[FFmpeg-trac] #10330(documentation:new): Opus documentation: lowdelay is too trivialized

FFmpeg trac at avcodec.org
Fri Apr 21 18:56:11 EEST 2023

#10330: Opus documentation: lowdelay is too trivialized
             Reporter:  Mingye Wang  |                    Owner:  (none)
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  documentation
              Version:  unspecified  |               Resolution:
             Keywords:  libopus      |               Blocked By:
  docs                               |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
Comment (by Mingye Wang):

 Huh, the description for `cutoff` is also wrong. On a setting of 0,
 `libopusenc.c` does not enforce a maximum at all and keeps the default
 `OPUS_BANDWIDTH_FULLBAND` in libopus's `opus_encoder.c`
 `st->maximum_bandwidth`. However, the `st->user_bandwidth` is still
 `OPUS_AUTO`, making it subject to a combination of auto-detection and
 bitrate-based selection in `opus_encode_native()`. The former will do
 nothing noticeable to the signal, but the latter does this:
     if (st->mode != MODE_CELT_ONLY && max_rate < 15000)
        st->bandwidth = IMIN(st->bandwidth, OPUS_BANDWIDTH_WIDEBAND);

 It's not a bad idea, but it needs to be documented. Try:

  The default is 0 (cutoff disabled). Note that libopus forces a wideband
 cutoff for bitrates < 15 kbps, unless CELT-only (lowdelay) mode is used.
Ticket URL: <https://trac.ffmpeg.org/ticket/10330#comment:1>
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