Interesting pic !

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 !!

formatting link

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

Reply to
Phil Allison
Loading thread data ...

Interesting and educational. There's lots more photos and info at

formatting link

Bob

Reply to
Bob Parker

"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

Reply to
Phil Allison

Perhaps the exposure time is short, but to make it even finer, a flash is fired 96 times a second. This technique is used in high speed photography.

Cheers, Nicholas Sherlock

--
http://www.sherlocksoftware.org
Reply to
Nicholas Sherlock

"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

Reply to
Phil Allison

I thought that happened with all film projectors, using, for example, the Maltese cross mechanism: . But IIRC on normal film, the audio is displaced by a few frames and recovered from continuously moving film.

Clifford Heath.

Reply to
Clifford Heath

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

Reply to
Bob Parker

** Did you even read your own URL ?

Says the film stops 24 times per second - not 96.

** Bit hard to recover any analogue audio from a stationary film.

...... Phil

Reply to
Phil Allison

The film runs at 24fps but there are 4 sprocket holes per frame

formatting link

Reply to
ER

** Is this profound - or just silly ?

..... Phil

Reply to
Phil Allison

I found a projector setup procedure

formatting link
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

Reply to
Bob Parker

Neither really, just pointing out that although the film runs at 24fps there are 4 sprocket holes per frame with 4 gaps filled with DD5.1 data hence the 96 times per second figure for the number of images camera takes.

Reply to
ER

" Exasperating Ratbag "

** What crapology !

Worse than merely silly - it is completely STUPID.

Another MORON who cannot read or follow a simple thread !!

PISS OFF - IDIOT !!!

....... Phil

Reply to
Phil Allison

Nothing stopping them using 4 CCDs to capture 4 audio frames simultaneously. It's not as though very many pixels are required in each one. They could even be optically routed to a single CCD using prisms.

Reply to
Clifford Heath

wow, whered that come from Phil?

Do you suffer from tourettes or something?

Reply to
Simon

"Simple Simon"

= Another Exasperating Ratbag

PISS OFF - IDIOT !!!

..... Phil

Reply to
Phil Allison

He suffers several diseases , closely akin to tourettes but with a bad case of a philthy past bad foot in mouth .

Reply to
atec 77

That makes sense, they need a constant speed portion for the analogue pickup anyway.

--
Bye.
   Jasen
 Click to see the full signature
Reply to
jasen

that's if the invest in a raster type sensor.

I expect they just have a linear sensor (like fax machines and scanners use) and simply read the film as it moves past, hmm, about 15000 rows per second (during the data blocks) sounds possible, i expect the blue data (sony?) is read the same way.

--
Bye.
   Jasen
 Click to see the full signature
Reply to
jasen

That would seem to make the most sense. Just capture each row of data bits as it flies past...

Reply to
Bob Parker

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.