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

[Jerry Gray-Eskue]

-----Original Message-----
From: Jerry Gray-Eskue [mailto:jerryge at cableone.net]
Sent: Saturday, February 06, 2010 12:06 PM
To: Ian Fritz
Subject: RE: SV: [sdiy] Mopho modulation DAC configuration, update rate,
etc


Thanks for the info, the one snapshot of the waveform made the FM a lot more
clear to me. I was expecting very small levels of FM and only minor
deviations of slope.
I guess I have not been on this board long enough to have seen the
discussions and have not seen any of the write ups.

- Jerry



-----Original Message-----
From: Ian Fritz [mailto:ijfritz at comcast.net]
Sent: Saturday, February 06, 2010 11:47 AM
To: Jerry Gray-Eskue; Synth-diy at dropmix.xs4all.nl
Subject: RE: SV: [sdiy] Mopho modulation DAC configuration, update rate,
etc


At 08:32 AM 2/6/2010, Jerry Gray-Eskue wrote:
>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.

Not sure I'm following what you are saying.  You are saying there is no
such thing as FM in analog VCOs?  My through-zero linear FM VCO gives
spectra that are very close to theoretical predictions.  Of course you have
to realize that technically the process is *phase* modulation, but that's a
minor point.


>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 ...

Why do you say minor?  They are major.  They can even reverse the direction
of the ramp in a through-zero unit.


>  but there is only one primary
>result of the analog calculations, the integrator hits the threshold level
>and resets or reverses direction.

Not true.  The shape of the waveform is changed drastically.


>There will be some variation in the harmonics due to irregular charge
rates,
>but I suspect these frequency components are not audible.

Why do you think that?


>The audible result
>will be the variation of the frequency of reaching the threshold and the
>harmonic content of the non FM wave form.

Again, not true at all, unless I totally misunderstand what you are
saying.  The FM'd waveform does not consist of linear segments.


>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.

Sorry, but you have gotten to the point of being incomprehensible. Audio
rate FM has been discussed extensively, both for exponential and linear
modulation, and it is used every day in analog systhesis.  There are good
writeups on the theory in Electronotes, for example.

There's a pretty picture of through-zero FM of a Saw core on my e-m thread,
along with a number of sound clips.
http://www.electro-music.com/forum/topic-29149.html

Again, sorry if I misunderstand what you are saying.

Ian