A question on loss of quality for digital video?

Discussion in 'Amateur Video Production' started by Brian, Sep 27, 2003.

  1. Brian

    Brian Guest

    If I copy a DV movie from digital camera to the hard drive, remove
    some parts then copy it back to the camera using firewire, is there
    any loss in quality?
    I was lead to believe that I could do this once or many times and
    there would be no loss of quality, but after reading that the video
    file is compressed on the hard drive (when loading into a video
    editor), then I'm thinking that there must be some loss of quality.

    Regards Brian
     
    Brian, Sep 27, 2003
    #1
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  2. Brian

    John Guest

    Brian,

    You didn't say how long the movie is, or how much space you have on your
    hard drive.

    You can copy it to the hard drive in DV format.
    There is no compression or quality loss that you will ever notice.
    But the files are huge, about 1.6GB per minute. That's 100GB per hour.
    I use an 80GB partition just for video capture.

    If you don't have enough space, then there will be some loss.
    But it will probably not be too noticeable.


    John
     
    John, Sep 27, 2003
    #2
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  3. Brian

    Brian Guest

    Thanks John,
    I think programs like Vegas Video,
    Pinnacle Studio, etc have their own codec. I was reading that one of
    the video editors compresses 5:1 which would mean some loss of
    quality.
    To store 1 hour of video needs 13 megs of hard disk space (most video
    editors seem to require this as I have been quoted this amount of hard
    disk space by a few people (3.6 mb/sec).
    As you need 100gb for one hour of video to get the original quality
    then at 13megs for 1 hour of video must require compression and thus a
    reduction in quality of video.

    Regards Brian
     
    Brian, Sep 27, 2003
    #3
  4. Brian,

    DV video is compressed only once, i.e. by the camera codec during shooting.
    After that it can be transferred to the PC, edited and transferred back to
    DV tape on an as-is basis, without quality loss (at least in theory, but
    you'll never notice the odd bit that's lost now and then). Only analog video
    must be compressed when capturing it on the PC and decompressed on output.
     
    Lou van Wijhe, Sep 27, 2003
    #4
  5. John,

    I think this isn't quite right. I took the following from my Pinnacle Help
    info:

    QUOTE

    Your camcorder compresses and stores video on the tape at 3.6 MB/s, which is
    broadcast quality video. With full-quality capture (Pinnacle offers a low
    quality preview capture too - Lou) the video data is transferred directly
    from the camcorder tape to your PC hard drive with no changes or additional
    compression.

    Due to the high quality, capturing at this setting does take up a lot of
    disk space. You may want to pick and choose small segments to capture
    instead of the entire tape. You can calculate the amount of disk space you
    will need by multiplying the length of your video in seconds by 3.6 MB/s.

    For example:

    1 hour of video = (60 seconds x 60 minutes) = 3600 seconds.

    3600 seconds x 3.6 = 12,960 MB or 12.9 GB of hard drive space.

    UNQUOTE

    So, it fortunately isn't as dramatic as you suggested.
     
    Lou van Wijhe, Sep 27, 2003
    #5
  6. Brian

    - Guest

    That's about right for DV compression.
    Yes but that "reduction in quality" occurs in the camera at the time
    of shooting. The DV data that the camera stores on the tape has
    already undergone the DV compression. It's part of the DV standard.
    If you just perform cuts then there will be no loss in quality when
    you copy it back to camera via firewire.

    The DV data that is copied from you camera to the computer has already
    been compressed by the firmware in the camera. This compressed data is
    written into the AVI file on your hard drive. If you were to copy it
    back to the camera it would be an exact bit-for-bit copy of the
    original data, so there would be no loss in quality COMPARED WITH THE
    ORIGINAL COMPRESSED DATA.

    When you perform simple cuts in an editing program, the program will
    simply remove the sections of data that you edited out, and the
    remaining footage will be copied back to the camera as an exact
    bit-for-bit copy of the original data (but with the edited-out parts
    removed).

    If you think about it, it would be a waste of time for the editing
    program to re-render (and recompress) footage which is already sitting
    on your hard drive.

    The difference is when you add an effect or transition to your video.
    The video editing program must GENERATE new frames containing the
    effect or transition. For an effect it must decompress the existing
    footage (but only over the time range where the effect is to occur) to
    get at the actual uncompressed video frame images (the "bitmaps" if
    you like), apply the effect and then compress the frames back again
    into DV data. This decompression and compression is performed by a
    software DV codec on the PC. So when an effect or transition is
    applied then the video goes through an extra decompress/compress cycle
    which reduces the quality very slightly.

    But if you just perform cuts then you can copy the DV data back and
    forth between camera and PC as many times as you like and there will
    be no quality loss (compared with the original compressed data that
    the camera generated).

    Colin
     
    -, Sep 27, 2003
    #6
  7. Provided you don't use any transitions, effects or titles else
    recompression is done. The quality depends on the quality of the codec
    on your system (Canopus, Main Concept and Matrox are considered best
    in class).

    cheers

    -martin-
    --
    filmmaker/DP/editor,
    Sydney, Australia

    "Today's mighty oak is just yesterday's nut that held its ground."

    SpamPal keeps my mailbox spamfree :) (http://www.spampal.org)
     
    Martin Heffels, Sep 28, 2003
    #7
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