Sync Playback
The key word here is sync. To have performers dance and mouth lyrics beat for beat with a prerecorded track requires 100% sync at all production and post-production levels.
A master soundtrack is created, and recorded with timecode. This track will generate the final version soundtrack that will be used by the editor in the finished product. Generated from this (final version) soundtrack will be two or more playback dupes, for use on the set.
On the set, the playback dupe is played back off of a digital recorder or computer. Do not use standard audio CD's as your playback source; their playback speed can be varied. The track is either amplified through a speaker system or silently broadcast to the performers via induction loop cueing. The camera films at crystal sync, sound speed or in dgital video. Timecode information from the playback track is displayed to the camera via a timecode clapstick slate.
Later on, in post, the editor can use the timecode slate reference to match up the picture footage with the corresponding location on the master soundtrack. Note that if the camera speed is 24 fps or true 24p video (not 23.976 which video sometimes calls 24p), then the soundtrack will be out of sync by one tenth of one percent. To deal with this problem, the playback track must be pulled up (speeded up) so that the playback speed of the music, on the shooting set, is actually one tenth of one percent faster than the master soundtrack that will be used in editing. Some playback recorders can do this on the fly; otherwise it must be done in the studio when the playback dupes are created.
If the video camera runs at 29.97 or even 23.976, then it becomes a real-time playback situation and does not require any pull-up on the set.
Moral of all of this: Do not attempt to be in charge of audio on a sync playback shoot until you get some real experience; and always consult carefully with post-production before you shoot so that you can determine what settings to use.
In conclusion
Production Sound is not an endeavor to be taken lightly. Electronic knowledge and a mastery of studio (music) techniques are not qualifications in themselves for successfully mixing dialogue on a shooting set. Life in the Recording Studio is based on achieving excellence under controlled conditions. Mixing on a Production Set is a matter of generating usable raw material, in what are usually uncontrollable conditions.
But with the right approach, equipment, and trained personnel -- a Production Mixer can achieve excellence.
