[LAD] stereospread.lv2 - Stereo spreading plugin (conjugate random phase all-pass filter pair)

Florian Paul Schmidr mista.tapas at gmx.net
Wed Nov 16 14:41:23 CET 2022

Sorry for top-posting.

Fons, you are, of course, absolutely right. My thinking wasn't clear.

Applying them one after the other would cancel the effect, but summing does not.

Kind regards,

Nov 16, 2022 14:15:45 Fons Adriaensen <fons at linuxaudio.org>:

> On Wed, Nov 16, 2022 at 12:51:44PM +0100, Florian Paul Schmidt wrote:
>> This ensures that what is a phase theta in the first filter becomes a
>> phase of -theta in the second filter, and summed that just gives a phase
>> of 0.
> 1. If I understand this correctly the L and R outputs have opposite phase
>    shifts. That means they will not sum to the input. Just assume the L
>    shift is 90 degrees. then R is -90, and they will just cancel.
> 2. If you measure this, you will also note amplitude differences between
>    L and R outputs. This is to be expected. Even if the two filters have
>    exact unity gain (and just a phase shift) at each frequency corresponding
>    to an FFT bin, the resulting filter will not be all-pass.
> 3. At high frequencies (above 1 kHz or so), it's actually the amplitude
>    differences and not the phase shifts that create the stereo effect.
> Ciao,
> -- 
> FA
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