Opened 20 months ago
Closed 14 months ago
#10330 closed defect (fixed)
Opus documentation: lowdelay is too trivialized
Reported by: | Mingye Wang | Owned by: | |
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Priority: | normal | Component: | documentation |
Version: | unspecified | Keywords: | libopus docs |
Cc: | MasterQuestionable | Blocked By: | |
Blocking: | Reproduced by developer: | no | |
Analyzed by developer: | no |
Description (last modified by )
Summary of the bug:
FFmpeg currently describes (at https://ffmpeg.org/ffmpeg-codecs.html#libopus-1) -application lowdelay
as simply "Restrict to only the lowest delay modes." This is true, but does not cover the whole picture. Compare the Official documentation of OPUS_APPLICATION_RESTRICTED_LOWDELAY: "Only use when lowest-achievable latency is what matters most. Voice-optimized modes cannot be used."
A better wording based on the above would be:
"Restrict to only the lowest delay modes by disabling voice-optimized codec. This is usually unnecessary, as basic Opus already has a delay of 26.5 ms. Try -frame_duration before using this."
How to reproduce:
Well, just navigate to the page. Or
ffmpeg -h enccoder=libopus
Change History (8)
comment:1 by , 20 months ago
comment:2 by , 20 months ago
A better wording here is...
Restrict to only the lowest delay modes. Specifically, disable speech-optimized codec (SILK) to remove a 4 ms delay.
That clarifies what you lose and what you gain.
For the short AVOption text, try
Restrict to only the lowest delay modes (disables SILK).
comment:3 by , 20 months ago
Description: | modified (diff) |
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comment:4 by , 20 months ago
Side note: "-application lowdelay" may also reduce decoding complexity.
And I noticed Opus seems to handle not anything above 20,000 Hz.
For clarity:
|*| OPUS_BANDWIDTH_FULLBAND = 20000
|*| OPUS_BANDWIDTH_WIDEBAND = 8000
See also:
https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/libopusenc.c
https://github.com/xiph/opus/blob/master/src/opus_encoder.c
comment:5 by , 20 months ago
Cc: | added |
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comment:6 by , 20 months ago
I honestly don't care about that. Most lossy encoders do the 20 KHz cutoff. I'm neither a human infant nor a dolphin...
comment:7 by , 19 months ago
Encoders that enforce 20 KHz cut-off are few. (IIRC, only Opus)
Which rendered Opus only meaningful for low bit rate applications.
(assuming Stereo)
Opus at 160 Kbps already matched MP3 at 320 Kbps, while raising the bit rate all the way to 500 Kbps still producing practically identical result...
comment:8 by , 14 months ago
Resolution: | → fixed |
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Status: | new → closed |
I tried to address this in commit:
commit 00eb4966aa8a71a70c4758254027fdc6925f8f86 (HEAD -> master, origin/master, origin/HEAD) Author: Stefano Sabatini <stefasab@gmail.com> Date: Sat Aug 26 15:45:33 2023 +0200 doc/encoders/libopus: clarify lowdelay and cutoff options Extend descriptions for the application=lowdelay and cutoff options. Based on notes by Mingye Wang. Address issues: http://trac.ffmpeg.org/ticket/10330 http://trac.ffmpeg.org/ticket/10343
Marking as closed, but feel free to reopen or suggest a better wording.
Thanks for the report.
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 defaultOPUS_BANDWIDTH_FULLBAND
in libopus'sopus_encoder.c
st->maximum_bandwidth
. However, thest->user_bandwidth
is stillOPUS_AUTO
, making it subject to a combination of auto-detection and bitrate-based selection inopus_encode_native()
. The former will do nothing noticeable to the signal, but the latter does this:It's not a bad idea, but it needs to be documented. Try: