I click auto, and the encoded file is 15% smaller than expected?

[broaddd]

Hi!

I use ffmpegx a lot, and I notice that if I encode a video using the bitrate and resolution set by the 'auto' button (after entering the ratio, framerate, length and desired encoded size, and setting the audio to passthrough at the original bitrate and CBR), the encoded videos are often 10-15% smaller than the desired encoding size.

Is this because: I should set the audio bitrate to VBR? the nature of the video (eg. dark, or not much action) just doesn't need the the bitrate set by 'auto'? or some other reason?

PS. Please don't just read this thread and move on - if it happens to you too, why do you think it is?

Cheers,
B

[Case]

In ffmpegX, the one way to get a desired encoded size consistantly with any precision, is three-pass encoding. Unfortunately, it's only available to DivX mencoder.

[major]

Depending on the format, the encoder may decide to allocate less bits if it has enough evidence that allocating more bits would be redundant.

[broaddd]

Is there a tool to kind of pre-scan source files and advise what bitrates will be needed at various resolutions, rather than the default bitrate that ffmpegx calculates?