[sdiy] What is a quadrature oscillator good for?

[Cornutt, David K]

From: Grant Richter [mailto:grichter at asapnet.net]
> Aside from frequency shifters and qudraphonic panning, is 
> there anything
> else a quadrature oscillator is good for?

There's a modulation technique used in satellite radio
known as quadrature phase shift keying, or QPSK for short.
I'll have to admit I don't fully understand it, but basically
the idea is that you shift the phase of a carrier signal
to one of the four polar quadrants (0/90/180/270 degrees)
to represent one of four states that you want to transmit.
(For instance, each transition might represent two bits
of binary data.)  For a while now I've wondered what that
would actually sound like if it were done at audio frequencies,
and what the synth possibilities might be.  For instance,
you take four outputs of a quadrature oscillator and feed
them into a voltage-controlled 4-to-1 switch.  Then, you
use another oscillator as a control signal, and its voltage
level selects one of the four outputs at a given moment.
I guess the output would sound something like what you might
get from a CZ-101, but I'm not sure.