My definition of a “bit constrained environment” is simply one where you do not have the luxury of providing enough bits to accurately represent the frame. This could be due to length, it could be due to resolution (wanting HD output) which affects size, it could be due to media limitations e.g., DVD cannot sustain more that 9MB while HD is recorded at 25MB / 50MB / 100MB. Whatever is causing you to render at a lower bitrate than originally recorded.
30 min of SD football footage can easily fit on a DVD at the max bitrate allowable so CBR will do fine. 90 min of footage cannot fit so VBR may yield better results but only if there are enough scenes without motion to steal bits from. 90 minutes of non-stop action will not allow VBR to have any benefit.
~jr
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