All pass filters?

[Martin Czech]

> 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.

m.c.