Metadata not being inserted to output (works for HLS, but SRT or FILE don't)

[christian-gama]

Description

I am trying to insert metadata via Telnet into an output, such as output.srt or output.file (the main goal is to make it work with SRT, the file is just an example), but neither of them works. However, using output.hls works fine, as stated in the documentation.

Even though I am successfully sending the Telnet command to Liquidsoap, a simple script like this does not work. The goal is to send a Telnet command to manually update the metadata.

I have tried version 2.3.1 as well but does not work.

Steps to reproduce

settings.server.telnet := true
settings.server.telnet.bind_addr := "0.0.0.0"
settings.server.telnet.port := 1935

source = mksafe(input.srt(host="0.0.0.0", port=8000, mode="listener"))

def print_metadata(m)
    list.iter(fun (lv) -> print("- #{fst(lv)}: #{snd(lv)}"), m)
end

source = server.insert_metadata(id="metadata", source)
source.on_metadata(print_metadata)

# There is no metadata when I use ffprobe in my audio
output.file(id="with_mp3", %mp3(bitrate=64,id3v2=true), "./audio_with_mp3.mp3", source)

# With FFmpeg I have no metadata as well
output.file(id="with_ffmpeg", %ffmpeg(format="mp3", %audio(codec="libmp3lame", b="64k")), "./audio_with_ffmpeg.mp3", source)

# I have tried with SRT as well, and using ffprobe in the SRT output, but no metadata as well
output.srt(host="0.0.0.0", port=8001, mode="listener", %mp3(bitrate=64,id3v2=true), source)

Expected behavior

Using server.insert_metadata works fine when I have an output.hls, but when I use other outputs, such as SRT or File, no metadata is present. I can see the metadata being printed due to the print_metadata function, so it is reaching the Liquidsoap server correctly.

Liquidsoap version

Liquidsoap 2.3.0+dev
Copyright (c) 2003-2024 Savonet team
Liquidsoap is open-source software, released under GNU General Public License.
See <http://liquidsoap.info> for more information.

Liquidsoap build config

* Liquidsoap version  : 2.3.0+dev

 * Compilation options
   - Release build       : false
   - Git SHA             : (none)
   - OCaml version       : 5.2.1
   - OS type             : Unix
   - Libs versions       : angstrom=0.16.1 base=v0.17.1 base.base_internalhash_types=v0.17.1 base.shadow_stdlib=v0.17.1 bigarray-compat=1.1.0 bigstringaf=0.10.0 bytes=[distributed with OCaml 4.02 or above] camlp-streams camomile.lib=2.0 cry=1.0.3 ctypes=0.23.0 ctypes-foreign=0.23.0 ctypes.stubs=0.23.0 curl=0.9.2 domain_shims dtools=0.4.5 dune-build-info=3.17.2 dune-private-libs.dune-section=3.17.2 dune-site=3.17.2 dune-site.private=3.17.2 duppy=0.9.4 ffmpeg-av=1.2.2 ffmpeg-avcodec=1.2.2 ffmpeg-avdevice=1.2.2 ffmpeg-avfilter=1.2.2 ffmpeg-avutil=1.2.2 ffmpeg-swresample=1.2.2 ffmpeg-swscale=1.2.2 fileutils=v0.6.6 gen=1.1 integers lame=0.3.7 liquidsoap-lang=2.3.0 liquidsoap-lang.console=2.3.0 liquidsoap_builtins liquidsoap_core liquidsoap_ffmpeg liquidsoap_lame liquidsoap_ndi liquidsoap_optionals liquidsoap_oss liquidsoap_runtime liquidsoap_srt liquidsoap_stereotool magic-mime=1.3.1 mem_usage=0.1.2 menhirLib=20240715 metadata=0.3.0 mm=0.8.6 mm.audio=0.8.6 mm.base=0.8.6 mm.image=0.8.6 mm.midi=0.8.6 mm.video=0.8.6 ndi ocaml_intrinsics_kernel=v0.17.1 posix-base=2.2.0 posix-socket=2.2.0 posix-socket.constants=2.2.0 posix-socket.stubs=2.2.0 posix-socket.types=2.2.0 ppx_compare.runtime-lib=v0.17.0 ppx_hash.runtime-lib=v0.17.0 ppx_sexp_conv.runtime-lib=v0.17.0 ppx_string.runtime=v0.17.0 re=1.12.0 saturn_lockfree=0.4.1 sedlex=3.3 seq=[distributed with OCaml 4.07 or above] sexplib0=v0.17.0 srt=0.3.3 srt.constants=0.3.3 srt.stubs=0.3.3 srt.stubs.locked=0.3.3 srt.types=0.3.3 stdlib-shims=0.3.0 stereotool str=5.2.1 stringext=1.6.0 threads=5.2.1 unix=5.2.1 uri=4.4.0
   - architecture        : amd64
   - host                : x86_64-pc-linux-gnu
   - target              : x86_64-pc-linux-gnu
   - system              : linux
   - ocamlopt_cflags     : -O2 -fno-strict-aliasing -fwrapv -pthread -fPIC -pthread
   - native_c_compiler   : gcc -O2 -fno-strict-aliasing -fwrapv -pthread -fPIC -pthread -D_FILE_OFFSET_BITS=64
   - native_c_libraries  : -lm -lpthread

