Obscura

30. MOS

With today's equipment, the term "Motor Only Sync" doesn't make much sense, and, since "Mit Out Sound" is a much more amusing explanation, I recommend it highly. There's even a version that credits it, in some detail, to Fritz Lang. I'll have to see if I can dig that up. But in the meantime, here's a post from rec.arts.movie.tech that sums it up better than I could:
=======================
Subject: Re: Origin of the term "MOS" ??
From: peterh5322@aol.com (Peter)
Date: 08 Jan 2001 17:57:33 GMT
--------

johnw248@aol.com (JohnW248) commented ...

>>
> What does it REALLY stand for??????

Motor only synch

It used used (or was) in the days when the sound department
ran the camera motors and a shot was to be made without film
running in the sound department recorder, hence it was a 24
fps synch shot but without sound and the camera report and
sound reports were marks M. O. S.

Not as romantic as the old Hal Roach camera assistant story
Mit Out Sound, but pay your money and choose your story.
>>

Indeed.

The "Mit Out Sound" legend is so old, one has to invoke the "The
Man Who Shot Liberty Valance" axiom to see the forest from
the trees.

That axiom states, "This is the West, sir! When the legend
becomes fact, we print the legend".

Whereupon the reporter tore up his notes, which notes
enumerated in excruciating detail the true history of Liberty
Valance's demise (namely, John Wayne killed Valance, James
Stewart did not).

I came (decades ago, admittedly) from a shop where the
cameras only had multi-duty 96 volt dc/240 volts ac motors
(240 volts is the most common three-phase "delta" ac voltage
while 208 volts is the most common three-phase "wye" ac
voltage).

The sound department operator controlled the 96 volts dc,
which was sent to the camera, and the camera's 240 volts
three-phase ac output was sent back to the sound department's
recorder (a Westrex in this specific case) where it provided ac
power for the recorder's drive (the ac power for the recorder's
electronics was provided separately).

A reed-type frequency counter was always across the 240 volt
three phase ac line, and this was the sole indication of the
camera's speed, as a 96 volt dc motor can actually run at
almost any speed from about 16 fps to about 32 fps, simply by
adjusting the dc voltage sent to the camera.

When the frequency meter read "24 fps" or "48 Hz", depending
upon the specific meter being used, the sound department's
operator would assert, "Speed!, and the shot could commence.

The selection of 96 volts is somewhat interesting and a
simplification of the selection of 96 volts, as opposed to 12 or
24 or anything else for that matter, is this ...

A shunt-wound dc motor designed to run on 120 volts dc has a
nominal rpm of just about 1800 (it is a pure coincidence that a
4-pole, 60 Hz synchronous motor also runs at 1800 rpm), and
the motor's rpm is almost linearly related to the applied dc
voltage.

Yet, 1440 rpm is needed for 24 fps on a direct drive camera,
such as a Fox Studio Camera, and some others.

So, we apply the simple linear equation ...

( 1440 / 1800 ) * 120 = 96,

and come up with 96 volts as being the nominal applied dc
voltage required to achieve and maintain 24 fps.

And, parenthetically, as required to send 48 Hz to the recorder
from the 240 volt output of this "multi duty" camera motor.

So, if the shot was to be made without sound, the sound
recorder's operator would still be required to perform every
function, including getting the entire system up to synchronous
speed, *except* for loading mag film into the recorder.

Under these circumstances, the operator's log would read
"Motor Only Synch", or, simply "MOS".

A similar situation would apply if this same camera was
operated in the studio, powered by an "interlock distributor".

In this case the sound department, which "owned" the interlock
distributor, would provide the 240 volts three-phase power to
the camera, which was usually 48 Hz in the case of a direct
drive camera, from its interlock distributor and the results
would be the same, "Motor Only Synch", or "MOS".

The only difference being that the speed did not need to be set
by the sound department's operator as an interlock distributor
always runs at synchronous speed.

Peter.
==================

I can vouch for the equipment descriptions and electrical details in the post above - he's not making this up. And, as you can see, there are also other versions of the "Mit Out Sound" legend.
I've also heard variations on "Minus Optical Stripe", but "MOS" predates the newsreel cameras that actually recorded an optical track on the film in the field. Optical tracks on studio films were added in post-production, and the whole film would have an optical track, even scenes that were shot MOS. (And it doesn't really make sense anyway, since the whole point of writing "MOS" on the slate is to tell the editor he doesn't need to look for a matching sound track. If you were using field-recorded optical tracks, it would be obvious that there's no optical track.)

posted by Cobb @ 9:45 AM