RCA Photophone was the trade name given to one of four major competing technologies that emerged in the American film industry in the late 1920s for synchronizing electrically recorded audio to a motion picture image.

RCA Photophone was an optical sound, "variable-area" film exposure system, in which the modulated area (width) corresponded to the waveform of the audio signal.

When Joseph P. Kennedy and other investors merged Film Booking Offices of America (FBO) with the Keith-Albee-Orpheum theater chain and Radio Corporation of America; the resulting movie studio RKO Radio Pictures used RCA Photophone as its primary sound system.

In 1925, GE began a program to develop commercial sound-on-film equipment based on Hoxie's work.

For nearly half a century, motion picture sound systems were licensed, with two major licensors in North America, RCA and Western Electric (Northern Electric, in Canada), which licensed their principal sound element (original track negative) recording systems on a non-exclusive basis.

Variable-density was preferred for Technicolor sound prints as this process utilized a silver gray-scale "key" record, thereby creating a CMYK color image, and the soundtrack was also a silver gray-scale record, which greatly facilitated variable-density (and made variable-area rather difficult).

As a consequence by the early 1970s several individuals and organizations were considering improvements to the dated technical standards used for optical sound on 35mm film.

In the mid-1970s, Westrex Corp. (a wholly owned subsidiary of Litton Industries since 1956, and the successor to Western Electric's cinema sound business unit) re-introduced the ca.

When encoded utilizing Dolby Laboratories' technology (itself originally being in part licensed from Sansui), the discrete L and R channels of Westrex's stereo variable-area system were renamed "Left Total" and "Right Total", and when decoded utilizing Dolby's Cinema Processor these produced the L, C, R and S sound image first commonly used by Fox's CinemaScope magnetic stereo system in 1953.

An alternative is LaVezzi's VKF ("Very Kind to Film") sprockets, which perform optimally on KS- as well as CS-perforated prints.

Nearly all original track negatives (OTNs) are now produced as stereo variable-area, and the former Western Electric (Westrex) system has been renamed Photophone and has become the de facto standard for analogue optical soundtracks, world-wide.

The fully degenerate case of stereo variable-area (i.e., no 4-2-4 encoding/decoding, and left total/left equal to right total/right) produces a conventional "dual-bilateral" mono 1.0 track.