But, the way mpeg2enc prevents the encoded data from going over that 9800 max (for VBR encoding) is by silently increasing the effective -q value for you behind the scenes. So for the peaks, to keep the
True, BUT - it is NOT "instantaneous"! By the time the encoder detects that the rate is too high and adjusts the effective -q the rate spike has already been passed to the output stream. So -b is in some senses not a "do not exceed" but "a strong hint".
Ah, that's an important detail. It might be a good thing if the man page would be a bit more clear about this.
The rate control is quite good for a single pass encoder but what's
really needed is a "back up and retry with a new effective -q"
(perhaps on a GOP basis) strategy _or_ a general 2 pass encoding
method (which of course almost doubles the encoding time). Back around
the beginning of the year there was mention of that being worked on
but that effort seems to have been suspended.
Perhaps it's a good idea to have mpeg2enc print a warning when the actual bitrate exceeds 9700 kbit saying that some hardware DVD players may not be able to play the stream perfectly.
The other thing to look at is the output of 'mplex'. Mplex prints out the "Average" and "Peak" rates. You can get those numbers fairly quickly by specifying "-o /dev/null" to mplex:
mplex -f 8 -o /dev/null input.m2v
Another *very* useful hint. Thanks! :)
values output is the actual -q that it used internally for that frame. If that value grew large, there's your artifact source.
Yep - and the other thing to watch for is how close the effective -q is to what you specified. If what mpeg2enc prints out is consistently a lot higher then you're very much rate limited and may want to consider filtering (perhaps smoothing/median).
Yes, I did. Shows nothing special. The actual q value never exceeds 1.25. Smoothing the frames a bit is a good idea indeed. I like soft images better than compression artifacts. :)
Dik
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