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

Question

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!

Answer 1

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.