Dear all,

during my architectural revision of the SoXPlugins
(mirroring the code of SoX in principle) I noticed a
similarity between the code for "chorus" and "phaser": both
have modulated delay lines.

For a chorus this is absolutely correct, but a phaser should
_not at all_ have modulated delay lines, it must have
_modulated phase shift stages_.

This might sound like an academical discussion, but the
sound differences are noticable: a flanger does a comb
filtering with equidistant spacing (e.g. 100Hz, 200Hz,
300Hz, 400Hz, ...), while a phase does a comb filtering
with factored spacing (e.g. 100Hz, 200Hz, 400Hz, 800Hz,
...) that sound much more harmonical.

There is a good explanation and also helpful diagrams at
https://www.uaudio.de/blog/flangers-and-phasers/

Hence the SoX phaser is not a phaser, but a flanger.

Funnily enough there is also a flanger available in SoX that
has similar code but differs in the fact that signal
feedback is done for even more dramatic effects and that
there can be a fixed modulation phase difference between
different channels.

What shall we do?

  * Leave everything as is and accept the inappropriate name
    for the phaser effect just to be compatible to the past?

  * Provide a real phaser as "phaser"?

    Its parameters should be

        phaser gain-in gain-out stage-count decay speed modulation-kind

    where the stage-count gives the number of allpass stages
    (typically 4, 8, 12 etc.) and modulation-kind is either
    -s or -t.

What do you think?


Best regards,
Prof. Spock



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