Re: benefits of constant bit rate versus variable?

Not to be too argumentative, but I personally shy away from 'absolutisms'. There are, IMO, merits to using either CBR or VBR. When disc space is an issue, then I completely agree that VBR is the only way that one can produce reasonably high quality (visual) files using lower average bitrates. VBR's advantage is that the encoder is given a range to encode--using lower bitrates for less complex scenes (like talking heads) and bumping the bitrate up to cover more complex or high motion scenes. VBR still has to 'hover' around an average, and the encoder has to 'choose' when and where the bitrate should be adjusted. Unless you are a compressionist using a higher end soft/hardware encoder, you are probably not doing segment by segment encoding, and are leaving this task up to the encoder. All encoders are not created equal, and that's why one can detect differences (sometimes significant) between encoders given the same material. The lower the average VBR bitrate one chooses, the more the 'men are separated from the boys' when comparing encoders.

CBR does have it's advantages over VBR, especially with consumer and prosumer software encoders, when disc space is not an issue. This is a quote from Ben W on Canopus' forum for Procoder (excerpted from a similar 'raging' debate over VBR and CBR over two years ago):

"Yeah, a 8000 Kbps CBR file will always be at least as good as a VBR with a 8000 Kbps peak. The difficult parts will be at the same data rate, and the easier parts could be at a higher data rate than the VBR. VBR is only needed when disc space is the limiting factor instead of throughput."

A CBR file encoded at the maximum setting for VBR will always be as good, and most likely better, than the VBR counterpart. Using CBR eliminates the need for the encoder to make 'choices' about when and where to adjust the bitrate--a high bitrate is used throughout. Using a high CBR bitrate ensures that all complex scenes and high motion scenes are encoded with adequate bitrates--no worries about the encoder making poor choices. Unfortunately those scenes that don't need as high a bitrate (like squirrel hunting with an elephant gun--effective, but overkill) will also be encoded at a high bitrate, which in turn eats up disc space. This gets us back to where I started--if the content will fit on the media in question without concerns for this 'wastefulness', then CBR is a good option to consider. CBR encodes are generally much faster, which is an added bonus.

Your reference to the quote by Barry Braverman compares CBR files encoded at the same rate as the AVERAGE of a VBR encode--and of course his conclusions are accurate, but are irrelevant when one considers the above.

Jim