[mythtv-users] Transcoding HD-PVR Recordings

Bob Sully rcs at malibyte.net
Wed Sep 22 04:05:50 UTC 2010

Doug Vaughan wrote:
> Bob,
> A difference in our approaches is that you transcode before you edit out
> commercials. That may give you some extra flexibility with Avidemux than
> my approach.

Doug: Perhaps, perhaps not.  I find that if the raw recording is less than
perfect (e.g. a drop in signal with resultant dropped frames or
pixellation), the transcode will fail and I will have to pull the raw mpeg
file into avidemux, which allows me to do the edit and convert to an .avi,
but the resultant file isn't navigable.  This happens in maybe one
recording out of ten or fifteen.  Converting to an .mkv fixes that.  Most
of the time the end result seems about the same ;-)

> It was also pointed out that "MKV File Creator" can convert an avi to
> mkv and that is true but I find ffmpeg is faster.

Used your line below.  Worked like a charm and is nice and quick :-)

> Someone else suggested that MythTV figures out commercial breaks better
> now. I have not tried MythTVs automatic commercial break detection for
> almost 2 years so things have likely improved.

I have noticed that this is true most of the time, especially on the "OTA"
HD channels - Myth does seem more accurate than it had been, say, back in
0.21, and now it's quite good.  But on some channels (some of the cable
channels such as AMC HD seem to be particularly fond of this) there may be
only one P frame between the program content and a commercial, and Myth
(I'm running 0.23-fixes. latest JYA builds) still occasionally misses one.

> My MythTV HDPVR recording profiles all have their settings maxed to:
> Average bit rates of: 13,500
> Maximum bit rates of: 20,200
> You mention about file sizes of your 1 hr HDPVR recordings at 1280x720p.
> I found out that different STB's send different bit rates to the HDPVR.
> For example when I use an MythTV HDPVR profile as descibed above I get:
> STB(SA3250HD) -1 Hr recording 2.1 to 2.2Gb
> STB(SA8300HD) -1 Hr recording 2.3 to 2.4Gb
> I assume that the SA3250HD has an older chip set which accounts for the
> lower bit rate. I have also heard that the cable or satilite providers
> transmit specific channels at different bit rates so to me file sizes
> vary according to these factors.
> a) The original content and how easily it can be compressed. Darker
> programming like the TV series Supernatural often has smaller file sizes.
> b) The provider's choice in what bit rate they transmit a specific
> channel.
> c) Your STB's chip set.
> d) The HDPVR recording profile settings you use in MythTV
> Please note that I am by no means knowledgable about what the providers
> do with their signal. So much of what I just wrote is based on my
> observations and what I have read.

I have an old 6200, so this may be the problem.  I just changed the
bit-rate settings to match yours in my rc.local file, so we'll see if that
makes any difference after I boot up next time.  I was not aware that
different STBs put out content at different bitrates.  I'll bet that's it.

> Here is an example of bit rates I get with my SA3250HD STB and the
> recording profile described above. This data comes from the utility
> MediaInfo and last night's recording of "The Event":
> The Event - S01E01 - Pilot.mpg
> Format                           : MPEG-TS
> File size                        : 2.42 GiB
> Duration                         : 59mn 49s
> Overall bit rate                 : 5 796 Kbps

> The ffmpeg command line I use to convert an avi container to a mkv
> container is very straight forward. $avifile" and "$@/$basename.mkv" are
> just the bash script variables.
> ffmpeg -v 1 -i "$avifile" -vcodec copy -acodec copy -y "$@/$basename.mkv"
> Doug

Worked great.  Thanks again!

Bob Sully - Simi Valley, California, USA

More information about the mythtv-users mailing list