[mythtv-users] Transcoding, mpeg-2, mpeg-4 ASP vs. AVC

[Niels Dybdahl]

>
> 1. Should I be seeing a drop in quality when I lower the bitrate?



Yes. Lower bitrates result in lower quality.

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.


As the filesize is proportional with the datarate, this indicates that your
original MPEG2 also was at 2200 kb/s, which is quite low for MPEG2 (I record
with a PVR-250 at 4500 kb/s and transcode to 1400 kb/s), so the starting
point for your compression is not very good.

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.


1600 kbps should be enough for MPEG4, but it also depends upon the number of
pixels. I do record at 480x576 pixels but as you are recording from DVB-T
you are probably at 720x576, so you would need 2100 kbps to get the approx
same quality as I have at 1400 kbps, because you have more pixels.
Mythtranscoding has settings to choose a different resolution, but it does
not seem to work on my system.
Noise is poison for MPEG compression, so as long as you are compression new
films you should be ok with a completely digital flow. Older film that from
analog media will probably have more noise and need higher bitrates.

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.


These are typical artifacts from MPEG4 encoding.

4. If not, what is the high-quality encoding for? Is it a two-pass vs.
> one-pass?


I do not think that mythtranscode can do two pass encoding, but I am not
sure.

I would keep the recordings at MPEG2 at 2200 kbps or check if mythtranscode
can reduce the number of pixels. You might try to go as low as 400x288
pixels, especially if you are watching on a CRT TV.

Niels Dybdahl
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