Walsh circuit now up

[Paul Maddox]

Tony,

   Ahh, yes I hadnt seen the idea of useing a negative
coefficient.... cool idea... Hmmm, so one DAC, multiplexed
32 times (one spare) to modulated coefficients, get the
micro to control all 31 coef's.. 8 step eg's, polphonic,
ooo funky.... the one thing would stop this from being made
into a poly module in about 3 seconds is the clocked FF's,
how would it sound without them? would the 'raceing' be a major
problem?  I mean the gate delay for decent 74 is tiny. but is it
tiny enough?

  Say, just out of curiosity how many IC's does this PCB have on?

  Paul (just thinking out loud)

*********************************************************
*   Modulus Synthesiser DIY page ;-				*
*				www.xavax.com/modulus	*
*  Email ;-							*
*	  Paul.Maddox at unilever.com				*
*********************************************************

-----Original Message-----
From:	Tony Allgood [SMTP:oakley at techrepairs.freeserve.co.uk]
Sent:	Wednesday, September 22, 1999 10:34 PM
To:	synth-diy at mailhost.bpa.nl
Subject:	Re: Walsh circuit now up

>Wouldn't it perhaps be a little less complex, when using an (E)EPROM
with
the precompiled walsh functions?

Yes, you could indeed so this. But, it is not the counter or the x-or
gates that take up the hardware for this project. Walshes are generated
easily enough but it is the VC mixing of them that is tricky. Remember
they are not 0 and 1, but +1 and -1.

To look at Paul's hex-VCA method, we still have a problem of allowing
negative value coefficients. That is, a negative CV must produce inverse
walsh functions. This could be done using the same circuit as mine but
with decent CMOS switches, eg DG271. However, because 74HC4066s are
about a tenth of the price and are faster, I have chosen to use them
instead. To get around the problem of negative coefficients without
using negative values of CVs, my CVs will produce walsh outputs with
negative coefficients, when they are lower than a reference voltage,
Vref. The reference can be varied to allow positive only, or biased
positive coefficient addition. Actually, since the ref is a voltage, it
gives some crazy results when this is swept over time. Example, you can
get a sawtooth changing to a pulse wave. I don't think that the hex-VCA
can attenuate a signal to some arbitary reference... but to be honest, I
don't know how that thing works anyway.

Regards,

Tony Allgood  Penrith, Cumbria, UK

Walsh synthesiser, SuperLadder, TB303 clone and rack mounted filter

http://www.techrepairs.freeserve.co.uk/projects.htm