Quite often it is essential to shoot musical numbers to playback, the logistics of recording music and shooting film or video simultaneously being too demanding

for reasonable budgets and time constraints. A good example of this is the LipDub as our contest pointed out.

To accomplish this, a playback tape is prepared, often a special mix emphasizing the elements to be lipsynced, and two recorders are usually needed, one for playback and one for recording. The recording machine records a slate from an open microphone and then rerecords the playback directly from the second machine, thus ensuring a reference for picture– sound sync. This also permits starting in the middle of a long number for a given shot, and a record of exactly what part of the song the picture is to match is recorded.
This method involves hand matching of the recorded portions with the original recording. Synchronization may also be accomplished by using time code on the pre- recorded tape and recording it to a track of the audio recorder as a reference. Some software can be used then to sync up the shots to the original recording.
For shooting to playback, a number of considerations apply:
The performers must be reasonably close to the playback loudspeakers, say within 20 ft, so there is no time delay associated with the air path for their lipsyncing.
For dance numbers, occasionally thumpers have been found useful, that is, low-frequency transducers that put vibratory pulses out into a dance floor so the dancers can follow the beat. The advantage of this approach is that ordinary recording can occur, and the low-frequency energy can be filtered out of the recording, leaving a synchronized dance number to music and also well-recorded direct sound.