Minimum difference between average and peak bitrage in mpeg2?

Question

is there a technical reason why compressor insists on certain minimum differences between average and peak bitrate when compressing to mpeg2?
when analyzing other footage, i regularly see material where the difference is much smaller than what compressor will allow me to do.
for instance; i'm seeing material that is compressed at 4.7 average with a peak at 5.2. compressor insists on setting the peak at 5.5 when the average is 4.7 ...

Answer 1

Well, compression theory says that, ideally, the max bit rate would be double the average bit rate. But since since life often gets in the way of theoretical perfection, Compressor 2 imposes a minimum 1 Mbps difference. This is partially touched upon in the DVDSP 4 manual. From page 64:
Since the VBR methods are most effective when the maximum bit rate is about double the basic bit rate, or at least 1 to 3 Mbps higher than the basic bit rate, the Max Bit Rate setting is forced to be higher than the Bit Rate setting.
Now, while Compressor 2 is coded for a 1 Mbps difference, it doesn't always follow that, in my experience. There are instances when, footage permitting, Compressor uses average bit rates lower than specified as well as never hitting the max bit rate that you've set (but it really does depend on the footage).