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DMH79
Inspiring
September 20, 2012
Question

Export Settings - Minimum Bit Rate Question (MPEG2-DVD)

  • September 20, 2012
  • 1 reply
  • 20139 views

Hi,

Curious...just came over recently from FCP and I don't remember Compressor giving me an option to adjust the MINIMUM bit rate when exporting HD projects for DVD. I figured I'd just leave it where the preset has it (at 2.8mbps minimum)...BUT...then I hovered over it and saw an interesting popup/explanation of what it does which kind of confuses me. It says:

Higher values set a higher minimum quality, but reduce quality of more difficult scenes.

The first part makes sense so I figured I'd raise it...but then the second part makes me think I should keep it low. Sort of confuses me. Any thoughts??? FYI: My projects are Weddings with a fair amount of action, etc.

....

Normally, these are my settings depending unless I can't fit the project onto the disc in which case I adjust:

DVD

CBR at 7.5mbps

...or...

VBR 2pass

Target Bit Rate: About 6.8 or 7mbps

Max Bit Rate: Usually 8mbps

BLURAY

VBR 2pass

Target Bit Rate: 25mbps

Max Bit Rate: 30mbps

Unless someone tells me that I should raise these settings higher for better quality output (if the project size allows of course) these are what I've been using to get maximum quality out of my videos without jepordizing playback due to bit rate max for each media (which I think I read was 10mb and 40 or 50mbps respectively. I was just thrown by the minimum bit rate description above.

Thank you in advance for your help!

    This topic has been closed for replies.

    1 reply

    S
    SAFEHARBOR11
    Participating Frequently
    September 20, 2012

    Hi,

    Some encoders offer the MIN bitrate setting for VBR, others don't. Even in Encore, DVD offers it, Blu-ray does not. Think of Variable Bit Rate encoding as such: if you have a very long video, that means the bitrate would be low to fit the project on disc, so quality will suffer. But, VBR can "allocate" the bitrate on as "as-needed" basis by first examining the video and determining which scenes need a higher bitrate (action) and which scenes might look fine at a much lower bitrate (static scene, less detail, etc.), then encoding based on that analysis.

    With an example of 3-5-7 VBR settings, a simple white on black title screen is going to be encoded at the minimum "3", a high-energy dance segment will be encoded at "7", and overall, the "average" will be "5". Because of this distribution of quality where it is needed most, the video should look better than if it had been encoded at CBR 5 for the whole thing.

    My thoughts and experience are this - for shorter videos, why use VBR at all? If you have a video of an hour or less for instance, you have the opportunity to encode the WHOLE THING at 8, which is tops! Why used VBR, which would then encode some scenes at a lower quality? Just use CBR 8 and get the best quality for all of it.

    With VBR 2-pass, you are likely doubling up the encoding times without any benefit in many cases.

    About Blu-ray, I believe the MAX allowable is 40Mbps, but to me, H.264 at 20 looks very good so 30 might be a good MAX, why push things and have possible playback issues? There is a point of diminishing returns - the video can only look "so good" and there comes a point where any difference from a higher bitrate cannot be discerned with the eye.

    Just my opinion

    Jeff Pulera

    Safe Harbor Computers

    R
    Rallymax-forum
    Inspiring
    September 20, 2012

    The whole notion of a minimum bitrate is crazy unless you have specific broadcast requirements that require you to pad out video to keep the connection alive.

    If the encoder can express the image with zero loss in less than the minimum bitrate why would you pad it with zeros to get the bitrate high enough to meet the min?

    The idea of a nominal/average bitrate is easy to understand.

    The Maximum bitrate is often misunderstood though.

    On some encoders the max rate sets the wiggle room (max - average) that can be used if there is a burst of required information.

    On other encoders it sets the maximum rate at which the video buffer is allowed to fill as per the specifications. eg Blu-ray is capped at 40Mbps. The Video encode itself MAY EXCEED THAT 40Mbps for a split second but will only LOAD into the video buffer at up to 40Mbps.

    For example, if you set constant bitrate at 20Mbps and no Maximum rate when the video first starts loading from the blu-ray disc to the video buffer it loads faster than the blu-ray maximum of 40Mbps. Thus you MUST have a maximum rate defined (for blu-ray compliance) even though it's a constant bitrate. Some encoders do this for you but some leave it up to the user to get right (and thus should provide a max slider and constant slider to set both).

    Jeff- absolutely agree. It's very rare that you need to use all 40Mbps available to you. 20 and 30 can look great (depending on the detail in the sceen and how much motion there is).

    VBR 2-pass does have value if you're trying to get down to lower rates like 10Mbps to fit a long title on a disc. If you don't action shots with sceen cuts will leave artifacts as they are bit starved.

    I'm actually a big fan of Constant _Quality_ (CQ) vs CBR or VBR. You then know what quality you're going to get on every frame and from experience will know how big it will come out to be. CQ is also considerably faster to render because you have no rate control computations to do. x264pro has a CQ option for this very reason.

    hope that helps.

    D
    DMH79Author
    Inspiring
    September 20, 2012

    This all does make sense and I appreciate your responses. However, I still don't totally understand my main question: why does it say what it does about minimum bit rate (see below) and what should I do? I don't want to reduce the quality of more difficult scenes but I also want a my minimum quality to be low and not look good during "simple" scenes. Yes, CBR is great, and I use it most of the time...but sometimes I have to squeeze in a bit more onto a DVD and I use VBR to get a little extra out of my video at a smaller size. In this case what should I do about the minimum bit rate? Leave it at 2.8 and risk a lower minimum quality (if that even effects the final video), or raise it up to 4 or 5 or whatever but risk apparently reducing the quality of more difficult scenes. Why would it even do that? Doesn't make sense.

    Again, it says:

    Higher values set a higher minimum quality, but reduce quality of more difficult scenes.

    Seems like a catch-22 to me.

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