 * Configured paths
   - mode              : default
   - standard library  : (set by dune-site)
   - scripted binaries : (set by dune-site)
   - rundir            : (set by dune-site)
   - logdir            : (set by dune-site)
   - user cache        : $HOME/.cache/liquidsoap (override with $LIQ_CACHE_USER_DIR)
   - system cache      : (set by dune-site) (override with $LIQ_CACHE_SYSTEM_DIR)
   - camomile files    : (set by dune-site)

 * Supported input formats
   - MP3               : no (requires mad)
   - AAC               : no (requires faad)
   - Ffmpeg            : yes
   - Flac (native)     : no (requires flac)
   - Flac (ogg)        : no (requires ogg)
   - Opus              : no (requires opus)
   - Speex             : no (requires speex)
   - Theora            : no (requires theora)
   - Vorbis            : no (requires vorbis)
   - WAV/AIFF          : yes (native)

 * Supported output formats
   - FDK-AAC           : no (requires fdkaac)
   - FFmpeg            : yes
   - MP3               : yes
   - MP3 (fixed-point) : no (requires shine)
   - Flac (native)     : no (requires flac)
   - Flac (ogg)        : no (requires ogg)
   - Opus              : no (requires opus)
   - Speex             : no (requires speex)
   - Theora            : no (requires theora)
   - Vorbis            : no (requires vorbis)
   - WAV/AIFF          : yes (native)

 * Tags
   - AAC               : no (requires faad)
   - FFmpeg            : yes
   - FLAC (native)     : no (requires flac)
   - Flac (ogg)        : no (requires ogg)
   - Native decoder    : yes
   - Vorbis            : no (requires vorbis)

 * Input / output
   - ALSA              : no (requires alsa)
   - AO                : no (requires ao)
   - FFmpeg            : yes
   - JACK              : no (requires bjack)
   - NDI               : yes
   - OSS               : yes
   - Portaudio         : no (requires portaudio)
   - Pulseaudio        : no (requires pulseaudio)
   - SRT               : yes

 * Audio manipulation
   - FFmpeg            : yes
   - LADSPA            : no (requires ladspa)
   - Lilv              : no (requires lilv)
   - Samplerate        : no (requires samplerate)
   - SoundTouch        : no (requires soundtouch)
   - StereoTool        : yes

 * Video manipulation
   - camlimages        : no (requires camlimages)
   - FFmpeg            : yes
   - frei0r            : no (requires frei0r)
   - ImageLib          : no (requires imagelib)
   - SDL               : no (requires tsdl-image & tsdl-ttf)

 * MIDI manipulation
   - DSSI              : no (requires dssi)

 * Visualization
   - GD                : no (requires gd)
   - Graphics          : no (requires graphics)
   - SDL               : no (requires tsdl-image & tsdl-ttf)

 * Additional libraries
   - FFmpeg filters    : yes
   - FFmpeg devices    : yes
   - inotify           : no (requires inotify)
   - irc               : no (requires irc-client-unix)
   - jemalloc          : no (requires jemalloc)
   - lastfm            : no (requires lastfm)
   - lo                : no (requires lo)
   - memtrace          : no (requires memtrace)
   - osc               : no (requires osc-unix)
   - ssl               : no (requires ssl)
   - sqlite3           : no (requires sqlite3)
   - tls               : no (requires tls-liquidsoap)
   - posix-time2       : no (requires posix)
   - windows service   : no (requires winsvc)
   - YAML support      : no (requires yaml)
   - XML playlists     : no (requires xmlplaylist)

 * Monitoring
   - Prometheus        : no (requires prometheus)

Installation method

From OPAM

Additional Info

I have this printed when I call telnet, printed from print_metadata function.

- StreamTitle: My Stream
- artist: Somebody
- title: Some title

FFprobe prints this for the audio file (something similar for SRT too):

[mp3 @ 0x560ad9ada700] Skipping 208 bytes of junk at 45.
[mp3 @ 0x560ad9ada700] Estimating duration from bitrate, this may be inaccurate
Input #0, mp3, from 'audio.mp3':
  Metadata:
    encoder         : Lavf59.27.100
  Duration: 00:07:05.95, start: 0.011995, bitrate: 64 kb/s
  Stream #0:0: Audio: mp3, 44100 Hz, stereo, fltp, 64 kb/s

[toots]

Hi,

The mp3 format does not support in-band metadata. Typically, icecast uses the (antiquated) ICY protocol for this.

I suggest looking at a format that supports in-band metadata. The most common would be ogg, with vorbis, flac or opus codec.

It's worth noting, however, that the spec for in-band ogg metadata is also not the best and notoriously badly implemented. You'd need to validate that the receiving side of the SRT stream can also properly decode ogg in-band metadata.

Other than ogg, I'm not sure of any format that can do it well except for HLS, which you already know about.

What you could consider is sending the metadat out of band, via a secondary call. But the, in this case, you'd need a method to insert them at the time timestamp.

[christian-gama]

Yeah, I used to manually inject the metadata using something similar to what Icecast does, padding the metadata into the audio buffer using a metaint value of 16000 (as recommended in their documentation). I just assumed Liquidsoap was doing something similar, so I thought I could rely solely on Liquidsoap to handle all the metadata. But I can manage it outside of Liquidsoap—it's not a big deal.

[toots]

Ha sorry I'm seeing that you're using ffmpeg now. Yeah, ffmpeg does not parse in-band metadata very well with ogg. But I'm working on changing that right now!

[christian-gama]

I noticed that too — FFmpeg is not great at parsing in-band metadata at all. I used to rely on a background job running FFprobe to keep track of the metadata.