5-2
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” from each
DSP, a threshold value is derived, above which a fixel is determined to be clear,
representing a digital “1”, and below which the fixel is determined to be a “0”. Having
reduced each 8-bit sample value to a single bit, each DSP packs groups of 8 bits of data
and 8 a bit of erasure information into a serial stream, and passes this block of 722 words
to the next stage.
Error Correction and De-interleaving
The System Services Card (FIFO, Resources, Error Correction, Delay) accepts blocks of
data alternately from each of the video processor DSPs, recombining the separately
processed alternate perf data streams into a single AC-3 coded data stream. Reed-
Solomon Error correction decoding is performed, using check bytes encoded during the
film recording process to correct errors in the data due to dirt, scratches, misalignment, or
illumination non-uniformity. The data, having been interleaved within a perf to reduce
the effect of block errors (dirt, etc.) is now de interleaved, returning the data to the format
used by the AC-3 decoder.
FIFO buffering and DAC Clock Generation
The data leaving the Error Correction process comes in bursts of 256 words
approximately every 10 milliseconds, depending on film speed. The DSP of the System
Services card implements a FIFO (First-In-First-Out) buffer to smooth out the bursts of
data from the Error Correction process into a steady stream of data to the Transform
Decoder section, while reconciling any difference in average data rates between the
incoming data and the output sample rate of the audio. Using fullness of the buffer as the
controlling parameter, the FIFO card slowly adjusts the frequency of the output sample
clock until equilibrium is reached.
Soundhead Delay
A delay is required in the DA20 to compensate for the time between when the data is read
(at the Digital Soundhead) and when the audio is to be reproduced in sync. with the
picture. Dolby Digital data is recorded 6 frames in advance of the analog sound track,
which is itself placed 20 frames in advance of the picture gate. If the Digital data is read
40 frames above the picture gate, for instance, the data must be delayed by 40+20+6 = 66
frames, or 264 perforations, or about 2-1/2 seconds. This delay is accomplished by
storing incoming data in RAM and sending it out at a later time, determined by the delay
setting.
AC-3 Transform Decoding
The Delayed data is now passed to a Zoran Z38000 DSP card which performs Dolby AC-3
decoding. A full explanation of this process is beyond the scope of this manual, but it
may briefly be described as a method of low bit rate perceptual coding of audio data
using frequency-domain information as the transmitted data, rather than conventional
time-domain PCM coding. The composite data stream is separated into three sets of two-
channel data. Each of the channel pairs is then fully decoded into PCM audio, and sent
serially to the DAC Card.
