All pass filters?

[John Speth]

> > This is something i just don't understand. I'm studying
> computer systems
> > engineering 2nd year, and i'm doing a lot of analogue electronics
> > subjects including filter design etc, but what the hell is an all pass
> > filter used for? If it passes everything then its not really
> filtering is it?
> >
> When all poles of the transfer function have a mirrored zero,
> then |H(s)| =1, thus allpass . But the phase of the signal will
> be frequency
> dependend. Allpass structurs are used for phase shifting or time delay
> therefore.

I think there's a difference between time delay and phase shifting.  Phase
shift is "time delay" *within a specific frequency band*.  Time delay as
it's implemented in, say, a BBD delay line, is constant across all
frequencies.  Phase shift will result in different time delays at different
frequencies.  I think it's fatal to one's understanding to discuss time
delay and phase shifting as though they're related.  Phase shifting is
studied in terms of frequency domain analysis while time delay is not.
Consider this: theory says you'll never be able to phase shift a fourier
frequency more than 180 deg in either direction.

John Speth
Object Engineering, Inc
mailto:johns at oei.com