When is it OK to use d*g*t*l chips in Analog Synths?

gstopp at fibermux.com gstopp at fibermux.com
Fri Jan 12 20:27:12 CET 1996

     Okay I admit that this thread does seem to indicate that there is a 
     feeling that "if there's digital chips in there, it's not totally 
     analog!". Like you open up a synth and see a NAND gate, somehow the 
     sense of sonic warmth diminishes a little.
     I don't see anybody actively expousing this view, and I don't propose 
     to, but hey it's fun to talk about. I mean, every once in a while it's 
     fun to consider what we would tell somebody if they asked us what 
     "analog synthesis" really means.
     Don's got a solid handle on the subject with his responses. It really 
     is the utilization, not the chip family, that determines whether or 
     not a function is "analog" or not. I mean, a 7493 is actually like a 
     bunch of discrete transistors, right? Well, not really, but....
     And what Kent said is also right on - audio grunge is a very gnarly 
     effect sometimes and it's easy to do with simple logic chips. The 
     "square wave" ring modulator mentioned is an EXOR function, as in the 
     ARP Odyssey - as "analog" a synth as they come. I still have my old 
     "tube fuzz" made with CMOS inverters as op-amps. (Hey Don - can you 
     use the new CMOS lines as op-amps? Like 74HCT4XXX? Are 4069's still 
     common?) Great fiddle stuff for you builders out there.
     See where my philosophical ponderings started is here at work. I 
     started putting entire digital circuit boards on single chips recently 
     for a project I'm on, and naturally it occured to me to stay a little 
     late once in a while and cook up something that would be useful in a 
     synthesizer (yes I'm using ALTERA parts). So far I've created a 
     digital keyboard interface with hi/low note priority on a single chip. 
     Next maybe will be some sequencer guts, or a big noise 
     generator/divider thingey. So you see my DIY experiments have waned a 
     little, and might get a little strange (I'll have a tough time trying 
     to keep up with Juergen, however!). One interesting note - standard 
     TTL designs *might not* work when ported over to EPLD simulations! I 
     figured out why, but it was not immediately obvious.
     Desperately trying to stay analog,
     - Gene
     gstopp at fibermux.com

______________________________ Reply Separator _________________________________
Subject: Re: When is it OK to use d*g*t*l chips in Analog Synths?
Author:  Kent Williams <kent at inav.net> at ccrelayout
Date:    1/12/96 8:18 AM

On Fri, 12 Jan 1996, Don Tillman wrote:
[A bunch of cool stuff you've already ready, with other cool stuff gene said]
Digital chips have some pretty interesting properties at the harsh 
end of the scale sonically.
Can't you
1. Make a crusty ring modulator imitator by running two signals into
   an XOR gate?
2. Make other interesting effects by raising signal peaks to TTL levels
   and then running them into logic gates? (i.e. AND,NAND,OR, NOR).  As 
   I understand it from my one class, logic gates can operate at well 
   above audio frequencies (i.e. MHZ).
Logic gates are kind of limited by their simple minded insistence on 
seeing their inputs as pulse trains.  And outputting a variable  duty 
pulse wave all day long.  But it could be fun.
Remember the power of ring modulated drum loops!

From: gstopp at fibermux.com
Date: Thu, 11 Jan 96 15:14:19 PDT

>Maybe this belongs in synth-DIY. Well, it is a philosophical question so maybe 
>it does belong here. Anyway I would like to propose that there are times when 
>digital logic can be used to replace an analog function in an analog 
>synthesizer and the musician won't be able to tell the difference. 

Oooh, sounds like fun...

>Suppose we're talking modular synths first. What function can be digitized, 
>without affecting the sound at all? Isn't this treason?

>In the first category, where the musician could not tell the difference, would 
>be the keyboard interface. 

A strictly analog keyboard interface has serious bugaboos and artifacts with 
probably no musical uses, while a digital keyboard interface gives you all sorts
of lovely features, so that's a no-brainer.

>Another example is the attack/decay flip-fllp inside an ADSR envelope 
>generator. Sure you can use discrete transistors, but TTL or CMOS could do the 
>same thing and no one could tell.

I'll claim that a single flip-flop in an otherwise analog circuit is not what we
mean by "digital".  It's operation is not time quantized, and the circuit needs 
a latching function no matter what.

>In the second category, where the musician could tell the difference, but it's 
>a desireable function anyway, would be things like sub-octave generators, 
>digital noise sources, and square-wave ring modulators. 

Sub-octave generators: same as above, not digital.

Digital noise sources: the only one I've heard is the one in my MemoryMoog, and 
it's lousy.  No contest there.  ("Ka-chunk, ka-chunk, ka-chunk, ka-chunk".  Good
thing I'm not a noise afficianado.)

Square-wave Ring Modulators: as before, not digital.  Also, it's just a special 
case of a regular ring modulator.

>It appears to me that digital and analog do mix, although where to draw the 
>line is rather hazy sometimes. 

Hmm, the line appears quite clear to me.  I mean, just because a chip is 
referred to as being from a digital product line doesn't, uh, condemn the whole 
circuit.  F'rinstance, you know the old trick of biasing CD4069 inverters in the
linear range as crunchy distorto amps? Or leaving pin 14 open and using it as a 
hex VCR FET?  Very analog applications of a "digital" chip.

-- Don


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