[FFmpeg-user] b and bt

Nicolas George nicolas.george at normalesup.org
Tue Nov 27 14:28:42 CET 2012

Le septidi 7 frimaire, an CCXXI, Tom Evans a écrit :
> I don't really understand this. I thought with multi pass encoding,
> the encoder can 'save up' bandwidth from scenes that do not change to
> use in high action scenes. Surely this results in multi pass encoding
> giving an output that is of identical or better quality, but lower
> bitrate, since the encoder on the n-pass can be more efficient.

If you are not targeting a specified average bit-rate (or total file size,
for any given duration it is equivalent), there is no bandwidth to "save
up": the encoder just uses the bandwidth it needs to achieve the desired

Unless I am mistaken, it is usually considered that the best
quality-for-size is achieved when the quality is roughly constant. With a
specified average bit-rate, that is approximately achieved by tweaking the
bandwidth allocation. But with an encoder that supports fixed-quality, there
is nothing to do: you set the quality, and get the necessary file size.


  Nicolas George
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