On Thu, 6 Apr 2006, Nicolas wrote:
> When you've determined the bitrate, how do you choose the value for -q ?
Pick a number between 2 and 8 :-) There are higher values (up to 31)
but in practice they're not used (not often).
Folks think of "-q" as being "quality" and indirectly that's true.
The encoder though does not make decisions based on "high quality"
or "low quality".
It might be helpful to think of "-q" as representing the amount of
compression applied to a frame. Low values of 'q' mean LESS compression
is being applied - this retains more information which of course
means bigger files (higher bitrate). High values of 'q' mean MORE
compression - the extreme of 31 is the most aggressive compression
offered. Very small files (low bitrate) but probably unwatchable.
Might be better to think of the specified value of 'q' as being
the "quantization floor" or "mimimum amount of compression applied"
rather than "quality". The value you specify is the value that the
encoder will NOT go BELOW. The encoder can (indeed MUST) go HIGHER
than the value you specify - that's the 'q=' lines logged by the
encoder.
Part of the "art of selecting '-q'" is realizing that too low of a
value does NOT improve the quality at all - if you can't see the
difference between -q2 and -q3 why waste the bits? Similarily for
'-q 4' vs '-q 5'. If you can't see the difference then why spend
the extra bits for 4 when 5 would work ok?
Using a lower value than necessary means that frames will be less
compressed and use more bits for no purpose - they're wasted. True,
it's "higher quality" but quality you can't see is pointless. Put
feature or short subject on the DVD with the bits that are saved by
setting the quantization floor up a point.
There aren't any firm/fixed rules for how to set the '-q'. If you
set it too low either 1) the encoder will raise it to what is necessary
to accomodate the bitrate constraints or 2) you're wasting bits
on invisible quality. If you set it too high then you're tossing
quality away ;(
Sorry to be so "vague" but it is a bit of an art form - there's no
"use -q N" magic rule.
You might find that '-q 5' will avoid the rate spike you're fighting.
Maybe '4' is not applying quite enough compression in a few places.
Might be worth a try as an experiment.
Cheers,
Steven Schultz
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