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- Phil Allison
December 2, 2006, 5:55 am
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Hi to all,
ever wondered how Dolby Digital 5.1 sound is ACTUALLY carried by a 35mm
cinema film ?
Turns out the 320 kbit/s DD data stream is encoded onto squares optical
grids of about 60 x 60 bits & very cunningly fitted in * between* the
sprocket holes !!
The embedded Dolby logo is cute dontcha think ?
A tiny CCD camera in the projector captures each 2.4mm square image, 96 of
them per second, so it can be processed into 5 channels of audio.
The other tracks you see are Sony SDDS, L/R stereo analogue and timing
pulses for DTS so the CD can be synched with the movie.
Such a multi-format movie can be shown by any cinema in the world and exact
copies are made by ordinary, chemical film processing.
Betcha you ALL knew that already ....
....... Phil
Re: Interesting pic !
"Bob Parker"
** I would still like to know how the CCD camera reliably acquires 3500
bits of data from 96 tiny squares every second.
Does the film come to a halt 96 times per second to give the camera a
stationary image to snap ?
Or, is the CCD exposure time so short that relative motion is effectively
frozen ?
Maybe clever spinning optics are used to make the image appear stationary to
the camera ?
........ Phil
Re: Interesting pic !
"Nicholas Sherlock"
** I suppose a high brightness LED could be pulsed at 96 Hz continuously.
The optical data bits are approx 1/800 of an inch square and the film speed
is about 18 inches per second, ie it moves the width of a data bit in
1/15,000 of a second.
The light pulse would need to last 20 uS at most to stop action !!
....... Phil
Re: Interesting pic !
I found a projector setup procedure
(http://www.film-tech.com/warehouse/manuals/CENMR3.pdf ) which seems to
indicate that the Dolby digital pickup is illuminated by continuous LED
light and is positioned where the film's moving at a constant speed.
Apparently the CCD is functioning as a very fast electronic shutter.
Bob
Re: Interesting pic !
I thought that happened with all film projectors, using, for example, the
Maltese cross mechanism: <http://en.wikipedia.org/wiki/Maltese_Cross_mechanism .
But IIRC on normal film, the audio is displaced by a few frames and recovered
from continuously moving film.
Clifford Heath.
Re: Interesting pic !
One of those articles mentions the digital area of the film being
scanned 24 frames away from the gate, where it's moving continuously not
being stopped for projection of each frame.
Maybe they pulse high intensity LEDs for a couple of microseconds to
grab each optical digital block?
There's probably info on some of the professional sites... I'll have
a look when I get time.
Bob
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