[sdiy] analogue phase modulation

[Eric Brombaugh]

On Jan 10, 2007, at 7:23 PM, Magnus Danielson wrote:

> From: Eric Brombaugh <ebrombaugh at earthlink.net>
>
>> 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.
>>
>
> OK, so if +/- pi radians is all it takes, then inserting my sawtooth 
> phase
> modulator
>
> http://rubidium.dyndns.org/~magnus/synths/schematics/sawphaseshift.pdf
>
> should be enought. Just do a normal sawtooth core, and sine waveshaper 
> and
> away you go.

Looking at the circuit description here:

http://rubidium.dyndns.org/~magnus/synths/schematics/

it's clear that's just an analog modulo adder so that ought to do it - 
it's essentially what I was doing in digits.

Simple, but not nearly as much fun as JH's idea though. :)

Eric