[mythtv-users] Recordedmarkup corrupt - consequences ?

Andrew Dodd atd7 at cornell.edu
Fri Feb 20 12:49:09 EST 2004

Quoting cmisip <cmisip at insightbb.com>:

> Thanks for replying.  What is a GOP and why do I need it?
Group of Pictures.  It's an MPEG-2 term.  MPEG-2 is pretty strictly structured,
with a fixed GOP length within any given file (15 frames for NTSC and 12 for PAL
IIRC, or is it the other way around?), and within a GOP, the order of I/P/B
frames is fixed.  Most importantly, GOPs ALWAYS start with an I-frame, and since
in compressed video that uses difference encoding you can only seek to a
keyframe (i.e. an I-frame in MPEG-2), a table of the beginning of each GOP and
its position in the file makes seeking much easier.  Without the GOP table,
seeking is slow, and with VBR files Myth's estimate of remaining time will jump

> How does one strip commercials with avidemux?
Load the video, edit out the commercials, save raw audio/video streams,
remultiplex them with mplex from mjpegtools.  AVIDemux now has support for
outputtting DVD program streams, but it didn't work well last time I tried it. 
mplex is much more consistent and reliable.

> I am also trying to convert the recordings into a format playable in a
> zaurus 5600.  By rebuilding the recorded markup with mythcommflag, does
> this automatically mark the beginning and end of commercials so
> nuvexport can use this information? I expect running mythcommflag on a
> system with about 420 gigs of storage (90 % full) will take a long time.
> I wonder if it is worth it.  I need to compress videos so they will fit
> on a compactflash.  So far I have been able to compress 1 gig to 100 mb
> with good quality.  Removing commercials automatically will further
> reduce the file size but I dont want to sit and manually mark the
> commercials.  
> What is the state of commercial detection in mythtv? 
Depends on your signal source.

Pure blank frame detection works EXTREMELY well if blank frames can be detected
reliably (i.e. channel is not noisy, brightness is set properly.).  Scene change
detection is close to useless.  Too many false positives/negatives.

420 gigs of storage - it's going to take you a long time to export that no
matter what.  mythcommflag is going to be a drop in the bucket compared to the
need to transcode everything.

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