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Interesting pic !

Discussion in 'Hobby Electronics' started by Phil Allison, Dec 2, 2006.

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  1. Phil Allison

    Phil Allison Guest

    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
  2. Bob Parker

    Bob Parker Guest

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

  3. Phil Allison

    Phil Allison Guest

    "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
  4. 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

    Nicholas Sherlock
  5. Phil Allison

    Phil Allison Guest

    "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
  6. 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.
  7. Bob Parker

    Bob Parker Guest

    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.

  8. Phil Allison

    Phil Allison Guest

    "Clifford Heath"

    ** 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
  9. Phil Allison

    Phil Allison Guest


    ** Is this profound - or just silly ?

    ...... Phil
  10. Bob Parker

    Bob Parker Guest

    I found a projector setup procedure
    ( 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.

  11. Phil Allison

    Phil Allison Guest

    " 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
  12. 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.
  13. Simon

    Simon Guest

    wow, whered that come from Phil?

    Do you suffer from tourettes or something?
  14. Phil Allison

    Phil Allison Guest

    "Simple Simon"

    = Another Exasperating Ratbag

    PISS OFF - IDIOT !!!

    ...... Phil
  15. atec 77

    atec 77 Guest

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

    jasen Guest

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

    jasen Guest

    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.
  18. Bob Parker

    Bob Parker Guest

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

    Phil Allison Guest

    "Bob Parker"

    ** The DD5.1 35mm film system is clearly STATED to use a CCD camera whose
    images are processed - so not a line scanner.

    High effective shutter speeds ( even 10uS ) are well possible with CCD
    technology, as is operation with a few uS pulse of light.

    Processing the "checkerboard" images into a data stream is then a simple DSP

    Let Occam's razor apply.

    ........ Phil
  20. Bob Parker

    Bob Parker Guest

    Google might be my friend, but searching for technical info on the
    actual Dolby 701/702 digital track readers and their CCD sensors ain't
    easy! I found heaps of info about how to install them into various
    models of projectors and connect and align them, though.

    The data block is read by a 512 element charged coupled device (CCD),
    which is, in essence, a television camera. The output of the CCD is an
    analog video "picture" of the spots on the film. This is converted into
    the digital video domain at a rate which tracks the speed of the film.
    This video data is scanned for synchronization information and to
    determine where the four corners of the data block are located. If two
    of the corners are known, the position of the entire block is known and
    the proper location of the data bits is known. Determining the whether
    or not a real data spot exists and preventing false data from entering
    the process is best understood by imagining a piece of window screen
    being placed directly over the data block. The corner bits verify the
    proper Position for the screen. Looking at the "screen", one sees in the
    small square openings exactly where the spots should be. Using a digital
    version of such a screen, the data spots are identified and then sent to
    a "thresholding" stage for further enhancement. Based on the average
    density of the spots, a threshold value is determined, above which the
    spot is recognized as a "1" and below which, a "0". This stage reduces
    the possibility that a scratch could be mistaken for actual data and
    establishes what, on the film, is a spot and what is not.

    I'm not sure if that clarfies anything or not. :)

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