[mythtv-users] Transcoding, mpeg-2, mpeg-4 ASP vs. AVC
Stephen Boddy
stephen.boddy at btinternet.com
Fri Oct 21 20:49:05 EDT 2005
I'm getting a little bit confused as to what MythTV is doing on the
transcoding front.
I'm recording UK SD DVB-T mpeg2 streams. I'm struggling to keep up with the
stuff I'm recording, so I'd like to transcode where possible.
I've got the system transcoding files, I know it is using the "From MPEG2"
profile. (When I change the bitrate the transcoded filesize changes.)
I understood mpeg-4 ASP to require approx. half the bitrate / filesize that
mpeg-2 does for an equivalent quality file. Then mpeg-4 AVC is even better,
requiring perhaps a third what mpeg-2 does.
I tried adjusting the settings, but I'm seeing a significant drop in the
quality, even with a modest reduction in bitrate.
From this I have a few questions for anyone willing to give it a shot.
1. Should I be seeing a drop in quality when I lower the bitrate? To
illustrate: A 1.5 GB mpeg-2 recording, transcoded to mpeg-4 with a 2200
kilobits/sec has roughly the same filesize and quality as the original
mpeg-2. Lowering that to 1600 kbps with high quality and 4mv enc. reduces the
file size (prob. to about 70%) but the quality suffers quite badly.
Background areas (like trees) that are fairly clear in the original become
smeared blurry blobs that pan in jerky little steps. Movements of foreground
objects (i.e. a shoulder and head shot) result in a blocky pixelating effect
as the face moves quickly, but then settles down once the movement stops.
2. So why am I not able to get better compression for the same quality when I
transcode to mpeg-4?
3. Does the transcode process actually use the better AVC codec? I took a look
at the details on the ffmpeg site (home of libavcodec) and according to:
http://ffmpeg.sourceforge.net/ffmpeg-doc.html#SEC19 they don't appear to
support H.264 which is essentially what AVC is.
4. If not, what is the high-quality encoding for? Is it a two-pass vs.
one-pass?
Thanks in advance for any explanations.
--
Steve Boddy
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