VideoHelp Forum
+ Reply to Thread
Results 1 to 7 of 7
Thread
  1. Hi. When I convert a video file I always set 'bitrate' the same as the original file. Usually, I convert to mp4 but have just started experimenting with AVI/xvid and have noticed something quite curious. I've opened up xmediarecode and loaded a WMV file with a constant bitrate of 4096. Under the 'video' tab, I set bitrate to 4100 but the converted video shows a bitrate of 2483. Further, unlike conversions to mp4, I only see 'Bitrate' - I don't see 'Bitrate Mode: Constant' (by the way, I use KMPlayer to check bitrate).


    I did a further conversion, upping the required bitrate to 8000 but the new video was much as before – bitrate 2528.


    Can anyone suggest why, when using AVI/xvid, I can't get the bitrate I've selected?


    Many thanks.
    Quote Quote  
  2. How does the resulting video look compared to the original? You may simply be "saturating" the codec. Ie, the codec is already giving you the best quality it can with only 2500 kbps. Some other Xvid settings could have an effect on the upper bitrate limit. Make sure the minimum quantizers are set to 2 or 1.
    Quote Quote  
  3. Hi jagabo. The picture quality is excellent and the min quantizers (whatever they are!) are 2. I know what you're saying, if the PQ is fine, don't worry. I'm just curious as it seems odd I can convert to mp4 at the bitrate I choose but xvid makes its own mind up...
    Quote Quote  
  4. If you reduce the min quantizers to 1 you will probably get bigger files, closer to the requested bitrate. But the visual quality won't be much higher. That is, some frames will end up using a Quantizer of 1 instead of 2 so they will have a little higher quality and require more bitrate. But the increase of visual quality between a Quantizer of 2 and a Quantizer of 1 is very small.

    Some other things you can try:

    1) Reduce the number of consecutive b-frames (BVOPs). B-frames are encoded at lower quality (higher quantizer) to reduce the overall bitrate requirement (the idea being that one or two of low quality frames between higher quality frames won't be noticed much at normal playback speeds). Changing from two b-frames to one b-frame or no b-frames will increase the bitrate requirement and increase the quality a bit.

    2) If you want to keep using b-frames change the b-frame Quantizer Ratio and/or Quantizer Offset so that b-frames have less quality loss. The b-frame quantizer is calculated as:

    b-frame Quantizer = i-frame Quantizer * Quantizer Ratio + Quantizer Offset
    With the default Quantizer Ratio of 1.5 and Quantizer Offset of 1, if I frames are using a quantizer of 4 the b-frames will use a quantizer of:

    bframe Quantizer = iframe Quantizer * Quantizer Ratio + Quantizer Offset
    bframe Quantizer = 4 * 1.5 + 1
    bframe Quantizer = 7
    3) If you are using the h263 Quantizer Matrix switch to the MPEG Quantizer matrix. It needs higher bitrate and retains a little more detail. Of use a custom Quantization Matrix. There are custom matrices which require even higher bitrates and retain even more detail.
    Quote Quote  
  5. jagabo, that was really helpful - you certainly know your stuff. I see from xmediarecode that I have no B frames at all - I didn't do that, must be a default setting. But I changed the Matrix from H263 to mpeg and that's resulted in a slightly higher bitrate/filesize (not that I could say there's an improvement in picture quality though).

    It's all interesting stuff, however. What about "Adaptive Quantization" - leave it Off?

    Thanks again.
    Quote Quote  
  6. Adaptive Quantization lowers the bitrate requirement and reduces quality a small amount.

    Try this: instead of using bitrate mode, use Target Quantizer mode (constant quality, you pick the quality, the file turns out with whatever bitrate is necessary for that quality). Set the Quantizer to 1 and encode with no b-frames. That is the highest bitrate Xvid can deliver (unless you disable some of the other features, like motion compensation or using custom quantizer matrices). Also try at 2 and 3. Most people consider 1 to be a waste of bitrate (very high bitrates but no visual improvement of Q=2), 2 to be nearly identical to the source, 3 to be a lossy but barely noticeable at normal playback speed.
    Last edited by jagabo; 3rd Feb 2011 at 17:56.
    Quote Quote  
  7. Thanks once again jagabo. As long as it's delivering the best quality, I'm happy to accept the lower bitrate generated by an Xvid conversion.
    Quote Quote  



Similar Threads

Visit our sponsor! Try DVDFab and backup Blu-rays!