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.
