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This topic comprises 2 pages: 1 2
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Topic: Dolby Digital Drop-out Adjustment
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John Hawkinson
Film God
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 05-05-2010 08:57 AM
Whoops! A while ago, Markus asked, "My real question is since Dolby Digital is already buffering the sound for my penthouse reader by nearly 3 seconds, couldn't it just compensate by doubling up known blocks so that the sound does not drop when it gets to the damaged area?"
That's not now it works. There's not a huge amount of redundancy in SRD. There is some, but generally not enough to compensate for four consecutive bad blocks.
Buffering doesn't help you -- when the splice goes through the reader, it fails to read the digital track. The fact that this is decoupled from the time at which that data plays does not help it get the data back.
For the special case of reel-end splices, Dolby implements "splice caching" where the data blocks for that time are included elsewhere in the reel (additional redundancy), so those splices don't cause SRD to drop out. But that level of redundancy is not generally available with SRD, so you're out luck.
--jhawk
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John Hawkinson
Film God
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 05-05-2010 04:28 PM
Ah.
The operation you ask about is not implemented.
The SRD processor has plenty of time to do it, however. The SRD track leads the analog track by 6 frames, which leads the picture by 20 frames. Even in the worst case of a basement reader, there are 250ms to perform such an operation, and that kind of operation would take less than 1ms to execute.
A lot depends on what you mean by "stretching." Generally speaking, let's imagine it needs 1 frame's worth of time to process the data. Once it has seen 4 frames of missing data (16 perfs), it would need to decide whether to drop to analog or to execute such a "stretch." It can't know whether there will be 4 more missing frames, or 40 frames, or never.
Many would argue that it is much better to fallback to analog immediately (10ms for a block or 42ms for a frame) rather than try to do any "tricks."
It is true that generally speaking repeating the last block probably sounds better than silence, and that empirically, sometimes you do get silence.
I believe Dolby also does do some psychoacoustical masking in the event of bad data, but that may merely be because of high error rates and not because of entirely unreadable blocks.
Anyhow, the short answer is it doesn't really do what you're asking for, and it is not because of time limitations. Philosophically I don't think there's widespread agreement that such a thing would be Correct to do, but they could certainly do a bit better than they do at present.
--jhawk
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