Intelevision technology,Wide Screen Signaling (WSS)[1] is digitalmetadata embedded ininvisible part of theanalog TV signal describing qualities of the broadcast, in particular the intendedaspect ratio of the image. This allows television broadcasters to enable both4:3 and16:9 television sets to optimally present pictures transmitted in either format, by displaying them infull screen,letterbox,widescreen,pillar-box, zoomedletterbox, etc.[2][3]
This development is related to introduction of widescreen TVs and broadcasts,[3] with thePALplus[4] system in theEuropean Union (mid 1990s), theClear-Vision[5][6] system inJapan (early 1990s), and the need to downscaleHD broadcasts toSD in the US. Thebandwidth of the WSS signal is low enough to be recorded onVHS (at the time a popular home video recording technology). It is standardized on Rec.ITU-R BT.1119-2.[3]
A modern digital equivalent would be theActive Format Description, a standard set of codes that can be sent in aMPEG video stream, with a similar set of aspect ratio possibilities.
For625 line analog TV systems (likePAL orSECAM), the signal is placed in line 23.[3] It begins with a run-in code and starts code followed by 14bits of information, divided into four groups, as shown on the tables below (based on Rec. ITU-R BT.1119-2) :[7][8][9][10][1]
| Bits (0 to 3) | Aspect ratio | Picture placement inside the broadcast area | Active lines |
|---|---|---|---|
| 0000 | - | - | - |
| 0001 | Full format 4:3 |  | 576 |
| 0010 | Letterbox 16:9 top |  | 432 |
| 0011 | - | - | - |
| 0100 | Letterbox14:9 top |  | 504 |
| 0101 | - | - | - |
| 0110 | - | - | - |
| 0111 | Full format 14:9 centreshoot and protect 14:9 (see note) | | 576 |
| 1000 | Letterbox 14:9 centre |  | 504 |
| 1001 | - | - | - |
| 1010 | - | - | - |
| 1011 | Letterboxdeeper than 16:9 centre |  | undefined |
| 1100 | - | - | - |
| 1101 | Letterbox 16:9 centre |  | 432 |
| 1110 | Full format16:9anamorphic |  | 576 |
| 1111 | - | - | - |
Note: The transmitted aspect ratio is 4:3. Within this area a 14:9 window is protected, containing all the relevant picture content to allow a wide-screen display on a 16:9 television set.
| Bit | Item | Group |
|---|---|---|
| 4 | Camera Mode (interlaced) / PALplus Film Mode (progressive scan) | 2 - Enhanced Services |
| 5 | Conventional PAL / PALplusMotion Adaptative Colour Plus encoding | |
| 6 | No Vertical helper / PALplus Vertical helper present | |
| 7 | Reserved /Ghost cancellation | |
| 8 | No subtitles / subtitles withinteletext | 3 -Subtitles |
| 9 | No open subtitles / Subtitles in active image area | |
| 10 | Subtitles out of active image area / Reserved | |
| 11 | Nosurround sound / Surround sound mode | 4 - Reserved |
| 12 | No copyright asserted or status unknown / Copyright asserted | |
| 13 | Copying not restricted / Copying restricted |
525 line analog systems (likeNTSC orPAL-M) made a provision for the use of pulses for signaling widescreen and other parameters, introduced with the development ofClear-Vision (EDTV-II), a NTSC-compatible Japanese system allowing widescreen broadcasts.[11][12] On these systems the signals are present in lines 22 and 285, as 27 data bits, as defined byIEC 61880.[3][13][14][15][1][16]
The following table shows the information present on the signal, based on Rec. ITU-R BT.1119-2 ("helper" signals are EDTV-II specific):[3]
| Bit | Item |
|---|---|
| 1 | Reference signal |
| 2 | Reference signal |
| 3 | Aspect ratio (4:3 full format / 16:9 letterbox) |
| 4 | Even parity for B3 to B5 |
| 5 | Reserved |
| 6 | Field type (First field / Next field) |
| 7 | Frame type (Reference frame / Other frame) |
| 8 | Vertical temporal helper (no / yes) |
| 9 | Vertical high resolution helper (no / yes) |
| 10 | Horizontal helper (no / yes) |
| 11 | Horizontal helper pre-combing (no / yes) |
| 12 to 14 | For TV station use |
| 15 to 17 | Reserved |
| 18 to 23 | Error correction codes for B3 to B17 |
| 24 | Reference signal |
| 25 to 27 | Confirmation signal |
{{cite book}}: CS1 maint: multiple names: authors list (link)