Problem calculating length of encoded mpeg2 file

Discussion in 'Video Cameras' started by Rob Beattie, Oct 17, 2004.

  1. Rob Beattie

    Rob Beattie Guest

    Tmpgenc has been outputting megs (encoded from avi files captured via
    my sony camcorder) which are larger than predicted.

    For a 95 minute avi with the average video bitrate set to 2600kb/s and
    audio set to 192 kb/s encoded to mpeg2(2 pass vbr) then I calculate
    that the mpg should be 95x60x(2600+192)/8 Kbytes = 1.9GB. What I get
    is a file that is 2.4GB.

    I am pretty sure that my calc is correct so why the huge difference in
    size?
    I have always assumed that the file size only depends on two things,
    the (video + audio) bitrate and length of the avi being encoded. I
    must admit that I usually get the files coming out around the
    predicted size but occasionally this is not so. The example given
    above is a low quality(even for vhs) vhs capture so is it possible
    that Tmpgenc may be ignoring the average bit rate and using extra bits
    to encode the video?
    i would be grateful if someone could confirm my calculations or
    suggest any settings that could be causing the file to be larger than
    expected.
     
    Rob Beattie, Oct 17, 2004
    #1
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  2. Rob Beattie

    mrlipring Guest

    Yep, it should be 1,989,300 bytes. There's some overheads for file
    structure etc, but not 20% overheads.

    You're doing 2-pass, so it should be more accurate than 1-pass,
    regarding output file size.

    Tmpgenc shouldn't ignore your bitrate settings, so that's out of the
    question. I don't use it all that much, so i guess someone more
    experienced should give you more help. I can't see why this would be
    happening. Have you tried encoding with another app? I guess the source
    could be so noisy, and the encoder is trying to compensate by raising
    the bitrate, but it shouldn't do it by THAT much.
     
    mrlipring, Oct 18, 2004
    #2
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  3. If you're usign VBR, then the bitrate is variable, not 'static'. It
    could well be the case that averaged over the entire clip, that VBR
    requires a greater bitrate in some sections than you might expect.

    This might be the case if the video consists mostly of high motion
    scenes which require more bits to encode - if there were sections of
    quiescent video, the VBR could 'take' bits from those to offset
    against the high motion scenes.

    If it's all-action then probably it can't and the bitrate will tend to
    whatever is required to keep up the framerate. What size do you get
    (leaving aside quality considerations) if you use CBR throughout?

    HTH
    CHeers - Neil
     
    Neil Smith [MVP Digital Media], Oct 18, 2004
    #3
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