https://www.nfsa.gov.au/sites/default/files/2025-02/Colour%20TV%20Mega%20Menu%203%20TVs_1000x745%20px.jpg
Bias line
The exposed line of soundtrack resulting from an unmodulated signal. 1
The normal rest position of a galvanometer gives an exposed line of half the available width. During quiet passages the average position is reduced by applying a direct current signal or bias current, to the galvanometer.
This current is adjusted so that without a signal the exposed area of the negative is about 0.05mm wide.
https://www.nfsa.gov.au/sites/default/files/uploads/2010/09/15/bias.jpg
References
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston
The National Film and Sound Archive of Australia acknowledges Australia’s Aboriginal and Torres Strait Islander peoples as the Traditional Custodians of the land on which we work and live and gives respect to their Elders both past and present.