DVI compression?

Discussion in 'Amateur Video Production' started by Don WA5NGP, Jan 18, 2004.

  1. Don WA5NGP

    Don WA5NGP Guest

    What is the compression that DVI does to raw video? I hear its like
    5X. Is it something similar to MJPEG or is it something simpler like
    run length encoding etc?

    Tks

    Don
     
    Don WA5NGP, Jan 18, 2004
    #1
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  2. Don WA5NGP

    Dan Maas Guest

    What is the compression that DVI does to raw video? I hear its like
    DVI is uncompressed.

    Do you mean DV? DV is like MJPEG with a compression ratio of about 5:1
    (or 9:1 if you consider the 4:1:1 chroma subsampling to be part of the
    "compression").

    Regards,
    Dan
     
    Dan Maas, Jan 18, 2004
    #2
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  3. Don WA5NGP

    Don WA5NGP Guest

    OK. Let me ask it this way. When I use a digital camcorder what is
    stored on the tape. DVI which is simple digitation of something like
    yuv 4:2:2 or is it DV that is similar to MJPEG or is that a
    selectable option on camcorders?

    Similarly, if I use "pass thru" to capture to my pc thru 1394 what is
    that? Or is like the above, selectable too?

    For the compressed version DV, is that MPEG I frames? This would mean
    to get the MPEG2 file the processor would need to do the PB frames and
    not the I frames or does the processor have to explode it back out to
    raw video and do the whole IPB series.

    tks


    tks

    Don
     
    Don WA5NGP, Jan 18, 2004
    #3
  4. Don WA5NGP

    RGBaker Guest

    DV is a codec that applies chroma down sampling & five to one compression to
    result in an mpeg/mjpeg like file (slightly more mpeg like, but with
    intraframe only compression) -- the specs are rigid and defined, no user
    options available.
    The pass thru delivers full spec DV.
    DV is mpeg like but not actually mpeg -- the frame is decoded to YUV in
    better systems, RGB in others & then compressed to mpeg2 if that is what is
    called for.

    GB
     
    RGBaker, Jan 19, 2004
    #4
  5. Don WA5NGP

    Dan Maas Guest

    When I use a digital camcorder what is stored on the tape.

    If it's a DV camcorder, then the tape will consist of DV-compressed
    frames. DV compression is like JPEG on each frame, with 4:1:1 chroma
    for NTSC or 4:2:0 for PAL.

    *DVI* is a connector that carries uncompressed video. There is no
    corresponding tape format.
    It's DV-compressed frames, the same as if it were from DV tape.
    The DV compression algorithm is very similar to MPEG I frames, but
    it's not directly compatible with MPEG. In order to compress to
    MPEG-2, the processor
    will have to decompress the DV back to raw video and then apply the
    whole
    MPEG-2 algorithm.

    Regards,
    Dan
     
    Dan Maas, Jan 19, 2004
    #5
  6. Don WA5NGP

    dylan_j Guest

    assuming NTSC: it's 720x480 pixel image, 29.97fps interlaced with YUV
    sampling of 4:1:1, then with lossy compression of 5:1 applied. This DV
    stream has a datarate of 25 Megabits per second, whgich is fixed. You
    can get better or worse codecs but the specs do not change. (in PAL is
    576x720, 25fps i'laced with 4:2:0 sampling a la DVD mpeg 2, but the
    per second datarate is the same.)
    I don't understand this - are you refering to "DVI Digital Video
    Interface" as found on computer display cards for connecting monitors?
    It's something completely different.

    I assume that DVI would be, like VGA uncompressed RGB, only digital,
    but I don't know.

    DV is like MJPEG in that it's discrete intraframe CBR compression, but
    unlike MJPEG, the compression rate of DV cannot be adjusted.

    "pass thru" what? When people use the phrase "pass thru" around here
    they are generally referring to using a DV camera with analogue inputs
    to convert other analogue footage (VHS ir Hi8 for example) into DV
    that can be captured via a firewire card, or going the other way,
    taking a firewire signal and converting it back to analogue to play on
    a monitor or record on VHS. Same thing applies as my first outline.
    It's not selectable at all, the specs for DV are fixed, though Final
    Cut Pro does offer an on the fly low quality capture mode via
    firewire.
    DV is not selectable in it's compression ratio - one second of DV
    footage WILL ALWAYS be 3.2MB. The best you can do it choose a codec
    that does a better or worse job at that data rate. The codecs built
    into cameras or bundled with NLEs or software (quicktime, windows) are
    generally not as good as the coedcs on good hardware DV cards (such as
    the matrox or canopus cards and bridges)

    I don't believe that MJPEG compression ratio was never selectable on
    any of the camera/tape recording systems it is part of, such as
    Digital Betacam where the MJPEG compression was fixed at 2:1.

    Not the same algorithm, but yes, in DV, EVERY frame is like an "I
    frame", there are no P or B frames.
    As far as I know, with cases of encoding a video source from one codec
    to another, at some stage the video is temporarily decompressed out to
    a fully RGB 4:4:4 stream before it's recompressed, even if you were
    going from mpg1 to DivX. Thats what it's called a coDEC it has to be
    decompressed to a full image.
     
    dylan_j, Jan 19, 2004
    #6
  7. Don WA5NGP

    Samuel Paik Guest

    An analog encoding of a digital signal. The digital signal primarily
    contains a DV bitstream.
    DVI is totally irrelevent here. DV is a intra-frame coding of
    digital video using DCTs, quantization, and entropy coding.
    At this level of description, it is the same as MJPEG.

    DVI is an interface made of a bundle of serial data lines generally
    used to transmit RGB pixel data from a display adapter to a display.
    It uses transition-minimized differential signalling to robustly
    transmit high bitrates over moderate distances at relatively low cost.
    Pass through: analog video goes into your DV camcorder where it is
    digitized and encoded into a DV bitstream, which is sent over a 1394
    connection.
    No. MPEG I frames use similar coding ideas as DV and MJPEG--DCT,
    quantization, entropy coding. The details in each of these three
    are different.

    Sam
     
    Samuel Paik, Jan 19, 2004
    #7
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