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

Fons Adriaensen fons at linuxaudio.org
Wed Nov 16 14:14:27 CET 2022

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.



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