[mythtv-users] Cutting commercials without expensive transcoding

papenfuss at juneau.me.vt.edu papenfuss at juneau.me.vt.edu
Sat Mar 13 15:59:01 EST 2004

	OK... aside from the apparent problem I have with finding the right 
tool for the job, I believe that avidemux2 is the closest so far... found only 
recently by following this thread, I may add.  I've got a test capture from my 
PVR-250 that I'm playing with mastering both DVDs and SVCDs.  Here are a few 
random issues that I would like to both share with everyone, and perhaps the 
ramifications of them:

- The hardware encoders don't have the benefit of a 2-pass encoding scheme
(being a causal device and all... :) or high-quality denoising, so they don't
necessarily make the right choices on I/P/B frames all the time.  It makes
sense to me that I frames should be used on large scene changes... but that's
not necessarily done with the raw captured streams and causes wasted bandwidth.  
For the best quality/bitrate ratio, post-processing (i.e. *re-encoding*) will
be necessary.

- The avidemux program still has some issues with processing streams.  Using 
the DVD-PS output file type produces DVD-compatible streams that are much 
larger than ones manually mux'd with tcmplex.  Unfortunately, extracting (or 
processing) video and audio streams separately generally tends to cause sync 
problems.  Also, is a DVD-PS (with NAV packets) acceptable as an SVCD stream?

There are a few different fundamental goals I can see would be useful for 
arching mythtv folks:

1. Simple, fast "VCR" functionality... quick and simple recording onto DVD from
a PVR-[23]50 set to record in a DVD compatible format.  This would preferrably
have commercial-cutting ability, but that's all.
Advantages: Quick, easy (especially if no commerical cutting), CPU-efficient.
Disadvantages: Not the most efficient use of space, can't specify 
"master-quality" recordings and then "archival-quality" DVD or SVCD since it's 
recorded/encoded in the final format.

2. Efficient/convenient, "Archive" functionality... Recording at high-quality
("Master copy").  Commercial cutting and transcoding in post-processing.  This
allows for the best quality/bitrate ratio and makes archiving to less expensive
media (e.g. SVCD) or more pack more episodes onto DVD's.  Especially important
for the SVCD/DVD decision, as the resolutions aren't compatible (480x480 for
SVCD, 352x480 for DVD, and 704x480 which is gross overkill for cable-quality

3. Space efficient, "Computer archive" functionality.  Similar to #2, but with 
the ultimate goal of MPEG4 archiving on computer media.  Once the requirement 
of set-top box compatibility is removed, anything mplayer can play is valid.  

	Did I miss anything?  :)


On Sat, 13 Mar 2004, James L. Paul wrote:

> Hash: SHA1
> On Thursday 11 March 2004 6:22 pm, Len Reed wrote:
> > Hi,
> >
> > I'm no video expert, so maybe I misunderstand the problem.  Or maybe I'm
> > missing a simple answer that's already there.
> >
> > I have a PVR250, so my .nuv files are mpeg2 with video and audio
> > multiplexed in them.  After I've marked the commercials (i.e., started
> > with myth's guesses and tweaked them using the built-in cut editor), I'd
> > like to do two things:
> >
> > 1. Remove the dreaded commercials in order to shrink the file.
> I also use PVR250 output. I still remove commecials manually with avidemux2 
> when I want to archive to CVD or DVD. I only cut on keyframe boundaries.
> > and optionally
> >
> > 2. convert to a plain mpeg2 for use in burning a DVD.  (The issue here
> > may be my poor DVD authoring s/w.  mplayer plays the unmolested .nuv
> > files.  OTOH, mplayer will play almost anything.)
> This has been discussed on this list, with specific steps. It's very easy. The 
> output of the PVR250 will work fine on a DVD with no transcoding. I just cut 
> out the commercials with avidemux2, remultiplex with DVD nav headers and pass 
> it to dvdauthor. I have burned many DVDs and CVDs from non-transcoded PVR250 
> output captured from my MythTV box. The video format is fine, the audio 
> format seems to work on most DVD players also. (Technically, I think standard 
> players aren't required to play MPEG audio is AC3 audio isn't present, but I 
> haven't heard of a case yet where it doesn't work anyway.)
> >
> > I've tried using various methods to accomplish #1: the built-in
> > transcoding (press 'x'), nuvexport, etc.  The problem with all of them
> > is that they do time-consuming transcoding.
> I'm not aware that anybody has a fully automated process for this from within 
> myth. There are people working on it though.
> > I don't want to change the bit rate or the format, I just want to lift
> > the parts that have the actual program, leaving behind the ads.  It
> > seems that that should take, in theory, about as much time as it takes
> > to read the parts of the original file that I care about and write them
> > out.  No serious computation, though maybe a bit of knitting around the
> > edges.
> The basic key to this is to capture in a DVD-compliant stream format in the 
> first place. You should only need to transcode if your captured stream is the 
> wrong resolution or bitrate.
> > So,
> >
> > a. Am I missing something that this makes this far more computationally
> > intensive than I think?
> Nope. It's more a matter of automation. We need a tool that can accurately do 
> keyframe edits based on the cutlist, remultiplex the results for dvdauthor, 
> then create and burn a DVD image.
> > b. Am I missing an existing tool that does what I want?
> We all are. ;) As is common for open source tools, the building blocks all 
> exist. We simply need to build something polished with them. MythTV is a 
> shining example of this process, and it's still relatively young yet.
> > Thanks,
> > Len
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