The Gstreamer encoder

The %gstreamer encoder can be used to encode streams using the gstreamer multimedia framework. This encoder extends liquidsoap with all available GStreamer formats (provided they are compatible with liquidsoap’s model of streams, see Caveats section below), which includes a huge array of encoders.

Presentation

A basic understanding of gstreamer’s pipelines and configuration should be expected in order to understand the following documentation.

The encoder’s parameters are as follows:

%gstreamer(channels=2,
                               audio="lamemp3enc",
                               has_video=true,
                               video="x264enc",
                               muxer="mpegtsmux",
                               metadata="metadata",
                               log=5,
                               pipeline="")

Without using the pipeline argument, the audio and video arguments are used to build the gstreamer pipeline used to encode. By setting the log parameter to a lower value or by using set("log.level",..), you should be able to see some example.

Basic examples

Here are a couple of examples:

An MP3 encoder that expects sources of type audio=2, video=0, midi=0:

% liquidsoap 'output.file(%gstreamer(audio="lamemp3enc",
                                                         muxer="",
                                                         video="",
                                                         log=3),...)'
                    (...)
                    2012/12/13 19:16:23 [encoder.gstreamer:3] Gstreamer encoder pipeline: appsrc 
                      name="audio_src" block=true caps="audio/x-raw,format=S16LE,layout=interleaved,
                      channels=2,rate=44100" format=time ! queue ! audioconvert ! audioresample !
                      lamemp3enc ! appsink name=sink sync=false emit-signals=true

A x264 encoder that expects sources of type audio=0, video=1, midi=0:

% liquidsoap 'output.file(%gstreamer(audio="",
                                                         muxer="mpegtsmux",
                                                         video="x264enc",
                                                         log=3),...)'
                    (...)
                    2012/12/13 19:14:43 [encoder.gstreamer:3] Gstreamer encoder pipeline:  appsrc
                      name="video_src" block=true caps="video/x-raw,format=RGBA,width=320,height=240,
                      framerate=25/1,pixel-aspect-ratio=1/1" format=time blocksize=307200 ! queue !
                      videoconvert ! videoscale add-borders=true ! videorate ! x264enc ! 
                      mpegtsmux name=muxer ! appsink name=sink sync=false emit-signals=true

An MPEG TS encoder that expects sources of type audio=2, video=1, midi=0:

% liquidsoap 'output.file(%gstreamer(audio="lamemp3enc",
                                                         muxer="mpegtsmux",
                                                         video="x264enc",
                                                         log=3),...)'
                    (...)
                    2012/12/13 19:18:09 [encoder.gstreamer:3] Gstreamer encoder pipeline: appsrc
                      name="audio_src" block=true caps="audio/x-raw,format=S16LE,
                      layout=interleaved,channels=2,rate=44100" format=time ! queue ! audioconvert 
                      ! audioresample ! lamemp3enc ! muxer. appsrc name="video_src" block=true
                      caps="video/x-raw,format=RGBA,width=320,height=240,framerate=25/1,
                      pixel-aspect-ratio=1/1" format=time blocksize=307200 ! queue ! videoconvert
                      ! videoscale add-borders=true ! videorate ! x264enc ! muxer. mpegtsmux 
                      name=muxer ! appsink name=sink sync=false emit-signals=true

An ogg/vorbis+theora encoder that expects source of type audio=1, video=1, midi=0:

% liquidsoap 'output.file(%gstreamer(audio="vorbisenc",
                                                         muxer="oggmux",
                                                         video="theoraenc",
                                                         channels=1,
                                                         log=3),...)'
                    (...)
                    2012/12/13 19:21:17 [encoder.gstreamer:3] Gstreamer encoder pipeline: appsrc
                      name="audio_src" block=true caps="audio/x-raw,format=S16LE,layout=interleaved,
                      channels=1,rate=44100" format=time ! queue ! audioconvert ! audioresample !
                      vorbisenc ! muxer. appsrc name="video_src" block=true caps="video/x-raw,
                      format=RGBA,width=320,height=240,framerate=25/1,pixel-aspect-ratio=1/1"
                      format=time blocksize=307200 ! queue ! videoconvert ! videoscale add-borders=true
                      ! videorate ! theoraenc ! muxer. oggmux name=muxer ! appsink name=sink 
                      sync=false emit-signals=true

For advanced users, the pipeline argument can be used to override the whole pipeline. For instance:

% liquidsoap 'output.file(%gstreamer(pipeline="appsrc name=\"audio_src\"
                        block=true caps=\"audio/x-raw,format=S16LE,layout=interleaved,
                        channels=1,rate=44100\" format=time ! lamemp3enc ! appsink name=sink
                        sync=false emit-signals=true",channels=1,log=3),...)'
                    (...)

Content type inference

When starting its sources and outputs, liquidsoap determines the content type of each source (audio, video and midi channels). During that process, encoders have to inform liquidsoap what type of sources they are expecting. It works as follows for the %gstreamer encoder:

  • If the audio parameter is a string different than "" then the encoder expects a stream with channels audio channels.
  • If the video parameter is a string different than "" then the encoder expects a stream with 1 video channel.
  • If the pipeline parameter is a string different than "" then the encoder expects a stream with channels audio channels and a video channels only if has_video is true.

The has_video parameter is only used when using the pipeline parameter.

Metadata

The %gstreamer encoder tries to also encode metadata attached to the stream. This requires that you specify a pipeline element named according to the metadata parameter (default: "metadata") that can be used with GStreamer’s tag_setter API. Here are two such examples:

An ogg/vorbis encoder with vorbis tags:

% liquidsoap 'output.file(%gstreamer(audio="vorbisenc ! vorbistag name='metadata'",
                                                         muxer="oggmux",
                                                         video=""),...)'

An MP3 encoder with id3v2 tags:

% liquidsoap 'output.file(%gstreamer(audio="lamemp3enc",
                                                         muxer="id3v2mux",
                                                         video="",
                                                         metadata="muxer"),...)'

In the last example, we tell the %gstreamer encoder that the element for injecting metadata is named "muxer" because, for id3v2 tags, the gstreamer muxer element is also the element used to inject metadata and the "muxer" name is implicitly added by liquidsoap to the muxer element. You can see that by printing out the constructed pipeline, as shown before.

Caveats

When using the %gstreamer encoder, one must think of it as an encoder for an infinite stream. This, in particular, means that not all containers (muxers) will work. For instance, the AVI and MP4 containers need to write in their header information that are only known with finite streams, such as the stream total’s time and etc.. These containers are usually not fit for streaming, which is liquidsoap’s main functionality.