Some theory

Discussion in 'Video Cameras' started by Just D, Dec 3, 2006.

  1. Just D

    Just D Guest

    Maybe the question is pretty simple... I'm just curious.

    Does anybody know what's going on? I downloaded the same fragments from the
    camcorder a few hours later, got the same set of DV files, sizes are
    absolutely equal. The byte to byte comparison shows that these files are not
    equal and are having some minor differences like 1 byte every Kbyte or
    something like that. Why? I was expecting that since these files have been
    downloaded from the DIGITAL into DIGITAL with the same device and software
    they should be absolutely identical. I'm not expecting we're writing
    something time sensitive inside these files during download, then why I'm
    seeing these differences? What it can be? One more time, I got two different
    sets of files with the same file sizes, so that they are looking equal until
    I compare the contents.

    Just D.
     
    Just D, Dec 3, 2006
    #1
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  2. Just D

    Jerry Guest

    Is this from a DV tape, there could well be issues around the video
    heads, as you say they are not huge and don't affect the playout -
    just the actual file size.

    Over to Tony Morgan...! :~)
     
    Jerry, Dec 3, 2006
    #2
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  3. Interesting.

    Is the difference an actual byte or just a single bit in the byte?

    There are also some reserved and/or undefined words (two bytes) in the dv
    headers which could contain random data.




    regards

    Stuart

    www.mckears.com
     
    Stuart McKears, Dec 3, 2006
    #3
  4. Just D

    Just D Guest

    I can make a screen shot if you want, although anybody can get this
    difference I guess. It was something like "P" or "Q" in the comparison
    window when I was comparing these files.

    "Stuart McKears"
    So there is mostly a one bit difference only and it's not very often, but
    regular. Maybe yes, some kind of reserved byte in the frame header or
    something.
    Maybe so, I hope that this is the reason.

    The lengths of all correlating files from both file sets are same up to one
    byte.

    Just D.
     
    Just D, Dec 4, 2006
    #4
  5. Just D

    G Hardy Guest

    Is the difference an actual byte or just a single bit in the byte?
    DV data is compressed (both audio and video), so I don't think it would be
    very tolerant of a one-bit corruption (although I've no idea how good the
    error correction is in a DV stream). The effect would be more noticeable in
    the video than in the audio.

    If the files are the same apart from a single byte at a consistent point,
    perhaps it's something like the system time that the capture took place? I
    suppose the way to test it would be to export a DV AVI from the timeline,
    wait a bit, then export another, then compre the two...

    .... in fact, I just did that - and sure enough the files are different. Two,
    two-byte differences in a one-second (25 frame) AVI.
     
    G Hardy, Dec 4, 2006
    #5
  6. Have a look at

    http://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/DVAVSPEC.RTF

    You should be able to track the offending bits/bytes down.

    Might take a bit of time but it'll give you something interesting to do over
    Christmas :)

    Of course, that might be Christmas 2007 as decoding raw data can be sometimes
    lengthy!!!!


    regards

    Stuart

    www.mckears.com
     
    Stuart McKears, Dec 4, 2006
    #6
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