On Thu, 28 Oct 2004, Richard Ellis wrote:
> On Thu, Oct 28, 2004 at 12:26:24PM +0200, Dik Takken wrote:
> > Quote from the mpeg2enc manual, -b option:
> >
> > "If variable bit-rate mode has been selected (see the -q option)
> > this is the maximum bit-rate of the stream."
> >
> > So, the "-b" value is not the average, but the upper limit when
> > "-q" is specified. So, "-b 9800 -q <anything-but-1>" should be
> > perfectly safe?
>
> 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".
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.
> Did you watch the output of mpeg2enc as it encoded (assuming you had
> it running so it output it's stats about each frame). One of the
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
> 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).
Cheers,
Steven Schultz
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