Devlog

While working on other ancillary data related features in GStreamer (more on that some other day), I noticed that we didn't have support for sending or receiving ancillary data via RTP in GStreamer despite it being a quite simple RTP mapping defined in RFC 8331 and it being used as part of ST 2110.

The new RTP rtpsmpte291pay payloader and rtpsmpte291depay depayloader can be found in this MR for gst-plugins-rs, which should be merged in the next days.

The new elements pass the SMPTE ST 291-1 ancillary data as ST 2038 streams through the pipeline. ST 2038 streams can be directly extracted from or stored in MXF or MPEG-TS containers, can be extracted or inserted into SDI streams with the AJA or Blackmagic Decklink sources/sinks, or can be handled generically by the ST 2038 elements from the rsclosedcaption plugin.

For example the following pipeline can be used to convert an SRT subtitle file to CEA-708 closed captions, which are then converted to an ST 2038 stream and sent over RTP:

$ gst-launch-1.0 filesrc location=file.srt ! subparse ! \
    tttocea708 ! closedcaption/x-cea-708,framerate=30/1 ! ccconverter ! \
    cctost2038anc ! rtpsmpte291pay ! \
    udpsink host=123.123.123.123 port=45678

Now you might be wondering how ST 291-1 and ST 2038 are related to each other and what ST 2038 has to do with RTP.

ST 291-1 is the basic standard that defines the packet format for ancillary packets as e.g. transmitted over SDI. ST 2038 on the other hand defines a mechanism for packaging ST 291-1 into MPEG-TS, and in addition to the plain ST 291-1 packets provides some additional information like the line number on which the ST 291-1 packet is to be stored. RFC 8331 defines a similar mapping just for RTP, and apart from one field it provides exactly the same information and conversion between the two formats is relatively simple.

Using ST 2038 as generic ancillary data stream format in GStreamer seemed like the pragmatic choice here. GStreamer already had support for handling ST 2038 streams in various elements, a set of helper elements to handle ST 2038 streams, and e.g. GStreamer's MXF ANC support (ST 436) also uses ST 2038 as stream format.