DVB decoders require a teletext subtitling descriptor in the PMT so that they know which PID is used for subtitling of which language. The DVB Subtitling specification ETSI EN 300 743 defines that for each subtitle service a subtitling_descriptor as defined in EN 300 468. The subtitling descriptor shall be included in the PMT of the program and shall be associated to the PID that conveys the subtitle stream.
Teletext Descriptor Structure (Tag: 0x56)
| Field | Length (bytes) | Description |
|---|---|---|
| descriptor_tag | 1 | 0x56 (Teletext descriptor) |
| descriptor_length | 1 | Length of the descriptor following this field |
| teletext_service_loop | N × 5 | Repeated for each teletext service |
- Each teletext service entry (5 bytes) contains:
| Field | Length (bytes) | Description |
|---|---|---|
| ISO_639_language_code | 3 | 3-character language code (e.g., “eng”) |
| teletext_type | 1 (4 bits) | Type of teletext (e.g., 0x02 = subtitles) |
| teletext_magazine_number | 1 (3 bits) | Magazine number (1–8) |
| teletext_page_number | 1 (8 bits) | Page number in hexadecimal (e.g., 0x29 for page 029) |
The teletext_type values relevant for subtitles include:
- 0x02: Teletext subtitles
- 0x05: Teletext subtitles for hearing impaired
Example
To represent Spanish subtitles on teletext page 888 and English subtitles on teletext page 889 in the Teletext Descriptor (tag 0x56) within the PMT, two teletext service entries are required:
| Field | Value | Description |
|---|---|---|
| descriptor_tag | 0x56 | Teletext descriptor |
| descriptor_length | 0x0A | 10 bytes (2 entries × 5 bytes) |
| ISO_639_language_code | 0x65 0x73 0x70 | “esp” for Spanish |
| teletext_type | 0x05 | Teletext subtitles for hearing impaired |
| magazine_number | 0x08 | Magazine 8 (from page 888 → 8) |
| page_number | 0x88 | Page 888 (hex 0x88) |
| ISO_639_language_code | 0x65 0x6E 0x67 | “eng” for English |
| teletext_type | 0x05 | Teletext subtitles for hearing impaired |
| magazine_number | 0x08 | Magazine 8 (from page 889 → 8) |
| page_number | 0x89 | Page 889 (hex 0x89) |
This page was last updated on 2025-07-23