[sdiy] analogue phase modulation

[Eric Brombaugh]

JH. wrote:
>> When I was doing the Modcan VCDO research I found that 314% mod index
>> was enough (funny - that number looks familiar).
> 
> 314% as in 3.14 periods, or as in 3.14 (= half of one period) ?
> 
> Ok, must be the latter, or there would be no significance in the Pi*100% 
> number.

 >Bing< Give that man a cigar!

> And indeed. I made some simulations with phase modulation of only +/- Pi, 
> and it gives a very rich spectrum already.

That's exactly the way to look at it: we're modulating +/- 1/2 cycle of 
the wave about some nominal center. As the carrier ratio increases this 
gives some nice wrap-around/foldover effects that introduce a lot of 
higher harmonics.

> So ...
> If we _only_ need sin / cos function fitting for [-Pi .. Pi], two 
> multipliers and a well-tracking
> QVCO (or an ordinary VCO plus a dome filter instead of QVCO), then building 
> DX7-style
> operators completely in the analogue domain will be very easy, and I wonder 
> why we've
> had so much trouble about it in the past.
> 
> Too good to be true? At the moment, I don't see any catch ...

 From my work in the digital domain I found that the timbre really falls 
apart if the modulation/carrier ratio isn't precisely controlled. 
Without a controlled ratio you'll get beating effects that, while 
interesting quickly become anharmonic and degrade into something that 
sounds more like simple ringmod. The Cynthia ZO has some capability for 
locking the modulation & carrier frequencies together to help with this.

Eric