[mythtv-users] Cutting commercials without expensive transcoding

[papenfuss at juneau.me.vt.edu]

> > 	That's quite true, but I'm talking about the best quality/bitrate
> > possible.  For NTSC captures, anything over 480x480 is wasted resolution. 
> > If you scale that to typical DVD movie MPEG2 bitrate of 3.5 Mbps, the
> > 480x480 shouldn't require more than 2.3 Mbps to obtain the same results. 
> > The PVR-[23]50's tend to look pretty blocky at such low bitrates.  If
> > captured at higher bitrate and then cut back, it can look better than if
> > captured at low bitrate to start with, since the encoder has the potential
> > advantage of 2-pass encoding, etc.
> 
> I think you have some misconceptions about the relationship of resolution to 
> bitrate in MPEG video. MPEG isn't simply compressing pixels, and reducing the 
> number of pixels doesn't simply reduce the need for bitrate. Regardless, I 
> understand what you are saying and while it's true that with tweaking you can 
> get the best possible results by using a 2-pass encoder for the final 
> version, I personally haven't found it necessary. By the way, what encoder 
> and switches are you using to encode MPEG2 in 2-pass mode? ;)

	Not so, although it may have come across that way.  My point is that
everything else being equal, double the number of pixels requires roughly
double the bitrate.  I know there are groups of pixels, 16x16 and whatever.... 
but think of that has having to encode more of the groups of pixels (i.e. more 
data to mess with).  Now, that said, the resolution/bitrate issue is a bit 
complicated with MPEG, but suffice it to say that higher resolution requires a 
commensurate increase in bitrate to avoid MPEG artifacts (blockiness, etc).  So 
for efficiency, there's no point in having to encode artificially contructed 
(read: oversampled) data that's not really there.

> > 	It doesn't seem like it should, but it does.  There's much more in a
> > DVD mpeg stream that audio and video... and lots of choices that can be
> > made. I did the same thing two different ways... a 1 minute clip muxed with
> > avidemux's DVD-PS, and the other 'tcmplex -m dvd'.  Here are the resulting
> > file sizes:
> > 21720   test.m2v
> > 1428    test.mp2
> > 23912   test_tcmplexdvdps.mpg
> > 27540   test_avidemuxdvdps.mpg
> 
> Wow, I'm surprised. Then again, I've never muxed with avidemux, I only use the 
> raw video/audio. I also rarely use tcmplex, and usually use mplex.

	I was trying to avoid separating the video/audio streams and remuxing 
for fear of sync problems (bad past experiences).  The tcmplex thing is a 
preference I obtained from one of the many HOWTO's I've read recently... don't 
really care either way.

> Why would you mux with DVD NAV at all for a SVCD target? They are only 
> placeholders that will be filled in during the DVD authoring process, no? 
> Without something like dvdauthor to generate the VOBU information and then 
> stuff relevant information into those placeholders, they are useless. I 
> didn't know vcdimager would even use a stream that has them. (Never tried.)
> 
	Again... just to get avidemux2 to spit out a mux'd MPEG-PS, rather 
than an AVI (which it prefers).

> > I'm just a nut...
> > :)
> 
> I'm surprised you don't capture in 704x480 then. :) It may be overkill for the 
> signal source, but I've had several perfectionists argue to me that it looks 
> better. My position is that for cable-TV captures, 704x480 looks identical to 
> 352x480 when viewed via an NTSC output device. Resolution has nothing to do 
> with image quality, as long as it's higher than the signal source. Bitrate is 
> everything for MPEG2.

	Mainly disk space.  Believe me... you don't want to get me going on the 
resolution thing.  That was last month's tangent.  I've got some really good 
test signal generating software and hardware now though... :)  Bottom line is 
my PVR-250 has softness of the picture that renders anything about 450 lines
wasted... even though the TV can distinguish about 550 lines.  I can, however, 
see a difference in the capture quality between 352x480 and 480x480... thus the 
desire to capture higher than 352x480.  The jump to 704x480 is just too big.

	Although, I've recenting been thinking about a hybrid resolution
solution... similar to the anamorphic widescreen idea.  If you look at the
default DVD VFilter in avidemux, it wants to rescale a 480x480 4:3 to 540x480
and then pad with black to 720x480.  Most DVD players (I've seen anyway) have
the Pan/Scan mode, which stretches the image to fit both horizonatlly and
vertically.  If one were to scale a 4:3 (1.33) capture to 540x480 (1.125)
padded with black bars on the *sides* to 720x480 (1.5), it could be watched in
Pan/Scan mode on a DVD player and would look correct.  It would be full-screen
on a 4:3, as the black bars on the side would be off the screen.  The black
bars wouldn't cost much bitrate to encode, and the 720x480 would then be a
standard DVD size.  Anyway, to truly make this friendly, it would have to be 
done on the card... i.e. possibly impossible driver hacking.  Just a thought 
though...


> Very true. My results with tcrequant are very dependant on the source 
> material. It's a very hard thing to automate.

	Agreed.  Not that it necessarily has to be, but I've yet to find 
something that ties it all together.

> > 	Of course should set-top boxes play MPEG4's sometime in the future,
> > life is good... :)
> 
> Yah. A couple do, but I think it's just playing files on an ISO disk ala the 
> MP3-playing boxes. I'm not aware of any format that allows for MPEG4 on a DVD 
> structure, official or otherwise. Yet. Unfortunately, I read something lately 
> about Microsoft getting one of their proprietary codecs into the next 
> standard.

	I figured they'd want to keep hold of the standard.  Next thing you
know, all next-gen DVD players will have to run WindersCE to be
"standards"-complient ... :)

> I knew things were raw when I got my burner almost 2 years ago. Things are 
> _way_ better, but not there yet. At least I've been able to make my own 
> custom menus and limited special features with dvdauthor, though with much 
> time and effort.

	No kidding.... right now still not worth the trouble for the most part.

-Cory
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