[Broken] RTVE.es videos [rtve.alacarta] aren't fluid

[gcapizzi]

Checklist

• [x] I'm reporting a broken site support
• [ ] I've verified that I'm running youtube-dlc version 2020.09.16
• [ ] 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

Verbose log

❯ youtube-dl -v --external-downloader aria2c --external-downloader-args '-c -j 3 -x 3 -s 3 -k 1M' --all-subs --embed-subs --postprocessor-args '-fflags +igndts' https://www.rtve.es/alacarta/videos/estoy-vivo/estoy-vivo-capitulo-1/4203918/
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'--external-downloader', u'aria2c', u'--external-downloader-args', u'-c -j 3 -x 3 -s 3 -k 1M', u'--all-subs', u'--embed-subs', u'--postprocessor-args', u'-fflags +igndts', u'https://www.rtve.es/alacarta/videos/estoy-vivo/estoy-vivo-capitulo-1/4203918/']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2020.05.08
[debug] Python version 2.7.16 (CPython) - Darwin-19.3.0-x86_64-i386-64bit
[debug] exe versions: avconv 12.3, avprobe 12.3, ffmpeg 4.2.2, ffprobe 4.2.2, rtmpdump 2.4
[debug] Proxy map: {}
[rtve.es:alacarta] Fetching manager info
[rtve.es:alacarta] 4203918: Downloading JSON metadata
[rtve.es:alacarta] 4203918: Downloading url information
[rtve.es:alacarta] 4203918: Downloading m3u8 information
[rtve.es:alacarta] 4203918: Downloading subtitles info
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: Estoy vivo - Temporada 1 - Capítulo 1-4203918.es.vtt
[debug] Invoking downloader on u'https://rtve-hlsvod.secure.footprint.net/resources/TE_GL23/mp4/4/2/1504716904424.mp4/1504716904424-audio_eng=96000-video_eng=1493000.m3u8?hls_minimum_fragment_length=6&hls_client_manifest_version=3'
[download] Destination: Estoy vivo - Temporada 1 - Capítulo 1-4203918.mp4
[download] 100% of 797.72MiB
[debug] ffmpeg command line: ffprobe -show_streams 'file:Estoy vivo - Temporada 1 - Capítulo 1-4203918.mp4'
[ffmpeg] Fixing malformed AAC bitstream in "Estoy vivo - Temporada 1 - Capítulo 1-4203918.mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel 'repeat+info' -i 'file:Estoy vivo - Temporada 1 - Capítulo 1-4203918.mp4' -c copy -f mp4 '-bsf:a' aac_adtstoasc -fflags '+igndts' 'file:Estoy vivo - Temporada 1 - Capítulo 1-4203918.temp.mp4'
[ffmpeg] Embedding subtitles in 'Estoy vivo - Temporada 1 - Capítulo 1-4203918.mp4'
[debug] ffmpeg command line: ffmpeg -y -loglevel 'repeat+info' -i 'file:Estoy vivo - Temporada 1 - Capítulo 1-4203918.mp4' -i 'file:Estoy vivo - Temporada 1 - Capítulo 1-4203918.es.vtt' -map 0 -c copy -map '-0:s' -map '-0:d' '-c:s' mov_text -map '1:0' '-metadata:s:s:0' 'language=spa' -fflags '+igndts' 'file:Estoy vivo - Temporada 1 - Capítulo 1-4203918.temp.mp4'
Deleting original file Estoy vivo - Temporada 1 - Capítulo 1-4203918.es.vtt (pass -k to keep)

Description

After post-processing, videos downloaded from RTVE Alacarta don't look fluid. This is due to the TS stream containing invalid PTS and ffmpeg correcting them in a way that results in non-fluid playback. More details can be found at this SO answer.

As the answer suggests, this can be fixed by using ffmpeg's igndts format flag. I'm trying to pass --postprocessor-args '-fflags +igndts' to youtube-dl but it's not helping, as ffmpeg options need to be specified before the file they apply on:

As a general rule, options are applied to the next specified file. Therefore, order is important, and you can have the same option on the command line multiple times. Each occurrence is then applied to the next input or output file. Exceptions from this rule are the global options (e.g. verbosity level), which should be specified first.

This is unfortunate, as output options only work when applied before the output file (but after the input file) and viceversa. We have no way to tell what the user wants, unless we introduce separate options for input args and output args.

My questions:

• Would it make sense to introduce a --postprocessor-input-args flag?
• How can we fix this for everyone using the RTVE extractor? Do we have a way to customise post-processing for specific websites?

Happy to help with PRs once we find out a way forward. Thanks!