[sdiy] What is a quadrature oscillator good for?

[David Cornutt]

On Tuesday, September 9, 2003, at 12:22  PM, John L Marshall wrote:

> I have assumed that common 56kbps modems use some form of Quadrature 
> (PM)
> and Amplitude Modulation (AM). Squeezing 56kbps down a less than 4kHz 
> audio
> channel requires mirrors and magic. The switching of phase and 
> amplitude
> take place at the data rate. Most of us have heard modem signals.
>
>
Well, there's a lot more than that taking place.  As you say, a 56K 
modem
is pretty much throwing the entire telephony textbook at it in order to 
squeeze
every last Hz out of the available bandwidth.  It winds up being in 
effect
a spread-spectrum technique, and the goal of any spread-spectrum
modulation method is to, in audio terms, produce "coherent" noise.
That doesn't imply (at least it doesn't to me) that QPSK itself has no
possible musical uses.

(As an aside: There was a parlor trick that I used to do with a 
Racal-Vadic
1200 baud modem that I had at work, circa 1985.  After a bit of 
experimenting,
I found that I could call the modem and whistle the carrier frequency 
at it
well enough to make it think I was another modem.  Once it locked up,
the idea was to make noise at it approximating the spectrum of its
normal noise-like modulated signal.  Making a hissing noise from the
back of one's throat, like imitating a hissing cat, worked well.  I 
could make
the modem spit out garbage characters for several seconds, until I ran
out of breath, at which point the modem would hang up.  Never could
get that second breath in time.  Would that I had played a horn in high
school and learned circular breathing...)