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  1. Hi, I have been making some test with GOP's in tmpgenc, and I cant find great diference between this two test :

    1) MPEG2, 25 fps, VBR avg 2500, min 2100, max 3000 standart GOP's
    2) The same settings, but IP gop's.

    In addition I dont undestand the benefits of B frames, why not only have I's and P's frames ?

    I'm doing something wrong ?

    thanks
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  2. Member
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    B-frames require the least storage memory and contain the least picture information. Thus, they are highly dependent on the I and P frames. For DVD's, they are required. The "IP" GOP structure is a "low delay" structure, and is not permitted in the DVD spec.
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  3. Член BJ_M's Avatar
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    B frames have the highest amount of compression .. with no B frames you would have larger file sizes .. I frames have the least compression and have the highest quality - though they take a lot of "room" - you would think that if you have all I frames you would have the highest quality but this is not exactly case at lower bit rates. certain patterns works better in certain encoders .. hence 1 14 0 , 2 5 2, 2 5 0 , 2 5 1, 1 5 2 are all popular .. but by no means best in all cases ..

    if there is not a lot of differeance (fast motion) scene to scene, switching to IP frames from IBP frames will not show a lot of differeance. at very high bit rates or when the mpeg files is going to be edited or re-encoded to a lower bit rate all I frames is a good thing (if you are not worrried about space) .... a lot of "pro" mpeg editors work only in I frames
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  4. Thanks for the posts,

    BJ : just Im looking for the best quality, and the space not is a problem IF the movie fit in two cd's. Can be an option reduce bitrate and use only I-P frames ?
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  5. Член BJ_M's Avatar
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    you should a test to see if your dvd player supports all i frame or IP frame only ... many do ... in fact most seem to now that i have tested .. pay special attention to audio sync and smooth motion ...

    in tmpgenc a setting of 2 5 0 or 3 3 will give very high quality - though of course there are other settings that are just as importaint .. in CCE 3 5 0 is a starting point ... you can push the I frame number to even more if your player can play it . when you are at the lower bit rates of a svcd i am really not sure using a high number of I frames will help a whole lot as you are really robbing most of the bit rate for a I frame (it is importaint to remember that bit rate is calculated for a GOP - not for a single frame).

    you would ussually increase your bit rate settings as you increase your number of I frames -- and reach a balance between file size and quality ..
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  6. OK, I have been doing some test with 1 min of video, with a lot of motion.

    I see the best quality in this two :

    a) VBR max 1900 min 0, GOP 1 14 0
    b) VBR max 1900 min 0, GOP 2 5 0.

    Im surprise when I see that a) is better than b), I thought that b) would be better because it have more I frames.

    Is this possible ?
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  7. Член BJ_M's Avatar
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    yes - this is what i refered to in another post about GOPS at low bit rates (1900 is very low) ... bit rate is per GOP - not per frame .. so at lower bit rates the I frames soak up most of the avaiable bit rate .. at a higher bit rate you have enough to "spread around" ..

    even though 1900 is very low bit rate - you can still get some really good quality by carefull set up of parameters .. dependent on source ..

    i posted in another thread a vcd sample i made of the thread starters low resolution quicktime movie ... i blew the QT image up , framed it , color corrected it and changed the frame rate by adding frames from 20 to 29.97 - and is very fast motion with lots of black --- but yet there is no blockyness or banding. AND i encoded it with CBR 1900 with tmpgenc. i used GOP 2 4 1 in this case w/a custom matrix ..
    http://www.simex-iwerks.com/test_oo/sahara_mpeg1.m1v
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