[mythtv-users] VDPAU performance issue with 1 specific video

Ronald Frazier ron at ronfrazier.net
Sat Nov 6 12:33:36 UTC 2010


On Sat, Nov 6, 2010 at 4:11 AM, Paul Gardiner <lists at laser-point.co.uk> wrote:
> OT I know, but how do the transcodes look compared to original and
> DVD? What resolution does this use, original or smaller?

I'm sure some people are going to tell you it's quite inferior to the
m2ts straight off the bluray. After all, there is a big difference in
bitrate... typically 20-40 mbps on the bluray, vs 5-10 mbps that is
typical from the transcode settings I typically use (though with the
constant quality settings, it will go as high or as low of a bitrate
as it feels it needs at any given moment to maintain quality, but 5-10
is what I typically see).

Myself, I don't see a difference from the full quality bluray, even
with up close inspection. I transcode at full 1080 resolution, and
I've often compared the transcoded file to the original m2ts and found
them virtually identical. From time to time I'll convince myself that
the transcode was not as good, only to throw on the m2ts and find it's
just the source that was blurry or blocky.


>Do you know any tricks for guaranteeing keeping in the lower range of 4-6GB so as to fit on a DVD?

This is completely against the idea of a constant quality setting.
Constant Quality tells the transcoder "I don't care how much space you
use, as long as you keep the quality up". If you wish to maintain a
particular size, then the transcoder can't guarantee the quality for
you. What you are looking for is the Target Size encoding method, not
Constant Quality. There you set a maximum size and it encodes to fit
that size. The downside of this is that the transcoded file may be
bigger than it needs to be (using a really high bitrate on material
that is not very complex), or on the opposite end of the spectrum, you
may be disappointed with the quality of the video. Especially when you
hit particular parts in the movie that are very demanding, the quality
may temporarily degrade. You can counteract this by using 2-pass
encoding, where it processes the video 2 times....once to figure out
where the most demanding parts of the video are (so it can allocate
higher bit rates to those parts and then compensate by using lower
bitrates in the least demanding part, thus averaging out) and the 2nd
time to do the actual transcoding. The downside of this is that, 1) it
still might not be the quality you desire, and 2) it takes nearly
twice as long to encode (my core i7 920 typically takes about 4-5
hours to transcode a movie, so with 2 pass it would be 8-10 hours).

That said, of all the blurays I've transcoded using the Constant
Quality RF 24 setting, the largest of the batch has been 7.8GB, so if
you were happy with these same settings, it would probably be very
rare that you'd miss your target and have to re-encode at a smaller
size to fit a dual layer DVD. If you are targetting single layer DVD,
then about 15% of my blurays are over that size.

And while we are on the subject, FYI...I encode my DVDs using Constant
Quality RF 20 which gives me 0.5 to 2.5 GB per movie (except for the
LotR extended edition trilogy, which ran 3.2, 3.7, and 4.0 GB per
movie).


More information about the mythtv-users mailing list