[sdiy] Companders

[René Schmitz]

Hi Scott!

>Yes, you're right, that's what originally got me thinking about the
>companding.  The fact that the PT2399's minimum delay wasn't going into the
>minimum delay range that a normal flanger uses diverted me down another path
>of experimentation, using a different device (rhymes with DDB) =) for that
>function.   I think the very low noise level of the PT2399 (< -90 dB) is
>allowing me to get away without companding, though companding may be a way
>to get some really clean longer delays out of it.  I am assuming that Scott
>Swartz went with companding to emulate the characteristics of the classic
>BBD in conjunction with the filtering that he employs in the circuit(?).
>Now, Harry really needs to talk to this guy - he *wants* the sound of the
>BBD =).

I don't take Harry serious about his BBD bashing. There is lots of trickery 
you can do with BBDs. People tend to forget that they are sampling systems, 
in which you need an antialiasing pre-filter and a reconstructing lowpass 
post-filter. That gets away of lots of the clock frequency feedthrough. 
Just you can't expect high bandwidth at low sampling frequencies, so
I usually use these only for very short (some mSec) delays only where they are ok. 

The datasheet quotes 74dB SNR for the TDA1022, and that noise can be reduced with 
a compander down to maybe >90dB. So what his point is is still unclear to me. 

When properly designed these things can work fine! 
(That means that one needs to think about Mr. Nyquist!)

>Anyway, this is one route that I'm taking to get that flange sound.  Waiting
>in the wings is experimentation with PT2395 and manipulation of the RAM
>address lines to get the really low delay range.

Another route, would be two longer delays in parallel. Modulate one with respect 
to the other. The benefit would be through zero possibilities, the downside a 
longer delay will be added to the signal, bad for feedback. 

>All of this when I really should be getting that basic synth together.  It's
>all just so much fun.

It seems to be common, when doing a major construction project to do smaller stuff 
on the side, when you really don't want to work on the big project. 

Cheers,
 René



-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159