Part 4 of the Understanding SMPTE ST 2110 Virtual Course
Traditionally, the ancillary data space (ANC) is used to transport embedded audio and various ancillary data types like Closed Captioning, Active Format Description (AFD) together with an active video in an SDI stream.The Ancillary data space (HANC & VANC) is the space not occupied by active video in an SDI frame.
Horizontal Ancillary Data Space (HANC) is the space between the end of active video (EAV) and the start of SAV, while Vertical Ancillary Data Space (VANC) is the space between the SAV and EAV both for data transport.
Currently, there are 79 registered ancillary data types which include embedded audio, SCTE triggers for advert insertion, time code, Closed Captioning, and the Active Format Description (AFD) among others. A full list of all registered ancillary data types is available online at smpte-ra.org.
SMPTE ST 2110 part 40 is the mechanism to transport ancillary data packets in RTP over IP networks.
SMPTE ST 2110-40 references other standards like the SMPTE ST 291-1:2011 which defines the structure of an ANC packet which carries ancillary data. RFC 8331- RTP Payload for Ancillary Data which provides detailed instructions on how to map ANC into RTP and SMPTE ST 2038:2008 which is used for the carriage of ancillary data packets in an MPEG2-TS.
SMPTE ST 2110-40 is based on RFC 8331 a draft IETF RTP Payload for SMPTE ST 291-1 Ancillary data with a few constraints of its use for live TV production.
All these four documents must be reviewed to fully understand how ancillary data is encapsulated into RTP.
ANC data packets in RTP
An ANC data packet consists of three 10 bit Ancillary Data flags (ADF), Data ID (DID), Secondary Data ID Number (SDID), Data Count, User Data words and checksum. See Figure below------------------
The first three words of the words are discarded that used to identify the start of the ANC packet on a digital interface or SPID as well as end-user data words are added to the payload of the RTP packet the checksum is included as well as edit and the RTP packet is ready for transport."
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Mapping the ANC data packets into RTP is relatively easy.
It used the same RTP header as defined in SMPTE ST 2110-10 see figure..., The ANC data packet as described in SMPTE ST 291-1:2011
RTP Packets in SMPTE ST 2110-40 uses the same RTP timestamp as the SMPTE ST 2110-20 the SMPTE Epoch, to keep everything in sync.
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