Edge of a 35mm film print showing the soundtracks. The outermost strip (left of picture) contains the SDDS track as an image of a digital signal; the next contains the perforations used to drive the film through the projector, with the Dolby Digital track between them. The two tracks of the analog soundtrack on the next strip are variable-area ( RCA Photophone), where amplitude is represented as a waveform. At present, these are generally encoded using Dolby SR matrixing to simulate four tracks. Finally, to the far right, you can see the timecode used to synchronize with a DTS soundtrack CD.
Sound-on-film refers to a class of sound film processes where the sound accompanying picture is physically recorded onto photographic film, usually, but not always, the same strip of film carrying the picture. Sound-on-film processes can either record an analog sound track or digital sound track, and may record the signal either optically or magnetically.
Analog sound-on-Film recording
The most prevalent modern method of recording sound on a film print is by stereo variable-area (SVA) recording, encoding a two-channel audio signal as a pair of lines running parallel with the film's direction of travel through the projector. The lines change area (grow broader or narrower) depending on the magnitude of the signal. The projector shines a light or LED, called an exciter, through a perpendicular slit onto the film. The image on the small slice of exposed track modulates the intensity of the light, which is collected by a photosensitive element, a photodiode or CCD.
Commonly, the audio signal recorded onto an SVA track is encoded through a phase matrix, which allowed the two-channel format to record a center and surround channel, and companding noise reduction, which allows a constant signal-to-noise ratio to be delivered over a wide dynamic range.
Earlier processes, used on 70mm film prints and special presentations of 35mm film prints, recorded sound magnetically on ferric oxide tracks bonded to the film print, outside the sprocket holes.
Sound-on-film formats
Almost all modern motion picture sound formats are sound-on-film formats, including:
Optical analog formats
- Fox/Western Electric (Westrex) Movietone, are variable-density formats of sound film. (No longer used, but still playable on modern projectors.)
- RCA Photophone, a variable-area format now universally used for optical analog soundtracks - since the late 1970s, usually with a Dolby encoding matrix.
Encoding matrices
Optical digital formats
Obsolete formats
Further reading
See also
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