary. You can shape the build up or decay of volume with your own voice depending how loudly or softly you sing. No point in asking the synth to do it as well.