Where did the term MOS come from and what does it mean? The term MOS is used when a scene is filmed without sound. Some people on movie sets define MOS as “Mit Out Sound” while some people refer to it as Motor Only Sync. It is a standard film jargon on a movie set. It is used during film production to indicate filming that has no audio track. The audio track is usually recorded on a separate audio divice from the camera. When the sound recorder is not rolling with the camera then that is MOS.

When sound is omitted while recording a shot, it saves a lot of time and allows the crew to do other work and keep moving as well as not having to take multiple takes because of exterior noise like planes and gardeners. This is what makes MOS more and more common during film shoots when the subjects of the take don’t need to speak.

An MOS takes can be fun as the can be used with miscellaneous sounds recorded on location, the musical sound track, voiceovers or sound effects that are created by a Foley artist. All of this is added in post production (editing).

Origin of the term

There are different sources that have explanations for the abbreviation of MOS. It could be “Mit Out Sound” some say a German director came up with the term and Mit is German for with. And according to Google translate this is true.

“When sound recording reached the point where the sound was recorded on a synchronized but separate piece of media (such as 35mm film, audio tape, or other media) a method of keeping the recording media and camera film “in sync” was needed. The solution was to use a special form of motor which has multiple “windings” in it, and which can be connected to another identical motor in such a way that turning one motor a certain distance will turn the other motor exactly the same distance. The motors did not have to be close together, and, with appropriate circuitry, did not have to be of the same size or power. These motors were called selsyn (self synchronous) motors. A system was created where a single sound recording room could be connected to any of the stages on a studio lot (you can still see the connection points on some of the oldest stages.) The sound mixer (sound man) on stage connected the control panel to the recording room and the camera. There was a selsyn motor on the camera and it was linked to a matching selsyn motor on the sound recording equipment at another point on the studio lot.
In order to use this system, the sound mixer used an intercom to the sound recordist to tell him to “roll”, or start the system. Since this was a very mechanical system, it took some time to start and get up to proper speed. When proper speed and synchronization was reached, the recordist would use the intercom to announce, “Speed” and the sound mixer would relay that to the director and crew on the stage. The expression is still used, but now simply means, “Sound is recording”.
It was the recordist who actually started and stopped the camera motor (the camera operator had a switch to ensure that the camera didn’t roll at an inopportune time such as loading, replacing lenses, etc., and to stop it if something went amiss). The actual power source for the camera motor was in the sound booth.[2][3]
When a shot was planned that did not require sound, the sound mixer would ask the recordist to “roll the motor only”. The recordist would start the camera motor without starting the matching “sound” motor and electronics. The procedure, allegedly, acquired the name “motor only shot”, thus MOS.”
From Wikipedia on MOS more info here https://en.wikipedia.org/wiki/MOS_(filmmaking)

Which ever you think is true the important part is to know what MOS means so you look competent on a set. The clearer you are on a movie set about what people are saying and doing the easier and more comfortable you will be. 

For more definitions of words in filmmaking see other posts. SYNC IN FILMSOUND IN FILM,Camera History

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