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
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