[sdiy] Multiplexed Expo Converter?

[cheater cheater]

Thanks for your reponses guys, especially Antti: you got closest to my
question. What *are* the places where EC's are used with audio-rate
signals?
Let's consider the most common uses of audio-rate signals in synths:

1 FM
2 PM
3 AM/RM

Am I missing anything important? I can't even come up with very
esoteric things...

1. FM
I thought 'linear' audio rate FM was supposed to be much better. What
can you guys tell me about expo vs linear fm? I have seen some VCOs
with 'linear fm in', I guess that just bypasses the EC?

Or is 'linear' audio rate FM better for the specific reason that
removing the EC makes the system simpler and hence easier to make very
good?

2. PM
Correct me if I'm wrong, but I think PM again doesn't need an EC, does
it? Even if it's DC modulation...

3. AM/RM
I don't think AM needs EC.. at all... correct me if I'm wrong, please.

With regards to EC multiplexing speed:
I was less concerned with the DAC and S&H quality.. and more concerned
with the quality of the EC! What about slew rates? Imagine you've got
a -5 and +5 signal, and those two are multiplexed.. the DAC could have
a perfect representation of this, but how would the EC handle it?
Aren't most ECs feedback-designs? What about ringing and limits
imposed on the slew rate by the topology?

D.

On Fri, Jul 31, 2009 at 2:05 PM, Antti Huovilainen<ajhuovil at cc.hut.fi> wrote:
> On Fri, 31 Jul 2009, Tom Wiltshire wrote:
>
>> What's the fastest pitch modulation? Nyquist demands at least twice that
>> frequency. I'd recommend oversampling by a few times (x4 maybe) just to be
>> sure.
>
> Nyquist criteria assumes good (read: high order) anti-alias filter after the
> hold. You won't have that in any reasonable multiplexed CV system because of
> the huge parts count. 4x oversampling with RC lag filter will of course
> help, but since we're dealing with audio rate modulation, you will still get
> unwanted sidebands due to inadequate bandlimiting.
>
> Antti
>
> "No boom today. Boom tomorrow. There's always a boom tomorrow"
>  -- Lt. Cmdr. Ivanova
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