Widescreen signaling
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.
625 line systems
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]
Group 1 (aspect ratio)
| b00 | b01 | b02 | b03 | Aspect ratio | Picture placement | Active lines |
|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | - | - | - |
| 0 | 0 | 0 | 1 | 4:3 | Full | 576 |
| 0 | 0 | 1 | 0 | 16:9 | Letterbox top | 432 |
| 0 | 0 | 1 | 1 | - | - | - |
| 0 | 1 | 0 | 0 | 14:9 | Letterbox top | 504 |
| 0 | 1 | 0 | 1 | - | - | - |
| 0 | 1 | 1 | 0 | - | - | - |
| 0 | 1 | 1 | 1 | 14:9 | Full-height 4:3, framed to be "14:9-safe" | 576 |
| 1 | 0 | 0 | 0 | 14:9 | Letterbox centre | 504 |
| 1 | 0 | 0 | 1 | - | - | - |
| 1 | 0 | 1 | 0 | - | - | - |
| 1 | 0 | 1 | 1 | >16:9 | Letterbox deeper than 16:9 | <432 |
| 1 | 1 | 0 | 0 | - | - | - |
| 1 | 1 | 0 | 1 | 16:9 | Letterbox centre | 432 |
| 1 | 1 | 1 | 0 | 16:9 | Full-height 16:9 (anamorphic) | 576 |
| 1 | 1 | 1 | 1 | - | - | - |
Group 2 (enhanced services)
| b04 | Mode |
|---|---|
| 0 | Camera Mode (interlaced) |
| 1 | Movie Mode (progressive scan) |
| b05 | Mode |
|---|---|
| 0 | PAL Standard |
| 1 | Colour Plus |
| b06 | Mode |
|---|---|
| 0 | No Vertical helper |
| 1 | Vertical helper present |
The above settings are related to PALplus.
| b07 | Ghost cancellation |
|---|
Group 3 (subtitles)
| b08 | Mode |
|---|---|
| 0 | no subtitles |
| 1 | Teletext subtitles |
| b09 | b10 | Mode |
|---|---|---|
| 0 | 0 | No subtitles |
| 1 | 0 | Subtitles inside active image |
| 0 | 1 | Subtitles outside active image |
| 1 | 1 | Reserved |
Group 4 (other)
| b11 | Mode |
|---|---|
| 0 | No surround sound information |
| 1 | Surround sound mode |
| b12 | Mode |
|---|---|
| 0 | No copyright asserted or status unknown |
| 1 | Copyright asserted |
| b13 | Mode |
|---|---|
| 0 | Copying not restricted |
| 1 | Copying restricted |
525 line systems
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]
| Bit | Item |
|---|---|
| B1 | Reference signal |
| B2 | Reference signal |
| B3 | Aspect ratio (4:3, 16:9) |
| B4 | Even parity for B3 ~ B5 |
| B5 | Reserved |
| B6 | Field type (first, next) |
| B7 | Reference frame (reference, other) |
| B8 | Vertical temporal helper (no, yes) |
| B9 | Vertical high resolution helper (no, yes) |
| B10 | Horizontal helper (no, yes) |
| B11 | Horizontal helper pre-combing (no, yes) |
| B12~14 | for TV station use |
| B15~17 | Reserved |
| B18~23 | Error correction codes for B3-B17 |
| B24 | Reference signal |
| B25~27 | Confirmation signal |
See also
References
- Loncaric, Matej; Tralic, Dijana; Brzica, Maja; Petrovic, Juraj; Grgic, Sonja (September 17, 2009). "Managing mixed HD and SD broadcasting". pp. 79–82 – via IEEE Xplore.
- https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1119-2-199802-I!!PDF-E.pdf
- http://www.divitec.se/sites/default/files/microvideo_wss-dec.pdf
- https://www.dmi.unict.it/~battiato/download/IISFAMemberbook2011.pdf
- ETSI EN 300 294 "Television Systems: 625-Line Television Wide Screen Signaling (WSS)"
External links
- Renesas AN9716, Widescreen Signaling (WSS) covering 625 lines and 525 lines standard.