Negative frequency

[Don Tillman]

   From: "Goddard, Duncan" <goddard.duncan at mtvne.com>
   Date: Mon, 25 Sep 2000 19:34:49 +0100

   > > A couple features I noticed: First, it can do thru-zero frequency PM.
   > > By that I mean if you shift the phase backwards quickly you can
   > > briefly take the frequency negative.
   > 
   ok, I've had all I can take of this thread. what exactly is the
   musical significance of "negative frequency"? in fact, what is
   "negative frequency"?  

Negative frequency is when the wavefrom goes just as fast in the
opposite direction.  Normally that's not too exciting -- triangles,
sines and squares are exactly the same going forwards and backwards, a
sawtooth just looks upside down, and any waveform will certainly have
the same harmonic content going backwards as forwards.

The interesting stuff happens with the transistion from positive
frequency to negative frequency.  A "thru-zero VCO" will have a linear
control voltage that will let you take the waveform down to zero Hz
and then backwards.

It's *really* difficult to implement a proper thru-zero VCO in
analog.  It's easy in digital -- the DX-7 oscillators do this, and it
is used as part of FM (PM, whatever) synthesis. 

What good is it?
  It makes great FM (PM, whatever) synthesis.

  A quadrature thru-zero oscillator is important for the Bode 
  frequency shifter.  

  You can PM modulate a low frequency sine wave through zero Hz for a
  nice chorus animation effect.  (The best DX-7 organ patches do this
  to emulate a rotating speaker.)

  And DJ scratching is effectively thru-zero FM.
  "Wheekwho-wheekwho-wheekwho."  :-)

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com