Produce anti-noise

[Greg Ewing (using news.cis.dfn.de)]

Gerrit Holl wrote:
> I think a phase shift is measured in time or as a fraction of the
> period, not in degrees.
 >
> But here, of course, we have no harmonic oscillation, and certainly not
> a period of 2*Pi.

It's perfectly correct to refer to an amplitude inversion as a
"phase shift of 180 degrees", even when you're not dealing with
a pure sinewave.

When electronic engineers talk about phase shifts, they're
speaking in the frequency domain, not the time domain. For
a non-sinewave signal, a 180 degree phase shift means to
decompose it into sinewave components, shift the phase of
each component by half of that component's period, and
then add the components back together.

In the time domain, this corresponds to simply turning
the signal upside down, and of course this is how it is
usually implemented in hardware. Electronics gurus only
describe it as a phase shift because they're used to
thinking in the frequency domain all the time (where
it's easier to visualise a lot of *other* things that
are more complicated in the time domain).

So the bottom line is, using a DSP to implement a 180
degree phase shift is massive overkill. Just swap the
wires to the speaker. :-)

-- 
Greg Ewing, Computer Science Dept,
University of Canterbury,	
Christchurch, New Zealand
http://www.cosc.canterbury.ac.nz/~greg