In television technology, widescreen signaling (WSS) is a digital stream embedded in the analog TV signal describing qualities of the broadcast, in particular the intended aspect ratio of the image. This can be used by a widescreen TV or other device to switch to the correct display mode.
PAL and SECAM Systems
For 625 scan line systems (PAL and SECAM), the signal is placed in line 23. It begins with a run-in code and start code followed by 14 bits of information.[1]
Group 1 (Aspect Ratio)
| b00 | b01 | b02 | b03 | Aspect ratio | Picture placement | Active lines |
|---|
| 0 | 0 | 0 | 1 | 4:3 | Full | 576 |
| 1 | 0 | 0 | 0 | 14:9 | Letterbox centre | 504 |
| 0 | 1 | 0 | 0 | 14:9 | Letterbox top | 504 |
| 1 | 1 | 0 | 1 | 16:9 | Letterbox centre | 432 |
| 0 | 0 | 1 | 0 | 16:9 | Letterbox top | 432 |
| 1 | 0 | 1 | 1 | >16:9 | Letterbox deeper than 16:9 | <432 |
| 0 | 1 | 1 | 1 | 14:9 | Full-height 4:3, framed to be "14:9-safe" | 576 |
| 1 | 1 | 1 | 0 | 16:9 | Full-height 16:9 (anamorphic) | 576 |
Group 2 (Enhanced Services)
| 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.
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)
| b12 | Mode |
|---|
| 0 | No copyright asserted or status unknown |
| 1 | Copyright asserted |
| b13 | Mode |
|---|
| 0 | Copying not restricted |
| 1 | Copying restricted |
NTSC Systems
525 scan line systems (PAL-M and NTSC) put the WSS data in lines 20 and 283. A start code consisting of the bits "10" is used, followed by 14 bits of information.
Word 0
| b0 | Aspect Ratio |
|---|
| 0 | 4:3 |
| 1 | 16:9 |
| b1 | Mode |
|---|
| 0 | Normal |
| 1 | Letterbox |
b2-b5 all have the value "0".
Word 1
b6-b9 are reserved and all have the value "0".
Word 2
b10-b13 are reserved and all have the value "0".
References
- ETSI EN 300 294 "Television Systems: 625-Line Television Wide Screen Signaling (WSS)"