[SIZE=+2]PCM (Wavetable) Synthesis[/SIZE]PCM or Wavetable synthesis exploits the principle of Pulse Code Modulation (PCM) in order to make a digital recording (sample) of a real instrument, and then replay the sample in the synthesizer. It could be argued that a PCM synthesizer is not in fact a proper synthesizer at all, since nothing is actually being synthesized. However, a PCM synth is considerably more complex than might at first appear, since the pitchof the note being produced has to be varied according to the note commanded (normally via MIDI). A PCM synthesizer achieves this by altering the 'sampling' frequency used to recreate the time domain waveform in the PCM decoder; to put this simply if a note (from a piano, say) is recorded at a fixed sampling frequency (which must of course be at least twice the highest frequency component of the note), then we can recreate a lower note by decoding at a slower sample frequency, or a higher note by decoding at a higher sample frequency. (The actual frequency relationships are actually quite easy to work out).
The problem with this approach is that most real (acoustic) instruments have a harmonic content that varies with pitch, so varying the sample playback frequency only works well for notes close to the fundamental pitch of the note originally sampled. The only way to get round this problem is to record multiple samples of the real instrument (say, one per octave), so that the replayed note is never too far (in pitch) from the original sampled note. As a result PCM synths need large ROM storage for the sampled waveforms and even a single instrument such as a piano (which is admittedly one of the more difficult instruments to synthesize) may need many megabytes of ROM.
The earliest PCM synths were, in fact, Drum Machines so they did not have the problem of varying the sample playback frequency to achieve different note piches (the LINN Drum, for instance). The now common Sampler is, of course, a PCM synth with the added facility of being able to record, store and edit the sample.