[FFmpeg-trac] #8751(undetermined:new): LADSPA latency compensation

[FFmpeg]

#8751: LADSPA latency compensation
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             Reporter:  ltratt       |                     Type:
                                     |  enhancement
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:  LADSPA       |               Blocked By:
  filter                             |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 A common idiom for LADSPA plugins is for them to report their latency with
 a "latency" output control port which the invoking program can use to
 correct the output. For example, sox has "ladspa -l" (see
 https://sourceforge.net/p/sox/patches/101/) and Audacity defaults to using
 this idiom (https://manual.audacityteam.org/man/effect_menu_ladspa.html).
 ffmpeg doesn't seem to do this automatically (which is fair enough, as
 it's not a guaranteed standard), but doesn't seem to have any way to do it
 optionally, nor any way?

 For example, consider the noise-gate plugin https://github.com/feuerbach
 /noise-gate. If it's used as such:

 {{{
   ffmpeg -i in.aac -af
 "ladspa=file=/usr/local/lib/ladspa/ng.so:p=noise_gate:c=c0=-35|c1=1500|c2=150|c3=50"
 -c:a aac -y out.aac
 }}}

 then 'out.aac' will be both substantially delayed relative to 'in.aac' as
 well as having the input cut off. You can see the noise-gate plugin
 defining the output port here for example:

   https://github.com/feuerbach/noise-gate/blob/master/ng.cpp#L246

 I think https://trac.ffmpeg.org/ticket/7616 is related to this issue,
 although the report there isn't clear about how common this idiom is.

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Ticket URL: <https://trac.ffmpeg.org/ticket/8751>
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