Re: How to determine VOB encoding quality?
From: Alessandro Angeli [MVP::DigitalMedia] (nobody_at_nowhere.in.the.net)
Date: 01/12/05
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Date: Wed, 12 Jan 2005 11:23:51 +0100
Tagman wrote:
> - I'm assuming that it is possible to select a bitrate in
> excess of what is required (based upon the video quality
> of the source).
You can, but smart encoders will not heed your advice and
use a lower bitrate if so many bits are not needed to
achieve the target quality.
> In your example - you stated that a DVD with a 5.4 mbps
> could be encoded with a wmv rate of 2.0 mbps with similar
> results. How did you calculate the 5.4 figure? How did
> you determine the 2.0mbps suggestion?
I calculated the bitrate of the source from the data you
gave: "1 hr video can be 3-1/2 g", and since bitrate ~=
8*size_in_bytes/duration_in_seconds, this gives 8Mbps (I
made a mistake before, I calculated the bitrate for 1.5h
because I can't read :-)). Then, from experience, you'll
discover that you get the same quality of DVD using 50-70%
of the bitrate and near-DVD quality using 25-50%. If you use
2-pass VBR of course. Single pass VBR or CBR will produce
visible quality degradation most of the time, unless you
perform a quality-costrained encoding with a low
compression, which will however produce very large files.
> 1) I guess my question should be "how can I tell which
> bitrate would be the appropriate selection to encode a
> vob file into a wmv file without incurring "Noticeable"
> video and audio degradation?"
Experience and these guidelines (gathered from experience as
well):
1. WMV9, MPEG4/ASP or MPEG4/AVC, encoded in 2-pass VBR, can
compress at 25-50% of the DVD bitrate (DVD uses VBR MPEG2)
for near the same quality and 50-70% of the bitrate for the
same quality (or close to it)
2. when you shoot for near-DVD quality, you may want to
reduce the horizontal resolution to 3/4 and crop any black
bars or border noise (which are BAD because of 10 and 8)
3. you ALWAYS want to deinterlace the video, because
interlacing is very BAD because of 8+10
4. OK dark video
5. OK low motion video
6. OK large uniform areas
7. BAD moving background
8. BAD small moving details
9. BAD flashing lights
10. BAD sharp edges (if the black bars are sharp and fall on
16 pixel boundaries, the sharp-edge effect is non existent,
which means it is better to crop the black bars on 16 pixel
boundaries and then letterbox the picture again, if backbars
must be preserved)
11. resolution must be multiple of 16 pixel (to avoid
padding, which is the same as a black bar, see 10) - better
yet, 32 (this one is not video-related, but
hardware-related)
If the video is long enough, a 2-pass VBR encoding is most
likely capable of distributing the bitrate in order to
compensate OK with BAD scenes, if OK and BAD scenes are
mixed. Short uniform movies are harder to compress because
the bitrate can not be distributed to compensate.
> 2) Once I determine what that setting should be - should
> I always stick with it - regardless of the source video -
> or do I need to constantly tweak the encoding bitrate
> each time by checking the bitrate of the source?
It depends on the content, like the previous guidelines
suggest, but long movies usually need less tweaking then
short ones (for the reason given above).
> I have a 23 minute 46 second video clip (1,426 seconds)
> The existing VOB size is 1,032,394kb
>
> How would I calculate the source mpbs rate in the case?
I'd try ~2.0Mbps in 2-pass VBR first. If I'm off in my
prediction, I'll correct it.
> Based upon this small 60 second sample - the average
> bitrate is calculated as 6329
> Do I need to take the peak bitrate of 10279 into account?
I usually only take the average bitrate into account. The
difference between the average and peak bitrate however is
an indication about the presence of harder-to-compress
scenes if it is high ("high" is not very well defined, let's
say something like 30% of the average bitrate), which may
require a slightly higher target bitrate when re-encoding.
-- // Alessandro Angeli // MVP :: Digital Media // a dot angeli at psynet dot net
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