In television technology, Wide Screen Signaling (WSS) is digital metadata embedded in invisible part of the analog TV signal describing qualities of the broadcast, in particular the intended aspect ratio of the image. This allows television broadcasters to enable both 4:3 and 16:9 television sets to optimally present pictures transmitted in either format, by displaying them in full screen, letterbox, widescreen, pillar-box, zoomed letterbox, etc.
This development is related to introduction of widescreen TVs and broadcasts, with the PALplus system in the European Union (mid 1990s), the Clear-Vision system in Japan (early 1990s), and the need to downscale HD broadcasts to SD in the US. The bandwidth of the WSS signal is low enough to be recorded on VHS (at the time a popular home video recording technology). It is standardized on Rec. ITU-R BT.1119-2.
A modern digital equivalent would be the Active Format Description, a standard set of codes that can be sent in a MPEG video stream, with a similar set of aspect ratio possibilities.
625 line systems For 625 line analog TV systems (like PAL or SECAM), the signal is placed in line 23. It begins with a run-in code and start code followed by 14 bits of information, divided into four groups, as shown on the tables below (based on Rec. ITU-R BT.1119-2) : +Group 1 - Aspect ratio 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 Letterbox 14:9 top 504 0101 - - - 0110 - - - 0111 Full format 14:9 centre shoot and protect 14:9 (see note) 576 1000 Letterbox 14:9 centre 504 1001 - - - 1010 - - - 1011 Letterbox deeper than 16:9 centre undefined 1100 - - - 1101 Letterbox 16:9 centre 432 1110 Full format 16:9 anamorphic 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. +Group 2 to 4BitItemGroup4Camera Mode (interlaced) / PALplus Film Mode (progressive scan)2 - Enhanced Services5Conventional PAL / PALplus Motion Adaptative Colour Plus encoding6No Vertical helper / PALplus Vertical helper present7Reserved / Ghost cancellation8No subtitles / subtitles within teletext3 - Subtitles9No open subtitles / Subtitles in active image area10Subtitles out of active image area / Reserved11No surround sound / Surround sound mode4 - Reserved12No copyright asserted or status unknown / Copyright asserted13Copying not restricted / Copying restricted
525 line systems 525 line analog systems (like NTSC or PAL-M) made a provision for the use of pulses for signaling widescreen and other parameters, introduced with the development of Clear-Vision (EDTV-II), a NTSC-compatible Japanese system allowing widescreen broadcasts. IEC 61880 prescribes the signals to be sent in lines 22 and 285 as 27 data bits. Other sources define lines 20 and 283 for this purpose.
The following table shows the information present on the signal, based on Rec. ITU-R BT.1119-2 ("helper" signals are EDTV-II specific): 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
See also PALplus Clear-Vision Active Format Description (AFD) Teletext
References
External links Renesas AN9716, Widescreen Signaling (WSS) covering 625 lines and 525 lines standard.
de:Wide Screen Signalling