Support LectureMix SRT streams in worker

[hmelder]

LectureMix's streamd outputs three SRT streams. A camera, presentation, and combined stream. Audio is encoded into AAC-LC, 2 Channel, 48khz, CBR and video into h264 1080p30.

Here is the output when probing the stream with ffprobe:

hugo@Hugos-MacBook-Pro-2 ~ % ffprobe "srt://[fdf7:2667:aa31::c50]:7000"
ffprobe version 7.1 Copyright (c) 2007-2024 the FFmpeg developers
  built with Apple clang version 16.0.0 (clang-1600.0.26.4)
  configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/7.1_3 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags='-Wl,-ld_classic' --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libharfbuzz --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-audiotoolbox --enable-neon
  libavutil      59. 39.100 / 59. 39.100
  libavcodec     61. 19.100 / 61. 19.100
  libavformat    61.  7.100 / 61.  7.100
  libavdevice    61.  3.100 / 61.  3.100
  libavfilter    10.  4.100 / 10.  4.100
  libswscale      8.  3.100 /  8.  3.100
  libswresample   5.  3.100 /  5.  3.100
  libpostproc    58.  3.100 / 58.  3.100
[matroska,webm @ 0x153e05d50] Unknown element A3 at pos. 0x1b904 with length 0x61b2 considered as invalid data. Last known good position 0x1dd, 20 unknown elements in a row
[matroska,webm @ 0x153e05d50] Seek to desired resync point failed. Seeking to earliest point available instead.
Input #0, matroska,webm, from 'srt://[fdf7:2667:aa31::c50]:7000':
  Metadata:
    encoder         : GStreamer matroskamux version 1.24.7
    creation_time   : 2024-12-06T16:47:56.284628Z
  Duration: N/A, start: 72.000000, bitrate: N/A
  Stream #0:0(eng): Audio: aac (LC), 48000 Hz, stereo, fltp (default)
      Metadata:
        title           : Audio
  Stream #0:1(eng): Video: h264 (High), yuv420p(tv, bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 30 fps, 30 tbr, 1k tbn (default)
      Metadata:
        title           : Video
        ENCODER         : "VA-API\ H.264\ Encoder\ in\ AMD\ Radeon\ Graphics"
hugo@Hugos-MacBook-Pro-2 ~ %

Should be just a matter of checking the URI scheme for srt and rtsp, and tuning the probing. Also, please stop re-encoding the video streams. SMP and LectureMix streams have a known codec config. Re-encoding is a waste of power, cpu-time, and increases latency dramatically.

To test this locally, I'd recommend spinning up a test server using GStreamer:

gst-launch-1.0 matroskamux name=m ! srtsink wait-for-connection=false uri="srt://127.0.0.1:7001?mode=listener" videotestsrc ! video/x-raw,width=1920,height=1080,framerate=30/1 ! x264enc tune=zerolatency bitrate=2048 ! m. audiotestsrc ! audio/x-raw,rate=48000,channels=2 ! fdkaacenc bitrate=96000 rate-control=cbr ! m.