[mythtv-users] Mytharchive errors
mythtv at rodsbooks.com
Sat Dec 6 04:36:17 UTC 2008
On Friday 05 December 2008 09:30:13 pm Harry Devine wrote:
> I'm not sure I'm following you 100% as far as what transcoding is
> actually doing.
A MythTV transcode operation can operate in one of two ways:
1) A lossless transcode does a few simple tweaks on the recording, which
reduces it in size by a few percent, but leaves it mostly unchanged.
This takes very little time to perform (more-or-less the file-copying
2) A full transcode converts the recording from whatever format it's in
(usually MPEG-2, but that depends on the recording hardware and
possibly the software settings) to either MPEG-4 or RTjpeg. This
operation takes a while (a significant fraction of the recording
time, or possibly even over that, depending on CPU speed and various
settings). MPEG-4 recordings consume less space than similar-quality
MPEG-2 recordings, which is one reason this type of transcoding is
popular. I get the impression that few people use RTjpeg. Its main
advantage is that it requires less CPU time than other encoding
methods, but the tradeoff is that file sizes are bigger to achieve
similar quality. Either type of transcoding necessarily reduces the
image quality because you're converting from one lossy format to
another lossy format.
When you configure a transcoder, you set these options -- IIRC, there's a
checkbox to perform a lossless transcode, and another option (which is
available only if you did NOT select a lossless transcode) chooses between
MPEG-4 and RTjpeg.
In addition to doing minimal or extensive conversions, as above, transcoding
removes sections of a recording you've flagged for removal -- so if, say, you
record an extra half hour because a show is often delayed because of a
sporting event, you can cut out the unwanted part and save some disk space.
> For example, I'm looking at the Transcoders area in my
> Frontend setup, and they are the default settings from when MythTV was
> installed. They all seem to have RTjpeg as the Video codec, and MP3 for
> the Audio codec (with a sampling rate of 32000). This is the same for
> Autodetect, High Quality, et al.
RTjpeg is a poor choice for transcoding; it'll create bigger files than the
original (assuming similar quality settings). I recommend you set these to
MPEG-4 and experiment with the bitrates to see what sort of quality you can
get. Try working on a short recording of a minute or two.
> So, if a DVD has to be in mpeg2 format, how do I set up one of the
> transcoders to do this so I can transcode it properly before trying an
> archive operation?
You don't, at least not within MythTV. MythArchive does its own transcoding
that's independent of the transcoding you can do manually (or automatically
after a recording is made). The MythArchive transcoding converts to MPEG-2,
so you can burn something to DVD after transcoding it to MPEG-4 or RTjpeg, or
if it was recorded in a non-DVD format (a tuner card that creates MPEG-4
natively or an MPEG-2 recording at higher-than-DVD resolution, for instance).
That said, it *IS* possible to either record in a DVD-compatible format
originally (using an SD source) or transcode the file OUTSIDE of MythTV into
a DVD-compatible format. (My own script for doing the latter is at
http://www.rodsbooks.com/mytranscode, but it doesn't integrate neatly with
MythTV.) When you do this, you don't need to transcode within MythTV, or at
most you should do a lossless transcode to remove unwanted parts of the
recording or to gain that small recording size reduction I mentioned earlier.
Thereafter, *IF* the recording is in a DVD-compliant format when you launch
MythArchive, you can select an option in MythArchive to skip MythArchive's
own transcoding step. This option is not available if MythArchive detects
that the recording is not in a DVD-compliant format.
> Also, I did see the following error in my
> mytharchive log, and I wonder if this would go away once the transcoding
> problem is straightened out:
> WARNING: frames rates do not match
> The frame rate for ntsc should be 29.97 but the stream info file report
> a fps of
My guess is that you've got an HD recording using a non-interleaved
(progressive) format. I'm not sure if that warning is related to your
problems or not. My experience is that automatic transcoding tools can easily
get confused by inputs they aren't expecting (or that their programmers
didn't anticipate, really). When this happens, they might or might not
complain; and complaints like the above might or might not indicate serious
problems. Sorry for not being very helpful on this score, but that message
alone just isn't very diagnostic, at least not to me. (Maybe somebody else
who's more familiar with MythArchive logs would be able to say with certainty
that the message is or is not important.)
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