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November 2000, Week 2


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Leo Enticknap <[log in to unmask]>
Reply To:
Film and TV Studies Discussion List <[log in to unmask]>
Wed, 15 Nov 2000 01:19:14 +0000
text/plain (60 lines)
In relation to R. Inglis' enquiry - sorry, but Don Larsson and Lang Thomson
are reproducing a widely understood but essentially incorrect myth here.

Early sound on film systems needing 24fps (i.e. a foot and a half of 35mm
film per second) consolidated the use of 24fps as a standard, but that
wasn't what started it off.  Bear in mind that Vitaphone worked at any
speed you like - the only crucial parameter was the speed of the record.

What really did it was projector shutter design.  The bottom line is this -
to fool the human brain into thinking that a lot of still pictures shown in
quick succession are in fact one continuously moving picture, you need at
least 48 expose and obscure cycles per second.  That means a picture has to
be projected, a shutter blocks the beam and it opens again 48 times every
second.  Now most projectors have what's called a two-bladed shutter.  That
is a concentric disc with two holes punched out of the edges.  That disc
rotates, driven by a shaft which is also linked to the projector's
intermittent mechanism.  But the film is only pulled down in one of every
two obscures, hence 48 cycles per second but only 24 frames of film per second.

Back in the 20s, it was common to have three bladed shutters.  Same
principle as above, only three blades and three holes.  Three cycles per
shutter rotation means you only need 16fps of film movement (16 x 3 = 48).
But, because more of the shutter surface area per rotation is blocking
light, the projector light source is less efficient, and that is the real
reason why projector designers gradually moved from three to two bladed
shutters during the 1920s.  The picture palaces of the 20s really stretched
projector lamp technology and the designers needed every speare foot-candle
they could get.  You need a third more light for a picture to look as
bright with a three bladed shutter as it does with a two.  Hollywood
complied by upping camera speeds.  Bear in mind that if the projector mech
speed goes significantly below that crucial 48 expose/obscures per second,
you'll see a horrid flicker on the screen.  Anyone who has tried to watch
BIRTH OF A NATION or a German silent (which were shot very slow right until
the  conversion) at 16fps on a two-blade projector will have the headache
to prove it.  Under pressure from exhibitors, Hollywood relented and
increased shooting speeds, basically to make their pictures look better on
the projectors that most exhibitors were using.  By the time sound came
along (and it is true that the De Forest system really needed 1.5 feet per
second, even though Vitaphone didn't) the argument was over and that just
cemented it.

Feel free to email me off list if you want any references for the points
mentioned above (havan't got 'em to hand, will dig 'em out if you wan't em,
can't be bothered if you don't!).

Leo Enticknap
Technical Manager
City Screen Cinemas (York) Ltd..
13-17 Coney St., York YO1 9QL.
United Kingdom
Telephone: 01904 612940 (work); 07710 417383 (brainfryer)
e-mail: [log in to unmask] (work); [log in to unmask] (home)

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