Specifications

APPENDIX D
THE DOLBY DIGITAL SUBSYSTEM
Basic Functions
The basic functions of the Dolby Digital subsystem 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
Sample Rate Conversion
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.
Thresholding and Bit Packing
Each dual DSP card then accomplishes Thresholding on the block of 5776 8-bit
values representing the center of each bit on the film, for each interperf area. Based
on the statistics of light and dark bits, which are accessible via “mailbox memory”