tepples wrote:
I'm curious as to what actually needs more than
two operators, in order to map the exact difference in capability between an OPL (such as the NSF scene's use of VRC7) and an OPN (such as the YM2612 in the Genesis).
The main way I think 4-op provides a massive upgrade:
A 2-op pair is very good at letting you select one aribtrary timbre / waveshape, but the modulator envelope only really lets you vary the strength of that timbre from 0 / dull / sine to full strength.
A 4-op pair lets you have two of these. Thus letting you have two parts of the sound with two different timbres each on their own envelope, rather than just a single timbre for the whole sound.
Where this matters a lot is trying to model the "attack" part of a sound differently from its sustain/decay portions. In real instrument sounds, or just in practical synthesized sounds, the onset of a note generally has very different timbral needs than the sustain. With 2-op you're trying to do double duty, either the attack or sustain timbre needs to (severely) compromise for the sake of the other.
In this respect I think the parallel 2-op algorithm (FM-AM) is the most immediately and easily useful configuration of 4-op. The other configurations are useful too once you know your way around this stuff, but this algorithm is definitely my "default" choice.
So... once you get to 6-op there's less advantage to be had. Two stage sounds correspond very well to modelling natural or practical instrument sounds. Finding a use for that third stage becomes more subtle. One way that's somewhat intuitive is the "multitimbral" sound of tying an extra voice to velocity or one of the expression controls (mod wheel, breath, etc.) which lets you get a component that adjust across a range of use. However, that particular kind of thing is probably most useful for live performance instruments; when the entire playback is computer controlled, you can kinda substitute for this by just controlling parameters directly.
A 6-op instrument is diminishing returns in this respect. I think the DX7 etc. had some very good uses for live performance in this way, but for video games it wasn't that helpful. More voices was probably a better idea than more complex voices... which is more or less what happened...
tepples wrote:
Understanding what to do with the extra two operators, other than a pair of 2-op voices tuned a fifth (3:2) or fourth (4:3) apart to improve perceived polyphony, might help composers using OPN for the first time.
Well, with a lot of notes at once, the need to have precise timbres isn't so great, so in that respect it is sometimes better to have twice as many 2-op voices. Trying to do it with a 4-op in FM-AM mode is really limited though. I don't actually recommend that; trying to write good contrapuntal melody under restrictions like that is a weird hell. Sometimes it's good to have a pad instrument that just plays a 5th (the General Midi sound set had one or two of these included), but it's very situational.
On OPL3 you had the option of just using 18 independent 2-op channels, though even its 4-op mode went with a 6 x 4-op + 6 x 2-op configuration, i.e. 6 "complex sound" channels, and 6 "simple sound" channels, which is a reasonable trade, though was also frustrating you couldn't just get 9 x 4-op for a uniform synthesizer. (I'm currently doing research on ESFM which kinda scratches that itch.)
If you're comparing to OPN2, yes this is stuck with 6 x 4-op, though even there one of its channels could be split as 2 x 2-op. Then it also has a DAC, and the Genesis had a separate PSG to back that up with a few extra "simple sound" channels... so there's a lot of extra stuff on top of the 4-op voices here.
The more limited OPN had 3 x 4-op + PSG, as well. There's a lot of variations on this combination of 4-op with other simpler generators.