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Thread: Setting the maximum variable bit rate in MainConcept AVC/MP4

  1. Default Setting the maximum variable bit rate in MainConcept AVC/MP4

    Hi,

    My footage was recorded in 720p, 60p, 11Mbps. The content are interviews.

    a) Now I intend to render by using MainConcept AVC/MP4 and the preset “Internet HD 720p”. In this preset the average (bps) is 8Mbps and maximum is set to 16Mbps. But since my original footage is 11Mbps, it seems logical to set the maximum bit rate to 11Mbps. Am I right about that? Or is there any parameter/procedure that I should be aware when setting the maximum bit rate?

    b) By downsizing from 11Mbps to 8Mbps (average) is something to be concerned in terms of quality loss?

    Thank you in advance,
    Cadu

  2. #2

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    The only way you will really know if your going to get quality loss is to try it. I've never been a big fan of variable bit rate but I don't usually have a consideration about file size. It's true that you won't get better quality by having the max bit rate higher than the original footage, so there is no point in having it set to 16Mbps. I think all you will do is increase the rendering time with no gain in quality.


    I think you seem to have a grasp of how things are but the missing element in your calculations is how will the final render codec deal with the original codec. In theory it shouldn't be an issue as the editing software opens the original footage uncompressing it so we can see it BUT in practise I have found this does have a bearing on the final result. This is why you really have to do some research yourself with the different formats to see what works best for you. I'm not a codec expert and I've never met anyone who is in real life or on-line, This is another reason why I say to your own testing to see what works best for you.

    Example, I have a camera that shoots DVCPro HD and found I get a better results rendering to .mpeg2 than .mp4 BUT another camera I have records .H264 and renders out well in .mp4. I don't know the technical reasons for this, it's just what I've found myself.

  3. Default

    Thanks for answering!
    I am doing some tests and comparing qualities when using different average/max VBR.
    However, I am wondering if CBR would be more effective, since the rendering time is shorter (comparing to VBR two-pass) and my footage is an interview, without significant variations in the movements during the 60 minutes of editing footage. The framing is basically the background of an office, and persons behind a desk, slight changes in position and hands movements.
    Theoretically, does it make sense that to this context CBR would be better than VBR?
    To the described footage content and using CBR, what is supposed to be reasonable bit rate value, bearing in mind a equilibrium between quality and file size? I will do my tests, but I would appreciate to know about experience of others when setting bit rate values...
    Cadu
    Last edited by cadu; 02-24-2013 at 10:07 PM.

  4. #4

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    Doing the two pass option with an interview video as you say is just adding time to the render without any improved quality.

    I mentioned earlier that I use CBR that's because I don't trust the codec to know what I want to be best quality and what "it thinks" can be a reduced bit rate. I usually set the CBR to the same value as the original footage. If it's mixed footage, I suppose it depends on what there is of what. I'd probably choose the higher bit rate of the best original footage, which may be over kill but I'm happy with that.

    I know CODECs are better than they use to be in a lot of respects but I'd rather be the master of my own decisions and not let the computers take over fully just yet.

    Good luck with the testing.

  5. Default

    As mentioned, you "probably choose the higher bit rate of the best original footage".
    Having in mind to maintain the bit rate of the original footage, I am wondering about:
    My original footage is 1280x720, 60p, 11Mbps.
    When rendering, I am maintaining same resolution (1280x720), but downsizing the fps to 30p. Since the fps are half of the original footage, it seems logical that I could downsize the bit rate to 5.5Mbps without quality loss. In other words, using the same 11Mbps would be overkill since I am using half of the fps when rendering. Does it make sense?
    Thanks,
    Cadu

  6. #6

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    Using CBR you are restricting your self to a set point in quality with a set point in file size. Using VBR you are getting the most from what you have. If you use VBR you will get the most from your bit rate when there is more movement, and only what is necessary when there is minimum.

    If you use VBR always do 2 passes, a single pass is a compromise, just like CBR is a compromise.

    David.

  7. Default

    I got the idea of VBR to obtain "the most from your bit rate when there is more movement, and only what is necessary when there is minimum".
    But what I am wondering is since my footage is totally linear in terms of movement (the framing is basically the background of an office and a person behind a desk, slight changes in his position - chair movement left/right - and hands movements) it seems that VBR 2 passes wouldn't make such a difference. I mean, if I had interviews+action scenes in same project VBR would be indicated, but since I have just linear interviews, by using CBR I could render in half the time of VBR 2 passes and achieve the same result basically. Does it make sense?
    Thanks,
    Cadu

  8. #8

    Default

    Quote Originally Posted by cadu View Post
    When rendering, I am maintaining same resolution (1280x720), but downsizing the fps to 30p. Since the fps are half of the original footage, it seems logical that I could downsize the bit rate to 5.5Mbps without quality loss. In other words, using the same 11Mbps would be overkill since I am using half of the fps when rendering. Does it make sense?
    Thanks,
    Cadu
    It may very well be that using 11Mbps is over kill and that you could get a good image with a lower bit rate and thus giving you a smaller file size if this is an issue for you BUT just because you are lowering the frame rate by half doesn't mean it's ok the lower the bit rate by half as they are two different things. The frame rate as you know is the number of separate images recorded per second. The bit rate is the amount of detail or information each separate image will have. So no matter how fast or slow the frame rate the bit rate needs to be high for a better image quality.

  9. #9

    Default

    Quote Originally Posted by cadu View Post
    I mean, if I had interviews+action scenes in same project VBR would be indicated, but since I have just linear interviews, by using CBR I could render in half the time of VBR 2 passes and achieve the same result basically. Does it make sense?
    Yes, if it makes sense to you. Now can we all go to sleep?

  10. #10

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    just like CBR is a compromise
    I don't see it as a compromise, if you select the same bit rate as the original footage you will maintain the best quality. As I have said above, I don't trust a computer to make some decisions for me when it thinks I want more or less bit rate. I do agree that it may be a compromise of file size if that is an issue for people. This is why it's best to do your own experiments with what ever codec you are shooting with.

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