[mythtv-users] Cutting commercials without expensive transcoding

[James L. Paul]

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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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