Dolby Laboratories CP500-300 Installation Manual download pdf (Page 102)

5-1

SECTION 5

THEORY OF OPERATION

5.1 DA20 Basic Functions

5.2 Discussion of Changeover Technique

5.3 Format Control

5.4 Auxiliary Data Channel

5.1 DA20 Basic Functions

The basic functions of the DA20 are:

Video Acquisition

Sync Finding

Thresholding and Bit Packing

Error Correction and De-interleaving

FIFO Buffering and DAC Clock Generation

Soundhead Delay

AC-3 Transform Decoding

D/A Conversion

Cinema Processor Interface

Dolby Digital data is printed in the “interperf” area, between sprocket holes, adjacent to

the normal analog tracks of standard 35mm release print motion picture film. Blocks of

76 X 76 “fixels” or film elements contain the audio data, along with synchronization, error

correction, and auxiliary data. The film is passed through a mechanical transport,

generally mounted on top of the projector, where the motion of the film is stabilized, and

the interperf area is illuminated with white light. A CCD (charge coupled device) optical

line scanner produces an electrical signal representing a video image of each interperf

area due to the horizontal scanning of the CCD and the vertical motion of the film with

respect to the CCD. This signal contains the information necessary to reconstruct the 6

audio channels.

Extensive Digital Signal Processing (DSP) techniques are applied to the video signal

described above, using a variety of specific hardware and general purpose DSP cards,

connected in a pipeline architecture, with each card performing part of the overall task,

and passing the results to the next card over a serial data path.

Video Acquisition

The analog video signal from the CCD is digitized by an A/D converter at a rate which

tracks the film speed. The converted samples are written into a bank of RAM to form an

image of the interperf area in RAM. Sequential interperf images are transferred to one of

four DSPs on two dual DSP cards where the images may be processed and data extracted.

Sync Finding

Each block of RAM is searched by the associated DSP card for synchronization patterns in

the four corners of the image. When sync. is found, the locations of the bits within the

image are calculated, and the values at those locations are obtained. An array of these

samples is then further processed.

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Dolby Laboratories CP500-300 Installation Manual Page 102