On Thu, 23 Nov 2006, Richard Rasker wrote:
> question I have deals with one of the elusive Holy Grails of video
> processing: how to render a given DV file into the highest possible
> quality DVD.
In addition to being a holy grail it is also known as a 'wild goose
chase' ;)
Actually it's not all that hard once you wrap your brain around that
fact that a few extra kbits/sec for video is NOT going to make
any difference at all in visual quality.
> mplex to merge both files, only to find that either the maximum bit rate
> and/or quantization value for DVD has been exceeded. After which I
The bitrate is the only thing that is (or can be) exceeded. There is
no such concept as "quantization value for DVD being exceeded".
> Sometimes it takes three attempts to reach the optimal values, and with...
>
> Example of the render pipe command:
> yuvcorrect -v 0 -T INTERLACED_BOTTOM_FIRST | mpeg2enc -M 2 -v 0 -r 32 -4
> 1 -2 1 -D 10 -g 15 -G 15 -q 5 -b 9600 -f 8 -o %
For one thing you could speed up the coding by deleting the "-r 32".
That is past the point of diminishing returns - i.e. you gain almost
nothing over the default "-r 16" but the encoding time is longer.
I'm curious why 'yuvcorrect' is being used. DV is always bottom
field first - is something earlier in the pipeline mangling the
field order tag?
-M is either useless (no speed gain) or worse (subtle threading
race can trigger an assert() error).
And 9600 is too high for a bitrate value. Try 8800 or even a little
lower.
> It would be nice if there was a tool to calculate which values for bit
> rate and quantization would still result in a valid DVD MPEG file, based
> on a given DV file, without actually having to go through the lengthy
> rendering process itself. Does such a 'video complexity analyzer' exist?
Yes, there is - it's called 'mpeg2enc' :)
mpeg2enc IS analyzing the file and mpeg2enc IS adjusting the
quantization to meet your specified bitrate.
All that you need to do is use a sane value for the bitrate and
the output will be fine.
I am not sure what you mean by "calculate quantization" - the encoder
will calculate and vary that to stay within the bitrate you've given.
IF you mean the quantization floor (amount of compression) that "-q"
represents then that's probably not what you think it is (granted the
documentation is a bit awkward and it is a confusing concept).
> I couldn't find anything, but I'm still rather unfamiliar with the
> actual intricacies of MPEG and the likes.
> So, are there any thoughts on this?
The topic has been discussed MANY times on this mailing list. A little
searching with bitrate related search criteria should yield quite a
few hits.
Basically the problem is folks see "10.08 megabits/sec" and the DVD
rate and try to use too much of that for video. That max rate has
to not only handle the muxing overhead (general guide I use is 2%)
AND more IMPORTANTLY video bitrate spikes. mpeg2enc's rate control
is excellent but rate spikes are a fact of life.
Assume the audio rate is 224Kb/s (a commonly used value for MP2).
So out of the 10.08 and subtracting overhead, audio and allowing for
10% rate spikes we end up with about 8680. So, to be completely
safe around 8600 to 8700 would be the highest value I'd use.
Maybe even be a little more conservative and use 8500.
Really - you won't see the difference between 8600 and 8800 or even 9000
Often the question/problem is how to calculate a bitrate in order to
fit a N minute movie on a DVD. For THAT there is a simple rule
that will provide a good starting point. I won't bore you with the
derivation (it's off in the DVD Studio Pro books/documentation) but
given a movie length N (in minutes):
bitrate (Mb/s) = 560 / N
For a 90 minute movie, 560 / 90 = 6.222 so "-b 6500" would be a
good starting point.
With dual layer media use 1012 instead of 560 ( the two layers are
not the same size, that's why the figure is not simply 2x the single
layer value).
Good Luck!
Cheers,
Steven Schultz
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