[mythtv-users] Transcoding Artifacts (MPEG-2 -> MPEG-4)

Nick knowledgejunkie at gmail.com
Thu Dec 15 13:38:49 EST 2005

On 14/12/05, Cymen Vig <cymenvig at gmail.com> wrote:
> I have been experimenting with transcoding the MPEG-2 output of my
> Hauppauge PVR-500 device to MPEG-4. The current settings are are below
> and result in approximately a 50% reduction in file size so a 1 hour
> recording is 2.2 GB in MPEG-2 and 1.1 GB in MPEG-4. I upped the
> bitrate and enabled some of the quality settings after the first
> transcoding attempt resulted in poor quality -- too many blocky chunks
> in the video for my taste. With the settings below the quality is
> better but I prefer the MPEG-2 recordings because the MPEG-4
> compression seems to highlight and make the video noise worse and
> there are some dancing jaggies occasional on faces and other things.
> I've also noticed that attempting to play the MPEG-4 files with
> mplayer results in the wrong aspect ratio as it thinks the video is
> 720x480 while it really appears to be 480x480. If I jump around in the
> file with fast forward/rewind it tends to kill X and leave my sound
> output in an usuable state until the system is rebooted.

I do all of my transcoding under Windows (2-pass to XVID, using
VirtualDubMPEG2) and my results are almost indistinguishable from the
MPEG2 source (esp when using DVB).

I deinterlace and then resize to either 640x480 (4:3) or 640x360
(16:9) depeding on the aspect of the video (AVI does not support
embedding the AR in the file in the way that MPEG files do). Typically
I will convert a 1hr show to 700MB or a 30/40minute show to 350MB.

I would try recording at 640x480 and then transcoding to see if this
clears up the aspect ratio issue. If you are able to use 2-pass
encoding, I would suggest that - if it is something you want to keep.
(Sorry I can't offer any concrete examples - I don't use transcoding
at all on my Myth box - normally I'll watch stuff before my disks
fills up, and anything I want to keep I transfer off. Once lossless
editing of MPEG2 files is locked down, I'll probably make the switch.)


More information about the mythtv-users mailing list