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  1. Member xzarkad's Avatar
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    As far as I know:
    the I picture in a GOP structure is an independant picture.
    The P picture extracts the differences.
    The B picture extracts again a difference.

    The logic seems that encoding only with I pictures would result automatically in the best quality. I have tried this, but the quality did decrease.
    I have read that lowering the P picture ( standard in TMPGEnc is 5 ) to 4 would increase quality. This is correct. But when I lower it to 3, there is again a loss of quality.
    I was wondering if setting the B picture to zero would increase the quality.
    I seems that no-one knows exactly how to deal with the GOP setting. At least; I have not found relevant information to this issue.

    Can someone tell me about this "secret"?


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  2. I can tell you this much, at least:

    A stream of I-frames will deliver a perfect picture if the bitrate is high enough to accomodate them because no inter-frame compression needs to be performed. But unless your recording is for capture or archival purposes only, you'll need to apply a significant degree of compression in order to lower the bitrate within range of what the playback format (generally VCD or SVCD) can handle. This is where P and B frames become significant.

    P-frames are interpolated only between successive I-frames. B-frames are interpolated between I or P frames both forward and backward from their respective reference frames. This you already know.

    But the role of B-frames is contested. In MPEG-2 they are entirely optional. Some people believe they are of benefit because they tend to smear prediction errors between the P-frames they're based on, leading to more fluid movement even if the movement is itself in error. Others believe they are simply a waste of bits because they can add to prediction errors in addition to propagating them.

    But whether you really need them or not depends entirely on the target bitrate you're trying to achieve -- the lower the bitrate, the more useful B-frames are in reaching it, because if bandwidth isn't an issue you don't really need them at all.

    B-frames, properly constructed, can yield the lowest overall bitrate because of their averaging property (or smearing property, depending on whether you happen to like them or not). A multi-pass, variable bitrate encoder can make good use of them to exploit redundancies between frames that cannot be detected by a single-pass, constant bitrate encoder. The B-frames become a little bit bigger and the P-frames become a little bit smaller with every pass, which really adds up over time because the P-frames require more bits to reproduce than B.

    Now for the GOP question. Again, if your bit budget is high enough to support I-frames only, and the recording media is dense enough to contain them all, you can dispense with P and B frames altogether. But if it's not, which is surely the case with [S]VCD, you begin to think of whether alternative IBP structures might yield better results than the default.

    Fortunately the answer is very simple: they don't. The default GOP structure 15/15/3 (IbbPbbPbbPbbPbb) is optimal for video encoded at 30fps at VCD bitrates, and 12/12/3 (IbbPbbPbbPbb) is optimal for 24. Of course the encoder can and will (or at the very least should) terminate the GOP in progress and insert a new I-frame whenever it can get the best results by doing so, in practice only multi-pass encoders have the luxury of considering the situation both ways in order to detect a situation like this.

    You can increase the number of P-frames, but if you exceed the maximum bitrate (1.15 Mb/s for VCD or about 0.6 Mb per GOP) the number of compression artifacts will increase as a result of your attempting to stuff 7 lbs into a 5-lb. sack.

    You only have so many bits per GOP to work with, and the default structure allocates them about as efficiently as they can be. Thus there isn't really a compelling reason to alter them unless you happen to change the frame rate, in which case you'll be getting more or less than the number of I-frames per second, generally 2, than you need.


    <font size=-1>[ This Message was edited by: KoalaBear on 2001-08-21 13:43:39 ]</font>
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  3. <TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font size=-1>Quote:</font><HR size=1 color=black></TD></TR><TR><TD><FONT SIZE=-1><BLOCKQUOTE>
    On 2001-08-21 13:42:16, KoalaBear wrote:
    I can tell you this much, at least:
    </BLOCKQUOTE></FONT></TD></TR><TR><TD><HR size=1 color=black></TD></TR></TABLE>

    Very informative! I commend you.
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  4. Member xzarkad's Avatar
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    Koalabear,

    damn..........I am impressed.
    That for taking the time to give this incredible clear information.
    Baldrick, or Sefy: if you are reading this, please add Koalabears information to one of your sections, because I did not have found such a complete explaination on this site, and this is to valuable to get lost.............

    The Dutchman
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