Sound-on-film

Later, Case Laboratories and Lee De Forest attempted to commercialize this process, when they developed an Aeolite glow lamp, which was deployed at Movietone Newsreel at the Roxy Theatre in 1927.

[2] Opposite with variable-density, in the early 1920s, variable-area sound recording was first experimented on by the General Electric Company, and later was applied by RCA which refined GE's technology.

A two-channel audio signal is recorded as a pair of lines running parallel with the film's direction of travel through the projector's screen.

The image on the small slice of exposed track modulates the intensity of the light, which is collected by a photosensitive element: a photocell, a photodiode or CCD.

Because traditional incandescent exciter lamps produce copious amounts of infra-red light, and cyan tracks do not absorb infra-red light, this change required theaters to replace the incandescent exciter lamp with a complementary colored red LED or laser.

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 (grey areas) with the Dolby Double-D logo, between them. The two tracks of the analog soundtrack on the next strip are bilateral variable-area, where amplitude is represented as a waveform. These are generally encoded using Dolby Stereo matrixing to simulate four tracks. Finally, to the far right, the timecode used to synchronize with a DTS soundtrack CD-ROM is visible.
Left: Movietone track with variable density. Right: Variable area track.