[mythtv-users] Cutting commercials without expensive transcoding

James L. Paul james at mauibay.net
Sat Mar 13 18:23:26 EST 2004


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On Saturday 13 March 2004 10:59 am, papenfuss at juneau.me.vt.edu wrote:
> 	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.

Re-encoding isn't going to restore quality that isn't there. Also, considering 
that the quality of the cable TV signal feeding the PVRx50 card is much lower 
than DVD quality to begin with, we're not talking about creating high-quality 
DVDs, I thought we were talking only about burning some commercial-cut TV 
shows to a standard format on plastic. ;)

> - 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?

I don't understand. Manually muxing has nothing to do with file sizes. I've 
had audio sync issues when capturing to elemental (non-PS) separate streams, 
but never a problem with the PS streams output by my PVR250's. Demuxing and 
remuxing my PVR250 streams has never produced any audio sync issues for me at 
all.

I assume that any DVD-PS stream is an incorrect format for SVCD, since the 
SVCD stream specs are not DVD-compliant. So this answer would be no. I don't 
see how it's relevant though, since the PVR250 doesn't output PS streams with 
DVD NAV packets if I understand correctly. I believe it's necessary to remux 
the PS stream to insert the NAV placeholders. And again, no, this should not 
create any audio sync issues.

> 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.

This is basically my own goal. I do it manually now. I don't feel the 
disadvantage as strongly as you imply it to be, since if I really want to 
shrink the size requirements I simply requantize the video instead of 
re-encoding.

> 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 captures).

Again, I've had good results with tcrequant instead of transcoding or 
re-encoding the whole stream. Some of the archive DVDs I've done of animated 
cartoons (think "Family Guy") requant very well and I've been able to fit 6 
hours or so on a single DVD with good 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.

Agreed. Many issues drop away if you aren't targetting playability in common 
set-top DVD players. :)

>
> 	Did I miss anything?  :)

Only things that come to mind right now are:

4. Instead of burning 1 show per disk, an interface for collecting media-ready 
titles that can be accumulated and selected for burning multiple captures to 
a single disk.

5. The obvious menu-generation features that would be welcomed by users of the 
preceeding #4. ;)

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