LA Synthesis is half analogue synthesis (subtractive synthesis using sawtooth and square waves) and half PCM synthesis. Basically, the MT-32 has the capacity to play 32 sound elements (Roland called them partials, though this is confusing terminology as partial means something else in DSP as well). Each element could be a PCM sample or an analogue waveform. Up to four elements could make up an instrument (though you were free to use from one to four elements) Each element had its own volume and pitch envelope, and for the analogue parts, a filter envelope. In addtion to the envelopes, each element also had a low frequency pitch osscilator. By adjusting the element parameters and the mixing methods, one could patch together instruments that sounded better than either the pure PCM synthesis or analogue synthesis of the day.
Comparing LA synthesis to modern wavetable synthesis we find several notable differences. First, the MT-32 can't rely completely on its PCM samples as they are incomplete snapshots of instruments. Wavetable synthesis normally has multiple frequency samples of an instrument's attack, loop and decay.
Comparing the MT-32 to FM synthesis, we find FM synthesis rather primitive compared to LA synth. FM synthesis is considered additive synthesis where waveforms (operators) are added together to make a complex, rich sound. In the MT-32's case, the opposite occurs. The MT-32 takes a signal rich sound (square or sawtooth) and applies a lowpass filter to extract the frequencies it needs.
I hope this clarifies it for you. If you're curious about another aspect of LA synthesis just ask and I'll elaborate on what I've learned from this project.
