[sdiy] group delay (of filters) and listening

[Magnus Danielson]

From: Don Tillman <don at till.com>
Subject: Re: [sdiy] group delay (of filters) and listening
Date: 07 Oct 2004 01:56:13 -0700
Message-ID: <m2k6u2q53m.fsf at till.com>

>    > 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.

Group delay is a direct consequence of the pole-zero locations for the system.

> (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.)

Group delay is the delay that the modulation of a carrier signal experience.
While the carrier will experience the straight phase delay, the modulating
signal will experience the difference in phase delay between its two side-band
components and thus becomming the derivate of phase delay.

> 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.

Exactly. Polarity change IS a real sonical artifact.

Cheers,
Magnus