SV: [sdiy] Mopho modulation DAC configuration, update rate, etc

[Jerry Gray-Eskue]

Not to step on any toes here, but I wonder if pure analog VCO's can really
be said be to "accurately" handle audio rate FM.
I may be making some invalid assumptions, but here we go:

Here is my point, the base frequency is determined by a charge rate into an
integrator, this may have some minor fluctuations in the slope when the
added variable charge rate of the FM is added but there is only one primary
result of the analog calculations, the integrator hits the threshold level
and resets or reverses direction.
There will be some variation in the harmonics due to irregular charge rates,
but I suspect these frequency components are not audible. The audible result
will be the variation of the frequency of reaching the threshold and the
harmonic content of the non FM wave form. It seems to me that with audio
frequency FM a lot of the higher frequency will be "filtered out" by the
integration. At most you only get one result for half cycle of the VCO, the
variation in frequency due to the RMS value of the FM signal during each
period.

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Tom Wiltshire
Sent: Saturday, February 06, 2010 6:24 AM
To: karl dalen
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: SV: [sdiy] Mopho modulation DAC configuration, update rate,
etc



On 6 Feb 2010, at 08:38, karl dalen wrote:
>
> Offcourse but there are basically no *modulation* frequency
> limit within the PA397, it can go into audio frequency range
> if one designs it in.

If the Mopho uses the same DCOs as the Prophet08, then I disagree,
Karl. I might be wrong (happened lots before) but I don't see how you
can do audio rate FM on a DCO.
Some people have argued that you can update the charging rate in the
middle of a cycle without causing a glitch, by doing some
recalculation of when the reset point would be, but I'm rather
sceptical that it can be done accurately. And FM is the absolute
worst-case, where you're changing the charging rate many thousands of
times during a single cycle (48KHz sample rate would be a minimum for
half decent FM). If you don't have some errors built up by the end of
that, I'd be amazed.
Delighted, but also amazed.

Aside from the DCO, he's got an analog filter and VCA in there, so he
can do audio FM on those, no problem.

T.


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