linear vs exp FM

[Sean Costello]

Paul Nugteren van wrote:

> I've read somewhere that the p5 could do to pulsewidth mod
> on osc1 by osc2 Is that right?

Yes, and a very nice effect it is. Gives good grit, without the pitch
problems of FM (see below).
 
> The two oscs in say a jp8 track each other
> quite evenly so why has the jp8 still got exp FM? Does it sound very
> much different than linear FM?

Other people have probably explained the difference between linear and
exponential FM, so I'll just put in a brief note about why exponential
FM is sometimes really annoying. Exponential FM allows for lots of tasty
FM sounds (probably more dramatic than linear FM in the average synth);
unfortunately, the pitch of the note goes up as the FM index increases,
which is unlike linear FM where the index has no effect on the
fundamental pitch. 

In a monosynth, this really isn't as big a deal as it might seem, if you
just want a good metallic sound where the FM index doesn't change during
the course of the sound. In a polysynth like the P5, however, this
change of pitch with FM index means that the pitch of each note is VERY
sensitive to the VCA that is controlling the amount of FM index. What
this amounts to is, if you have a single voice on a P5 that is tuned to
the pitch you want while using a large FM index, the other notes will
probably be out of tune with that note, due to miniscule variations in
the VCA's controlling the FM index for each note. As far as I'm
concerned, FM on a P5 is really only useful for special effects, not for
any sort of pitched music. 

Of course, you can get FM-like sounds in other ways, using FM of the
pulse width, for example, or FM of the filter cutoff. However, if you
want that super metallic super nasty FM sound, a P5 or other
exponentially controlled analog probably isn't the best way to get it. 
Yamaha FM synths are currently dirt cheap right now - slap a homemade
MS-20 filter on one of these (or Paul Perry's version of the same), and
you're in business!

Sean Costello