ANSI SCTE-21-2012 pdf download STANDARD FOR CARRIAGE OF VBI DATA IN CABLE DIGITAL TRANSPORT STREAMS
5.1 Use of Picture User Data The use of additional new data types within video user data is discouraged unless the data is inseparable from the video content and unlikely to be replaced or modified prior to decoder’s passive receipt of this data. In other words, it shall not be necessary to process the data before reception in the decoder and no processing should be required after reception other than using it for NTSC waveform reconstruction in the decoder.
The NTSC VBI data defined in this standard is an example of data that is by not any practical means separable from video and is unlikely to be replaced or modified. This type of VBI data is considered a part of the input video waveform that is digitized, compressed, transported, and reconstructed with an NTSC output (when the video mode is NTSC format). The VBI data encoded in video user data syntax is carried only to allow reconstruction at a receiver of the full original NTSC waveform as it was input to the MPEG-2 encoder. In all other cases, applications residing in digital receivers should receive their pertinent data via elementary stream components of a digital service (i.e. via separate PID streams) and must not rely on the user data structure.
5.2 Closed Captioning Digital television services in North America carry closed caption information as DTV Closed Captioning (DTVCC) standardized in the CEA-708 standard (Ref. [6]). CEA-708 provides carriage of CEA-608 caption data as a part of the DTVCC data structure. Placement of this structure within the video user data is standardized by ATSC A/53 Part 4 (Ref. [5]).
Many television services carry closed caption information in line 21, field 1 of the VBI. According to the CEA-608 standard, closed caption data is also carried in line 21 of field 2. As noted, these captions are also carried by CEA-708. Note 1: ANSI/SCTE 43 (reference [12]) specifies the ordering of closed caption structures that conform to this specification and SCTE 20, 2012 (reference [13]). Note 2: SCTE 20, 2012 (reference [13]) specifies another format for coding and transmission of closed captions that may be used in addition to what is specified here. Certain system service providers use the CEA-608 closed caption format to carry additional data in VBI lines other than line 21. The user data syntactic constructs described in this document allow multiple VBI lines per display field, including the standard line 21 closed caption usage.