Reading CEA-608/708 closed captions from SDI¶
Closed captions (CC) are text representation of the audio part of a TV program, movie, etc. It is a transcription or translation of the dialogue, sound effects, some relevant musical cues, and other relevant audio information in case when sound is unavailable or not clearly audible. Initially, closed captions were designed for deaf and hard of hearing people.
Closed captioning information is encoded within the stream and a viewer can turn them on or off if necessary. Different video protocols support different captioning standards.
For more information, see: Subtitles.
Flussonic is able to detect CEA-608/708 closed captions in SDI source streams and to read them. It is done automatically, so there's no need to configure it explicitly.
What Flussonic does is read the CEA-608/708 captions from an SDI stream, performs repackaging and then carries them within the MPEG-TS stream as an H.264 SEI NALU: H.264 file consists of a number of NAL Units, i.e., Network Abstraction Layer Units, SEI refers to Supplemental Enhancement Information.
CAE-608 is a streaming, character-based format that allows for the transmission of up to 4 simultaneous channels of data. Flussonic adds 4 text tracks to those 4 channels (one for every channel). As a result, we have 4 text tracks and one video track carrying CEA-608/708 closed captions. You can later play those text tracks via WebVTT or TTML together with HLS, DASH, etc. video streams.
Flussonic captures SDI streams with the help of Decklink SDI, Stream Labs SDI, AJA SDI and Magewell SDI cards.