Even in the silent film era, films were shown with sounds, often with musical accompaniment by a pianist or an orchestra keeping time with the screen action. The first synchronization was a turning recording device marked with a white spot. As the white spot rotated, the cameraman hand cranked the camera to keep it in sync with the recording. The method was then repeated for playback, but with the projectionist hand cranking the film projector. "Single-system" sound recorded sound optically to part of the original camera film, or magnetically to a stripe of magnetic coating along the film edge.
"Double-system" sound used independent cameras and sound recorders. The first sync sound standard used recorders and cameras both powered by AC (alternating current) motors - essentially clock motors.
Later the 50 Hz or 60 Hz sine wave, called a Pilottone, was recorded on a second parallel track of an audio recorder.
In double-system film, speed variations of camera and recorder, as well as the elasticity of the magnetic recording tape, requires some positive means of keying the dialogue to its appropriate film frame. The inclusion on the sound recorder of a second, parallel, sync or "Pilotone" track, has been the most common method in use until today. In video recording, synchronism is electronically generated and generally called dual-system sound 
If location, a camera is driven by a DC motor, with some sort of governor control to hold it fairly accurate at 24 fps, a sync pulse generator geared to the movement or motor shaft could be employed to provide the sync pulse output. A cable conducts the sync pulse from camera to sound recorder. The sync pulse is typically a sine wave of 50 or 60 Hz with an RMS amplitude of approximately 1 volt.
This double-system audio recording could then be transferred or "resolved" to sprocketed magnetic film, with sprocket holes that match one to one with the original camera film. These two sprocketed media could be run through a "Moviola" or flat-bed editing table such as the Steenbeck for synchronous sound editing. 
With the introduction of the Bulova "Accutron" watch that used a tuning fork as a time reference (watches later used an oscillating electronic crystal), the camera no longer needed to be connected to the sound recorder with a cable. The camera speed was controlled by one oscillator, and a second oscillator in the recorder generated the Pilotone.
This method was developed in the 1960s by pioneering filmmaker Richard Leacock. It was called Direct Cinema. Filmmakers abandoned the studio and went out on location to film, often with hand-held cameras. 
In 1972, Bell & Howell brought out a consumer version of a double-system Super-8 sound filmmaking system called "Filmosound". A compact cassette recorder was attached to the camera with a cable that transmitted a single pulse to the recorder every time a new frame of film was exposed in the camera. On playback, the cassette recorder pulse was used to control the projector speed.
At that time, Ricky Leacock, a professor in the MIT architecture department film section, developed a Super-8 film production system with a crystal-controlled camera, a crystal-generated Pilotone cassette recorder, a sprocketed magnetic film recorder, a flatbed editing table, and a projector. The MIT/Leacock System was funded with a $300,000 grant from the founder of Polaroid, Edwin Land.
In 1973, the one-pulse-per-frame technique was used to control recording directly onto sprocketed magnetic film in the Super8 Sound Recorder. The Super8 Sound Recorder could also "resolve" sound that had been recorded onto cassette tape with this new "digital" sync pulse.
Sync sound in Asia
Most Bollywood films from the 1960s onwards do not employ this technique and for that very reason the 2001 films Lagaan and Dil Chahta Hai were noted for the use of sync sound. The common practice in Bollywood is to 'dub' over the dialogues at the post-production stage. Currently, Indian sound designers and sound mixers like Resul Pookutty, Nakul Kamte, Dileep Subrahmaniam, Debashish Mishra, Indrajit Neogi, Arun Nambiar, Baylon Fonseca, Manas Choudhary and many others among the fresh talent are endeavoring to change this.
The first Indian talkie, Alam Ara (1931), made the first use of sync sound in India and subsequent Indian films were regularly shot in sync sound with the silent Mitchell camera, until the 1960s. With the arrival of the Arri 2C, a noisy but more practical camera particularly for outdoor shoots, 'dubbing' became the norm and was never reversed.
Use of this technique has increased in recent times with the development in film techniques and instrumentation like Arri Blimp cameras. This system has been used in the Hindi movie Jodhaa Akbar.
- A Very Short History of the Transition from Silent to Sound Movies at Wonderstruck
- Steven Ascher and Edward Pincus, The Filmmaker's Handbook, Penguin, 1999
- Dual-System Sound – article at Videomaker
- American Cinematographer Manual, Fourth Edition, p.183
- Mag Film – definition at Film & Sound
- Cinema verite/Direct Cinema – description at Berkley University
- Super8 Sound Catalog, 1975 edition
- Robert O. Doyle, "The Need for Sync Sound Standards", American Cinematographer, November 1975