The A and B rolls are made by the negative cutter in a synchronizer or sync block. There must be at least three gangs on the synchronizer. The cut workprint? (with the edge number code) is put in the first gang, and the A roll is started in the second gang, and the B roll started in the third gang. The negative cutter assembles the film, matching the code and length of the workprint with the corresponding original film, which has been left intact after the initial workprint was made. The sync block insures that all three rolls match with accurate to the single frame.
The negative cutter copies the workprint with the original film, alternating from A roll to B roll, with the blank (black) leader filling the space on the B roll when the A roll has picture, and visa versa.
The A and B rolls are both printed on the same roll of print stock. First the A rolls is set up at the "printer start" mark, the lights are turned off in the room, and the A roll and the printer stock are run through the contact printer. In this first pass, the shots on the A roll are exposed on the print stock. The print stock is then re-wound in to the feed magazine, and set up again with the "printer start" mark matched to the printer start on the B roll, and the print stock is exposed again.
Everything depends on the accuracy of the sprocket system to keep the two rolls in their proper orientation. For example, if in the setting up the B roll, the technician made a single frame mistake at the printer start mark, the B roll would overlap the A roll picture one frame at one end of the shot and there would be a single frame of black at the other end.The whole job would have to be done over on new print stock.
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The split reels holding the A & B rolls are separated by a plastic core which creates space between the reels so that they match the spacing of the sync block gangs.