On Mon, 25 Feb 2008, [UTF-8] Bernhard Fr??hmesser wrote:
> I was wondering if anyone knows the programm Compressor from Apple?!
I've used it and the later version (Compressor2). Not very often
for video, mostly for AC3 audio encoding. Haven't seen the latest
version (3).
> I am asking because i was curious and did a test with a 6 minutes video,
> one i encoded to MPEG2 using "Compressor" under MAC OSX and then i used
> mpeg2enc on the same video on my Linuxbox.
> I am sure i used the same settings on both computers but the file
That's impossible ;)
That of course requires some explanation so here we go...
You almost certainly specified a "-q" with mpeg2enc. That is a flooor,
a minimum below which mpeg2enc will NEVER go. mpeg2enc will raise "-q"
(increase the amount of compression) to keep the bitrate below the
maximum. In its extreme and buggy form this increase will cause the
effective quantization to be 30, 40 or more and poor quality.
However, if the encoding produces a bitrate LESS than what you specified
on the command line then mpeg2enc will NOT lower '-q'!
This has the effect of transforming the '-b' (bitrate) option into
a "maximum" (try not to exceed) RATHER than an "average".
SO if you don't specify a low enough value of '-q' the average bitrate
will be less (often a lot less) than the value specified by '-b'. You
may specify "-b 8000" but depending on the -q you may end up with an
output file closer to 3500.
Compressor on the other hand uses the bitrate you specify as an
AVERAGE and will not only increase the effective quantization ("-q")
to keep the bitrate low enough, Compressor will also DECREASE the
quantization to increase the bitrate towards the specified average
value.
This means it's really not possible to say much more than "I specified
the same bitrate" to both compression programs. How that that value is
used is different. And the concept of '-q' and the other options
from mpeg2enc are not present in Compressor (just very limited choice
of GOP size).
Compressor is doing what mpeg2enc _should_, in my opinion, be doing:
use the specified bitrate ('-b') as an average (not maximum) value
and doing completely dynamic and _INTERNAL_ adjustment of quantization
("-q" should be removed from the list of options).
> with mpeg2enc has only 114MB so far i don't see much quality difference.
Which brings me to the point I believe I've made before.
Pushing -q too low doesn't necessarily increase visual/perceived
quality. It will raise the bitrate but will you see the difference?
Maybe, maybe not.
Too much time is wasted fiddling with "-q" in the name/quest of
"quality". This is because mpeg2enc doesn't do it automatically
but realistically most folks could just stick a "-q 4" in place and
be done with it. Maybe some day -q will go away but I'm not holding
my breath (you turn blue that way :)).
> So i was wondering what this could come from? Any ideas?!
I think, in your case, the video doesn't need as high a bitrate as
you specified and the values you used with mpeg2enc caused a lower
bitrate to be used. It's also possible you're seeing a mild version
pf the bug that can causes mpeg2enc to push -q up too high.
You can get a good idea of the average bitrate like this:
mplex -f 8 -o /dev/null input.m2v
Try that on the two .m2v files you have and see what the average bit
rate and frame sizes are.
Cheers,
Steven Schultz
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