"Active" VBR streams, regardless of what you specify, can only vary by so much. ATI encodes operate a bit more like CVBR (constrained variable bitrate). It has a range, but it's not a huge range. A lot of DVD recorders work this way too (LiteOn, for example, and Panasonic).
You can specify 15,000k if you wanted, but if you load the captured video into a bitrate analysis tool, you'll notice the spikes are small. When you specify a value that is too high, it also has a tendency to go too low. It can over-compensate.
You've got to remember that an "active" VBR capture only has a buffer than is so big.
Compare this to a VBR encode done complete in software, loaded from an AVI file. It usually doubles the encode time, often taking an hour at minimum, to analyze the file for the best VBR allocation. WHen you capture to VBR, you skip this step. You get a 1-pass quickie guestimation based on what's loaded in the buffer.
Now that's sounds sloppy, and it really is, but it's still better than CBR (unless you set the CBR bitrate to the max allocation spec for the resolution, such as 4000-5000k at 352x480). VBR is meant to not take up space, only use what is needed, when needed. Sloppy or not, it works fine almost all of the time, as long as you give it a good bitrate.
Think of VBR capturing as CBR with a bonus. It will use a small bonus if given the option, when needed, but it'll basically capture CBR at the average you specify most of the capture time.
Certain capture chips (like LSI chips found in JVC DVD recorders) have a very large buffer, which explains how it can both filter video noise, as well as do a decent VBR encode. The JVC has a range of about 1200k up from max, and it really does use it.
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Motion estimation is how MPEG works during the encode phase. This is really hard to explain, but I'll try my best.
Estimation is a scientific was of saying "educated guess". When a football player is running across the screen, it guesses from frame to frame. This deals with temporal compression. There is also a relation here to deciding it's time to use more or less bitrate.
In some encoders, this is also a basis for encoder-controlled GOP length and GOP type. ATI MMC only does the GOP type specified, it is not "smart" here like Procoder or other advanced MPEG encoders.
I don't really know what else to say here. This is one of those super-techie MPEG topics. Give this a gander:
http://www.cs.cf.ac.uk/Dave/Multimedia/node259.html
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Basically, the settings shown in the ATI MMC guide only apply to the ATI Theatre chipset, and devices of a similar nature. Other hardware can vary, as can software encoders (TMPGEnc, Procoder, etc).
TMPGEnc "CQ" mode is more similar to CVBR. It uses a % instead of hard Mb/s (k) bitrate values, but it's the same thing. For example, 85% of 4000k is 3400k. This is a setting that matches the ATI MMC almost identically.
I use a lot of ATI MMC with 352x480, 3420k average, and 3900k max. I've set it for all kind of things higher before, and rarely did it use anything higher. Feel free to try on your own. Remember I have the Theatre Rage/100 chipset, the so the 200 may be a bit more advanced. Now that cards have come down a bit in price, I may grab a 9800 AIW in the near future.
Practically, give yourself a max about 400k-600k above the target.
