Existing MPEG-2 TS content does not always contain the ‘caption_services_descriptor’ to signal when captions are present. Caption tracks should be sourced in this case. This can be done as follows:

Section 3.2. The CEA 708 caption service [CEA708] as identified by:
- A 'Caption Service Descriptor' [ATSC65] in the 'Elementary Stream Descriptors' in the PMT entry for a video stream with stream type 0x02 or 0x1B.

- For stream type 0x02, the presence of caption data in the ‘user_data()’ field [ATSC52].

- For stream type 0x1B, the presence of caption data in the ‘ATSC1_data()’ field [SCTE128-1].

Section 3.3. Language: @kind is “captions”:

- Content of the 'language' field for the caption service in the 'Caption Service Descriptor’.

- If a ‘caption_service_descriptor()’ is not present:

o @language of the audio track where @kind is “main” for the first caption service
o The empty string for all other caption services

[ATSC52] http://www.atsc.org/cms/standards/a53/a_53-Part-1-6-2007.pdf
[SCTE128-1] http://www.scte.org/documents/pdf/Standards/ANSI_SCTE%20128-1%202013.pdf