5-3
D/A Conversion
The three two-channel PCM audio serial data streams are converted to six analog audio
channels by the DAC card. Oversampling of the data is performed to simplify the
requirements of the reconstruction filters following the DACs. Sample rate of the DACs
is determined by the clock oscillator controlled by the FIFO card, as described above.
Sixteen, eighteen, or future twenty-bit DACs may be supported by the DA20, as the
transform decoded data is accurate to around 20 bit resolution.
Cinema Processor Interface
The six audio signals from the DAC card are sent to the Cinema Processor Interface Card,
where they are buffered, metered, and sent to the associated Dolby Cinema Processor.
This card is also responsible for format control, changeover timing, and status indication.
5.2
Discussion of Changeover Technique
Projector changeover sensing is provided through 9 way female D connector J9 on the
DA20 rear panel. In single projector applications, all terminals may be left unconnected,
and the DA20 will assume projector 1. If reversion to analog tracks is desired, the motor
start signal must be provided, otherwise the DA20 will not revert to analog, as without a
motor running, the DA20 assumes no analog track is playing. The motor start signals are
used to signal the DA20 in advance of a changeover in order that the video front end can
switch to video from the incoming reel prior to the actual picture changeover. The
changeover to the incoming projector CCD signal will be made at a time after the Motor
Start signal equal to (7 seconds minus the fixed delay value.) Removal of the Motor Start
signal prior to video changeover resets the changeover timing, allowing the operator to
“jockey” the placement of film in the projector for between 1.5 and 7 seconds, depending
on delay setting, without initiating a changeover. Verification of changeover is confirmed
by the Changeover signal at the actual instant of changeover. If no Motor Start signal is
asserted, the video input follows the changeover signal.
5.3
Format Control
The DA20 is controlled by means of the Cinema Processor’s format selection controls.
The ID0-ID7 lines from the Cinema Processor are monitored by the DA20, which senses
format changes by detecting changes in the state of these lines. When Format 10 (Dolby
Digital) is selected by the cinema processor, the audio output of the DA20 will be
decoded digital audio if valid digital data is being read by the soundhead. When this is
not possible (as when playing an analog only print) the DA20-CPXX system will play
analog audio in format 05 (SR).
Should the digital data be corrupted through perforation damage, concealment
techniques will be applied for up to 4 perforations (40 ms), after which time the DA20
will revert to passing analog audio. Corrected digital data must remain valid for a period
of time before switching back to the digital track.
