Add "postprocessing" section to readme

Checklist

• I’m reporting a bug unrelated to a specific site
• I’ve verified that I’m running yt-dlp version 2022.02.04. (update instructions)
• I’ve checked that all provided URLs are alive and playable in a browser
• I’ve checked that all URLs and arguments with special characters are properly quoted or escaped
• I’ve searched the bugtracker for similar issues including closed ones. DO NOT post duplicates
• I’ve read the guidelines for opening an issue

Description

I am trying to download audio only and convert that audio to PCM.

For some reason, when a format argument (in my case, -f s16le) is specified in --postprocessor-args, m4a fixup (where it apparently adjusts the file to not be a DASH one or something like that) doesn’t seem to actually happen, although it says it does and it doesn’t give an error.

I’ve omitted the --extract-audio --audio-format wav stuff as that falls over at the end as a result, but I don’t think it’s connected to this as I can see the m4a file is clearly wrong after the first command has run.

Command 1 (with -f s16le)

yt-dlp -o "%(id)s.%(ext)s" -f bestaudio/best --postprocessor-args "ffmpeg:-ac 2 -f s16le -ar 48000" --compat-options youtube-dl https://www.youtube.com/watch?v=54rwawswQpM/

Command 2 (without -f s16le)

yt-dlp -o "%(id)s.%(ext)s" -f bestaudio/best --postprocessor-args "ffmpeg:-ac 2 -ar 48000" --compat-options youtube-dl https://www.youtube.com/watch?v=54rwawswQpM/

Verbose log

Command 1 output:

[debug] Command-line config: ['-o', '%(id)s.%(ext)s', '-f', 'bestaudio/best', '--postprocessor-args', 'ffmpeg:-ac 2 -f s16le -ar 48000', '--compat-options', 'youtube-dl', 'https://www.youtube.com/watch?v=54rwawswQpM/', '-vU']
[debug] Encodings: locale cp1252, fs utf-8, out cp1252 (No ANSI), err cp1252 (No ANSI), pref cp1252
[debug] yt-dlp version 2022.02.04 [c1653e9] (win_exe)
[debug] Compatibility options: no-youtube-unavailable-videos, format-spec, seperate-video-versions, no-attach-info-json, playlist-index, no-youtube-channel-redirect, list-formats, embed-thumbnail-atomicparsley, no-clean-infojson, embed-metadata, no-direct-merge, *filename, no-playlist-metafiles, no-keep-subs, abort-on-error, format-sort, no-live-chat
[debug] Python version 3.8.10 (CPython 64bit) - Windows-8.1-6.3.9600-SP0
[debug] exe versions: ffmpeg 5.0-full_build-www.gyan.dev (setts), ffprobe 5.0-full_build-www.gyan.dev
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
Latest version: 2022.02.04, Current version: 2022.02.04
yt-dlp is up to date (2022.02.04)
[debug] [youtube] Extracting URL: https://www.youtube.com/watch?v=54rwawswQpM/
[youtube] 54rwawswQpM: Downloading webpage
[youtube] 54rwawswQpM: Downloading android player API JSON
[debug] Sort order given by user: hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, codec:vp9.2, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id, res, hdr:12(7), vcodec:vp9.2(10), acodec, asr
[info] 54rwawswQpM: Downloading 1 format(s): 140
[debug] Invoking downloader on "https://rr5---sn-aigl6nl7.googlevideo.com/videoplayback?<REDACTED>"
[download] Destination: 54rwawswQpM.m4a

[download]   0.0% of 5.29MiB at Unknown speed ETA Unknown
[download]   0.1% of 5.29MiB at Unknown speed ETA Unknown
[download]   0.1% of 5.29MiB at Unknown speed ETA Unknown
[download]   0.3% of 5.29MiB at  7.50MiB/s ETA 00:00     
[download]   0.6% of 5.29MiB at  3.87MiB/s ETA 00:01
[download]   1.2% of 5.29MiB at  4.85MiB/s ETA 00:01
[download]   2.3% of 5.29MiB at  6.05MiB/s ETA 00:00
[download]   4.7% of 5.29MiB at  7.97MiB/s ETA 00:00
[download]   9.4% of 5.29MiB at 11.61MiB/s ETA 00:00
[download]  18.9% of 5.29MiB at 15.98MiB/s ETA 00:00
[download]  37.8% of 5.29MiB at 18.95MiB/s ETA 00:00
[download]  75.6% of 5.29MiB at 18.36MiB/s ETA 00:00
[download] 100% of 5.29MiB at 18.43MiB/s ETA 00:00  
[download] 100% of 5.29MiB in 00:00               
[FixupM4a] Correcting container of "54rwawswQpM.m4a"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:54rwawswQpM.m4a" -map 0 -dn -ignore_unknown -c copy -f mp4 -movflags "+faststart" -ac 2 -f s16le -ar 48000 "file:54rwawswQpM.temp.m4a"

