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Bias Lines

From Chace Audio

Bias lines are the vertical lines found running the length of the soundtrack area of optical film. Shining light through a valve onto unexposed optical film creates the bias lines; there can be one or two bias lines depending on the format. In quiet or silent sections of the track, the bias lines will be thin and will not waver. When sound is present, the light valve will open and close, causing the bias lines to modulate to form a waveform and a sine wave. The larger the waveform, the louder its volume; the more rapid the sine wave, the higher its frequency. The sound we hear is a combination of the two.

The main purpose of the constant bias line is to have a reactionary base line for silence that provides a starting point for sound and frequency modulation. In these terms, they perform much the same function as the bias frequency on magnetic tape or film.

If no bias line were present on optical film - only areas of modulation followed by blank sections of track - the track would sound unnaturally clamped and gated when printed, as no light would pass through the quiet areas when reading the sound track because those areas would be completely black on the print. This clamping and gating would also occur if an optical sound track print (OSTP) is over exposed or if the light valve on an optical recorder is not set up correctly, thereby creating abnormally thin bias lines on an optical sound track negative (OSTN). When an OSTN is created with this flaw, it cannot be corrected.

Without program, bias lines appear as barely visible vertical lines in the sound track area of this optical positive. (Click on the photo for a magnified view).

biaslines.jpg

During program, the bias lines modulate into a waveform. (Click on the photo for a magnified view).

biaslinesmodulating.jpg