On Fri, 26 Dec 2003, Daniel Silva wrote:
> I'm encoding an mpeg2 bitstream using the following command:
>
> mpeg2enc "inputfile" -f 3 -a 2 -b 7500 -q 5 -V 220 -o "outputfile"
>
> When we use a quantization step (5 in this case), the bitrate used by
> the mjpegtools will be a VBR. But I notice that the maximum bitrate used is
> not the one we give as a parameter (7500 kbit/s) which led to poor quality
And, as was mentioned in the last reply, without some more information
such as the framesize and so on there's not a lot that can be said.
If you're encoding 160x120 thumbnail sized images then the specified
bitrate is not needed and thus will not be approached.
For SIF sized images ~1Mb/s is about all that is needed in many cases -
you could specify a max rate of 10000 and only 1000 would be used.
Are you encoding for one of the standard formats such as DVD or SVCD?
What is the pipeline prior to the mpeg2enc command? Where did the
data originate (was it an analog source, DV camcorder, MJPEG card, ...)?
We really don't have enough information to do much more than say
that the max rate is not used because it is higher than needed.
> I'm doing a study of the mjpegtools and I have to look into the witten
> code to discover why this happen (why the bitrate never reaches the one
> specified in the -b field). The idea was to change the code in order to
The most likely reason is that the rate needed is lower than the
maximum specified.
> make the control of the bitrate only when the maximum bitrate desired was
> reached because right know it is controlled right from the beginning, and
> therefore the 7500 Kbits/s aren't reached.
> Does anyone know if this is possible?
Almost anything is possible of course. But not always necessary.
The rate is constrained by _2_ factors: 1) the maximum specified and
2) the number of bits required. If #2 is lower than #1 then the
setting of the maximum is irrelevant.
Cheers,
Steven Schultz
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