[sdiy] What is a quadrature oscillator good for?

[John L Marshall]

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.

Take care,
John
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----- Original Message ----- 
From: "Andre Majorel" <amajorel at teaser.fr>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, September 09, 2003 7:30 AM
Subject: Re: [sdiy] What is a quadrature oscillator good for?


> On 2003-09-08 10:13 -0500, Cornutt, David K wrote:
>
> > 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.
>
> Would that be FM or AM ? Is the switch instantaneous, and how
> often does it occur ?
>
> -- 
> André Majorel <amajorel at teaser.fr>
> http://www.teaser.fr/~amajorel/