Aspect ratio signaling in an analog television signal
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In television technology, widescreen signaling (WSS) is a 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 letterbox, widescreen, pillar-box, zoomed letterbox, etc.[1][2]
This development is related to introduction of widescreen TVs and broadcasts[2] and with the PALplus system in Europe (mid 1990s) and the need to downscale HD broadcasts to SD in the US. The bandwidth of the WSS bits is low enough to be recorded on VHS (at the time a popular home video recording technology).
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.
For 625 scan line systems (like PAL or SECAM), the signal is placed in line 23.[2] It begins with a run-in code and start code followed by 14 bits of information, divided into groups as shown on the tables below:[3][4]
525 line systems (like NTSC or PAL-M) made a provision using pulses for signaling widescreen and some other parameters in a similar manner to PAL and SECAM. Players output them and NTSC compatible TVs (including multiformat) recognize them. On these systems the signals are present in lines 22 and 285.[2]
The following table shows the information present on the signal:[2]
