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

[Stephen Boddy]

On Saturday 22 October 2005 17:20, Niels Dybdahl wrote:
> > 1. Should I be seeing a drop in quality when I lower the bitrate?
>
> Yes. Lower bitrates result in lower quality.
>

Probably didn't phrase that correctly. I meant that if I have a better 
compression codec, then should reducing the bitrate reduce the 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.

Thinking on this, I never actually checked to ensure that the mpeg-2 stream 
was at 2200 kbps. It is just whatever the incoming DVB-T stream is. But I 
would have thought that regardless of the codec, the same length of content 
at the same bitrate will give a pretty similar filesize. If I have a more 
efficient codec, then surely I should be able to lower the bitrate without 
losing quality.

> > 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.

Yes, I suspect it is 720x576. Again, it boils down to an mpeg-2 of size x 
should be able to be reduced in size to y using the more efficient mpeg-4 
codec, without losing quality.

> > 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.

Unfortunately it's a big-ass DVI fed HiDef DLP Rear Projector. Losing res is 
not really an option. ;-)
-- 
Steve Boddy