[sdiy] group delay (of filters) and listening

[Harry Bissell Jr]

Group delay can be important (and real...) in time
sensitive
applications.

I wanted to convert a guitar pitch 'faster'. What
better way than to shift the frequency UP and then
convert.  Problem is... the dome filter needed to
maintain the 90 degree shift down to 80Hz (low E)
would
introduce a group delay of 18ms or so... longer than
the single cycle time of the note itself (12ms).  So
you cannot get something for nothing - go figure.

H^) harry


--- Don Tillman <don at till.com> wrote:

>    > Date: Tue, 5 Oct 2004 13:40:05 +0200
>    > From: "Czech Martin"
> <Martin.Czech at Micronas.com>
>    > 
>    > After all, it is not important how the group
> delay was achieved,
>    > but only it's shape.
> 
> Oh?  Are you sure?
> 
> I'll claim that group delay is not an actual
> physical phenomenon, but
> an interpretation of phase measurements.
> 
> (If you have a device that provides a small delay
> without any other
> side effects or distortions, if you measure the
> phase of the output,
> compared to the input, you'll see the phase changing
> with frequency,
> linearly.  There's no "phase distortion" going on,
> the resulting
> waveform looks identical to the original waveform,
> it's just delayed a
> little bit.  Group delay is an attempt to interpret
> phase measurements
> in this light, as a delay that that changes with
> frequency.)
> 
> But a measured phase value could be due to any
> number of mechanisms;
> from a direct signal, from an inverted polarity
> signal, from a delay
> line, from a simple filter circuit, from a multiple
> stage filter
> circuit that shifts the phase and delays the signal,
> or from any
> combination.
> 
> These will all carry different sonic artifacts.
> 
>   -- Don
> 
> -- 
> Don Tillman
> Palo Alto, California
> don at till.com
> http://www.till.com
> 
>