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Old August 16th, 2006, 09:49 PM   #1
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Losing quality from dubbing?

If I'm dubbing a MiniDV tape from one deck to another, will there be a degredation of quality on my new dub. I ask this because my engineer was reluctant to make dubs, saying that I'll lose quality. His reasoning was this: The first deck has to DECODE the DV signal recorded to tape, then it sends it to the next deck, which then has to re-encode this signal back to the MiniDV codec. During this re-encoding process, it can throw away information that the original tape had captured.

Is there any truth to this, or will it dub over EXACTLY is its original was encoded?
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Old August 16th, 2006, 11:33 PM   #2
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As long as you record via Firewire the dub will be a bit-for-bit copy of the original. If you went analog then, yes, you'd lose a generation, but using FW makes it no different than copying a file from one hard drive to another.

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Old August 17th, 2006, 01:59 AM   #3
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This is a point that raises a few subtle issues.
Firewire is a digital copy and in theory, it is just like copying a file from one media to another.
BUT and this is an interesting but, the firewire transfer does not have any hand shake or way to have a block of data retransmitted. So if in fact any errors occur during the transfer, or during the copying of the file to the media on the other end, then in fact you do not get a bit for bit copy. The dv codecs are very good at correcting and reconstructing errors, but it is not like copying to a disk drive where if there is an error the date is re sent, and were you can impliment a read after write to make sure of the written date.

This is why for instance on another thread that did not get any comment re DVRACK for instance I was asking for a simple implimention in the software to report the amount of error correction the dv codec is doing.

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Old August 17th, 2006, 03:07 PM   #4
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Well the error correction would be done in the playout device. Once the data is sent down the 1394 interface that information is probably not available. Much the same thing happens when data is digitally transmitted over reliable connections, for example as far as I know there's no error detection / correction on the IDE interface between disk drives and the controller. The disk itself does do a lot of error correction but that's invisible outside the disk (I think). SCSI drives do report that errors were corrected and the bus interface does have a parity bit although mostly not used / ignored.
The type of protols you're referring to are mostly used on communication paths that are error prone, RF and modems being a good example. These protocols do have an overhead which is probably why they're not used on things like firewire or SDI.
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