Motion Picture Audio

There are several different audio formats used for motion picture sound. The oldest and most reliable is the analog optical soundtrack photographed on the film next to the picture, also known as "sound on film". This modulated track passes through a beam of light modulating the light. A photo-electric pickup device senses the modulated light producing a small voltage. The voltage from the pickup is amplified through a standard analog amplifier. Photo-electric pickups vary from the original cells that required aprox 100VDC to operate to the most recent stereo cells that produce excellent fidelity.
Other types of sound on film include magnetic stripes along the sprocket holes for 4-track audio but is no longer used. And digital audio photographed on the edge of the film in the form of speckled spots. However, the sample and bit rate is limited by film resolution and the audio quality tends to sound harsh.
DOLBY is registered trademark of Dolby Labs


The optical track shown is mono, both tracks have identical modulation. On a stereo track, one track would be the left channel and the other track the right channel. The left channel feeds the left amplifier and speaker. The right channel feeds the right amplifier and speaker. Then, to get a center channel you mix the left and right channels in phase via a summing (mixing) amplifier that isolates the channels to a center amplifier and speaker. To get surround you mix the left and right channels out of phase with a difference (mixing with inputs out of phase) amplifier to a surround amplifier and speakers. The Dolby© surround decoder can produce some amazing directional sound with this simple system.
Noise Reduction, Dolby® noise reduction helps reduce surface noise on the optical track caused by light scratches or dirt. New prints can be run without noise reduction without objectionable noise.
Noise reduction also adds dynamic range to the audio but sometimes to the extreme where low voices can hardly be heard and explosions blow your ear drums. In some cases, the audio might be more enjoyable without the added dynamic range. Films can be run without noise reduction.

Other film audio formats involve digital processing of some sort, audio on CD for instance. Digital audio does provide some extra audio channels for surround. For the most part, the general public won't know the difference between a good analog or digital system.