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

Cymen Vig cymenvig at gmail.com
Wed Dec 14 16:05:07 EST 2005

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.

Does anyone have some advice on the settings below? Would the
interlaced settings have any affect on the video coming from a
Huappauge PVR card? How do the hardware MPEG-4 encoders such as the
Plextor PX-TV402U compare to software transcoding? I am going to
continue to bump up the bitrate on the MPEG-4 transcode settings as
currently I am not happy with the video quailty. It is *very* close
most of the time but every once in a while some obvious artifacts can
be seen.

So any advice on improving quality? As it stands now, I am leaning
towards giving the system more drive space and skipping transcoding.


Video source: NTSC cable
Video display: Nvidia Geforce4 MX SVideo out to Phillips 27" CRT TV

Transcoders->Autodetect from MPEG2:
Profile name: MPEG2
  Reize video while transcoding: unchecked
  Width: 480
  Height: 480
Video compression:
  Codec: MPEG-4
  Bitrate: 3200
  Maximum quality: 2
  Minimum quality: 15
  Max quality difference between frames: 3
  Scale bitrate for frame size: checked
  Enable high-quality encoding: checked
  Enable 4MV encoding: checked
  Enable interlaced DCT encoding: unchecked
  Enable interlaced motion estimation: unchecked
Audio compression:
  Codec: MP3
  Sampling rate: 32000
  MP3 quality: 7
  Volume (%): 90

MPEG-2 Encoders (PVR-250, PVR-350):
Profile: Default
  Enable auto-transcode after recording: unchecked
  Height: 480
  Width: 480
Video compression:
  MPEG-2 Hardware Encoder
  Stream Type: MPEG-2 PS
  Aspect ratio: 4:3
  Bitrate: 4500
  Max. Bitrate: 6000
Audio compression:
  MPEG-2 Hardware Encoder
  Sampling rate: 3200
  Type: Layer II
  Bitrate: 384 kbps
  Volume (%): 90

Hardware: Celeron D-340 (2.93 Ghz), 1 GB RAM, 300 GB Seagate drive
with 80 GB used by non-myth video (Seagate drive is noisy -- not
recommended), Hauppauge PVR-500, Charter analog cable.

Software: Gentoo w/ MythTV SVN (current as of last night)

Thank you,
Cymen Vig

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