[sdiy] Multiplexed Expo Converter?

[Tom Wiltshire]

On 31 Jul 2009, at 12:00, cheater cheater wrote:

> Hi guys,
> This might be relevant to the DIY Polysynths thread going on at the  
> same time.
>
> I heard that Korg used to multiplex the Expo Converters in their
> polysynths. I think it's a pretty good idea to keep things in sync, so
> that even if the EC floats, every part floats in the same way at the
> same time. This is also good because it simplifies the design.
>
> There are several questions to this, though:
> 1. what could the performance be, at best, in samples per sec?

That depends how fast you can run the demultiplexer, and how fast you  
can shove samples out to the DAC. 100s of KHz is probably achievable  
these days. Whether it is worth it is a different question. By which  
I mean that you might struggle to make it work for a relatively small  
benefit.
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. Multiply by the number of channels, and that's your  
required input DAC rate. Now check your favourite supplier of DACs  
and see what they've got for a reasonable price that'll go that fast  
at the resolution you want. If you can't find anything, ease the  
limitations a bit in the places you care about least, and try again  
until you've got something realistic.

> 2. when are EC's used with input signals that are audio-rate?

Probably not often. FM is the obvious audio-rate application, and the  
received wisdom is that linear FM is better than exponential FM. So  
you *could* have FM applied to your demultiplexed CVs after the EC.
(Have I got that the right way around? I'm feeling a bit uncertain...)

> can this stuff be re-designed so that it doesn't require an EC at all?

I doubt it. Most "musical" applications require pitch to be  
exponential, based on octaves. Given that no-one has an oscillator  
that uses exponential pitch directly, a convertor of some kind is  
always required.
I've even come up against this problem when designing digital  
oscillators in software. I have a number that represents exponential  
pitch, and have to perform a pitch-to-frequency-increment calculation  
for the NCO oscillator core. This is essential an EC in software.

HTH,
T.