Contemporary with these developments, the movie industry was engaged in a frantic race to develop practical methods of recording synchronised sound for films. Early attempts — such as the landmark 1927 film The Jazz Singer – used pre-recorded discs which were played in synchronisation with the action on the screen. By the early 1930s the movie industry had almost universally adopted the "sound-on-film" technology (developed by Western Electric and others) in which the audio signals picked up by the microphones were modulated via a photoelectric element to generate a narrow band of light, of variable width or height, which was then captured on a dedicated 'audio' strip on the edge of the film negative, as the images were being filmed. The development of sound of film also enabled movie-industry audio engineers to make rapid advances in the process we now know as "multi-tracking", by which multiple separately-recorded audio sources (such as voices, sound effects and background music) could be replayed simultaneously, mixed together, and synchronised with the action on film to create new 'blended' audio tracks of great sophistication and complexity. One of the best known example of a 'constructed' composite sound source from this era is the famous "Tarzan yell", originally created for the RKO Picture series of Tarzan movies, starring Johnny Weissmuller.