3:2 Pulldown. This is where it gets frankly amazing. You all know that for NTSC they do the perverse 3:2 pulldown, wherin you use a film frame for the first 3 fields, then grab the next frame and use it for 2 fields, and then start again. (Grabbing the next frame is referred to as 'pulling it down' hence the 3:2 pulldown name. They used to just repeat the 4th frame which gives the awful juddering you can see on some early telecines.
Now to do this on Laserdisc, they player has to know which two adjacent fields actually make up the frame (othewise you might get one field from one frame of film, and the other field from a diferent frame - not good). So how does an analogue system cope with this? Easy, encode the required information in the VBI (the vertical blank interval). When making the disc, you store the info in the VBI, its often referred to as a 'white flag'. When you hit the pause button on a CAV disc, it reads the flag, and the laser assembly actually does a one track reversal (i.e. 2 fields) and can then redisplay the current frame. It is set in the VBI area outside of picture info, or CC info (its at line 11 or 274 depending on the field). If you get it wrong, the pause feature will have a 'jiggling' frame for 40% of the frames! You can see this on some discs. Sometimes just the 'picture number' is used instead, which is also encoded into the VIL.
Now to do this on Laserdisc, they player has to know which two adjacent fields actually make up the frame (othewise you might get one field from one frame of film, and the other field from a diferent frame - not good). So how does an analogue system cope with this? Easy, encode the required information in the VBI (the vertical blank interval). When making the disc, you store the info in the VBI, its often referred to as a 'white flag'. When you hit the pause button on a CAV disc, it reads the flag, and the laser assembly actually does a one track reversal (i.e. 2 fields) and can then redisplay the current frame. It is set in the VBI area outside of picture info, or CC info (its at line 11 or 274 depending on the field). If you get it wrong, the pause feature will have a 'jiggling' frame for 40% of the frames! You can see this on some discs. Sometimes just the 'picture number' is used instead, which is also encoded into the VIL.