Dolby Laboratories CP500-300 Installation Manual download pdf (Page 104)

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.

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Dolby Laboratories CP500-300 Installation Manual Page 104