Command 2 output:

[debug] Command-line config: ['-o', '%(id)s.%(ext)s', '-f', 'bestaudio/best', '--postprocessor-args', 'ffmpeg:-ac 2 -ar 48000', '--compat-options', 'youtube-dl', 'https://www.youtube.com/watch?v=54rwawswQpM/', '-vU']
[debug] Encodings: locale cp1252, fs utf-8, out cp1252 (No ANSI), err cp1252 (No ANSI), pref cp1252
[debug] yt-dlp version 2022.02.04 [c1653e9] (win_exe)
[debug] Compatibility options: no-attach-info-json, embed-metadata, embed-thumbnail-atomicparsley, no-youtube-unavailable-videos, no-clean-infojson, no-direct-merge, list-formats, no-youtube-channel-redirect, no-keep-subs, *filename, no-playlist-metafiles, seperate-video-versions, playlist-index, format-spec, no-live-chat, format-sort, abort-on-error
[debug] Python version 3.8.10 (CPython 64bit) - Windows-8.1-6.3.9600-SP0
[debug] exe versions: ffmpeg 5.0-full_build-www.gyan.dev (setts), ffprobe 5.0-full_build-www.gyan.dev
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
Latest version: 2022.02.04, Current version: 2022.02.04
yt-dlp is up to date (2022.02.04)
[debug] [youtube] Extracting URL: https://www.youtube.com/watch?v=54rwawswQpM/
[youtube] 54rwawswQpM: Downloading webpage
[youtube] 54rwawswQpM: Downloading android player API JSON
[debug] Sort order given by user: hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, codec:vp9.2, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, hasaud, lang, quality, tbr, filesize, vbr, height, width, proto, vext, abr, aext, fps, fs_approx, source, id, res, hdr:12(7), vcodec:vp9.2(10), acodec, asr
[info] 54rwawswQpM: Downloading 1 format(s): 140
[debug] Invoking downloader on "https://rr5---sn-aigzrne7.googlevideo.com/videoplayback?<REDACTED>"
[download] Destination: 54rwawswQpM.m4a

[download]   0.0% of 5.29MiB at 341.28KiB/s ETA 00:15
[download]   0.1% of 5.29MiB at 1023.83KiB/s ETA 00:05
[download]   0.1% of 5.29MiB at  2.33MiB/s ETA 00:02  
[download]   0.3% of 5.29MiB at  5.00MiB/s ETA 00:01
[download]   0.6% of 5.29MiB at  3.87MiB/s ETA 00:01
[download]   1.2% of 5.29MiB at  4.85MiB/s ETA 00:01
[download]   2.3% of 5.29MiB at  6.05MiB/s ETA 00:00
[download]   4.7% of 5.29MiB at  7.73MiB/s ETA 00:00
[download]   9.4% of 5.29MiB at 11.61MiB/s ETA 00:00
[download]  18.9% of 5.29MiB at 15.50MiB/s ETA 00:00
[download]  37.8% of 5.29MiB at 17.20MiB/s ETA 00:00
[download]  75.6% of 5.29MiB at 17.80MiB/s ETA 00:00
[download] 100% of 5.29MiB at 17.94MiB/s ETA 00:00  
[download] 100% of 5.29MiB in 00:00               
[FixupM4a] Correcting container of "54rwawswQpM.m4a"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:54rwawswQpM.m4a" -map 0 -dn -ignore_unknown -c copy -f mp4 -movflags "+faststart" -ac 2 -ar 48000 "file:54rwawswQpM.temp.m4a"

[marcello-pietrobon]

Just confirming that this thread has been very useful, and that it would be very useful to add some sections with practical examples on how to use --postprocessor-args , --use-postprocessor, and similar.

Beside that, the yt-dlp documentation is excellent.

[Zirro]

There are two postprocessors involved here - FixupM4a and ExtractAudio. I think you’ll want to use --postprocessor-args 'ExtractAudio:-ac 2 -ar 48000' to only pass the arguments to the latter instead.

I also removed the -f flag since s16le appears to be the default for WAV files, and doesn’t seem to be an accepted value for this flag either way. Perhaps what you were looking for was -c:a pcm_s16le?

Here’s the full command with these modifications:

yt-dlp -o '%(id)s.%(ext)s' -f 'bestaudio/best' --postprocessor-args 'ExtractAudio:-c:a pcm_s16le -ac 2 -ar 48000' --compat-options 'youtube-dl' 'https://www.youtube.com/watch?v=54rwawswQpM' -vU --extract-audio --audio-format wav

EDIT: -f does seem to take s16le under some circumstances, after all. This page appears to have examples for what you’re trying to achieve (using both -f and -c:a): https://trac.ffmpeg.org/wiki/audio types