Mad Man Rambles about Noise Generators

[Don Tillman]

   Date: Mon, 3 Jun 1996 21:31:50 -0700
   From: chordman at ix.netcom.com (Scott Gravenhorst, Synthaholic)

   >Don Tillman writes:
   >When I first proposed a VCO driven feedback shift register to AH a
   >long while ago, the idea was to make the harmonic structure of the VCO
   >waveform far more complex.  So for that, the string of 0's is just
   >more fundamental tone.

   I wasn't on the list then, but this is intriguing.  Of what size 
   register were you thinking?  I can see where one could take advantage 
   of the sort term repeatability of a very short (2,3,4,5,6 bits) 
   register.  One would want to be able to patch the bits to the XOR gates 
   as well as into the data inputs of a DAC to change the wave shape.  Am 
   I on the right track?  Sorry if you're repeating something explained 
   earlier.  This sounds cool.  

The conversation at the time was about getting lots of waveforms out
of a VCO, and I pointed out that if you want the most waveforms for
your entertainment dollar then using a sawtooth VCO and waveshaping
the saw is just fine.  But if you really care about truly interesting
waveforms and behavior then it's best to use a special-purpose VCO
that works differently.  Incorporating an FSR into the VCO was one of
the examples I gave.  (Not a great example, since the only special-
purposeness of the VCO is that it's pitched up higher, but what the
hell...) 

I was assuming a 3, 4 or 5 bit shift register. 

I thought that since FSRs typically divide by (2^n)-1 they might
suggest the stretched harmonics of a string, but later I realized
that's bogus thinking.

Anyway, I've never built one, so I've never heard what this sounds
like.  

   Any good book suggestions for error correction ?

Not offhand; it's a area I'm not up on.  But this sort of error
correction is used a lot in disk controllers, network controllers,
communications systems, and even digital audio.  So if you check out a
university engineering library or technical bookstore, I'm sure you'll
find some good stuff that covers the finite field theory math that
goes along with this.

By the way, my first introduction to this was an article in a very,
very old issue of Radio-Electronics magazine, gee it had to be in 1970
or so, a construction project for a device called a "Muse".  It was a
pseudo-random FSR driving a VCO as a way to do computer-composed
music.  A large sloping front panel had lots of controls for tweaking
the random pattern. 

  -- Don