VBIInfo
From DaphneWiki
Vertical Blanking Interval (VBI) Information
Laserdiscs have things like frame numbers and stop codes encoded in the VBI portion of the NTSC (or PAL) signal. This info is available in the official laserdisc standard document (IEC 60857) and is summarized here.
24-bit biphase coded signal
A 24-bit value can be stored on one line, and laserdiscs have various 24-bit values stored that represent various things. Each 24-bit value must have the most significant bit set to 1 by definition. The first nibble of each value is the "key", and the first bit after the first nibble is the 'stop bit' (see more about this below). Each 'bit cell' is 2 microseconds long with the digital level between 0 and 100 IRE. I think what that means is that the 24 bits will take 48 micro seconds, and the square wave that defines these bits can go from totally black (0) to totally white (100).
This screenshot represents line 17 of frame 587 from the Dragon's Lair NTSC laserdisc. It has been stretched vertically to make is easier to see. It resolves to a value of F80578. The red lines (not present on the video signal) show where the boundary of each bit cell is. Notice that there is black space before and after the bit cells. This is one reason why the first bit must always be a 1, so that the first bit cell can be reliably found.
http://www.daphne-emu.com/graphics/F80578_manchester.png
24-bit values (NTSC)
| Description | 24-bit value | Lines |
| Lead-In | 88FFFF | 17, 18, 280, and 281 |
| Lead-Out | 80EEEE | 17, 18, 280, and 281 |
| Picture Number (frame number) | A BCD-encoded number OR'd with (F00000 OR (bNoStop << 15)), where bNoStop is 1 if we don't want to stop or 0 if we do. For example, picture number of 10 with bNoStop set, would be F80010. | 17 and 18 OR 280 and 281 depending on which field is the first field for the picture. |
| Picture Stop Code | 82CFFF. Must come one field after a picture number code, to enable stopping on that picture. | 16 and 17 or 279 and 280. |
| Chapter Number | A BCD-encoded number OR'd with (800DDD OR (bNoStop << 15)), where bNoStop is the same as in the PictureNumber description. For example, a chapter number of 1, with bNoStop set would be 881DDD. | 17, 18, 280, and 281 of active fields that do not have picture numbers. Stop codes also take precedence for lines 17 and 280. |
