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The Sound Negative
In professional films, the sound usually consists of lip-synchronised dialogue (p. 129) and is recorded on a separate sound negative which is edited to match the picture negative. When the sound track consists of music, sound effects or commentary, however, you can record the sound later in synchronism with the projection of the cutting copy. Ideally the sound negative will then require no editing, but you can correct any timing errors by adding or removing parts where necessary.
You can check the sound negative quite simply by running it through any sound projector. Correctly threaded, it will sound quite normal. However, because it is a negative it is particularly susceptible to the effects of dirt and scratches. The noise reduction bias applied during recording is designed to minimise the effect of dirt on the positive. Consequently the bias, which produces slow changes in track width or density, actually emphasises any defects in the negative. So handle it with great care.
The necessity for editing of the sound negative has been further reduced by the widespread adoption of magnetic recording in many of the recording studios. Sound is recorded first on magnetic tape which can be checked by
Noise reduction bias cuts down the light transmitted by the sound track in quiet passages and so reduces the effect of dirt and scratches. These variable density (left) and variable area (right) tracks both show how the light valve has gradually re-closed after recording the loud sounds.
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