[mythtv-users] Questions about codecs / nuvexport

[Christopher Friend]

 > Firstly am I correct in thinking divX is the version that was open
 > source so divX on Linux is effectively frozen and will never improve
 > in quality.

XviD is an opensource mpeg4 codec, similar to the proprietary DivX. I'm 
not sure of the technical similarities, but basically most stuff that's 
compatible with DivX files are compatible with XviD, and the compression 
options / quality of both are almost identical.

 > Secondly is it possible to decrease the bit-rate/width/hight or DivX
 > or Xvid and still get good results on a 20 inch monitor/tv? If so
 > what bitrate/hight/width are worth trying?

err... everytime i think about with/height stuff, i always wonder how i 
came up with my original answer of "it doesn't matter" but whenever i 
can be bothered to think about it, i always end up at the same place. 
basically, changing the width / height shouldn't matter too much as the 
bitrate controls the amount of infomation going into each frame, so 
regardless of size it should always look the same. but i have read that 
changing the image away from default res can reduce quality (probably 
something to do with the positioning of pixels or whatever within the 
scene) but then again i've also heard that producing smaller sized vids 
make them look better quality. if you're playing back on a monitor, 
probably the pal (7xx * 5xx i forget) or ntsc (i never knew...)  would 
be technically the best answer.  but i play back 624 * 4somethings fine. 
They don't look perfect, but again that's probably a bit rate thing. and 
then i always wonder how i can forget stuff so simple. meh... *forgets 
it all again*

as for bit rate, reducing it dramatically is gonna cause stuff to look 
worse, though i'd say no worse on a tv than on a monitor. 1000 kbps is 
alright, you might just notice a bit of blockyness in some of the plain 
coloured areas, bump it to 1500 k and you shouldn't notice anything. 
i've got a comparison of bitrate image quality on the nuvexport wiki 
page (link at bottom).

 > VCD may be the quickest but is the worst quality, similar if not worst
 > than VHS.

yeah, some of the codec options in nuvexport are more geared towards 
very low size / low quality vids for watching on portable devices. for 
general use, xvid or divx are probably best. the rest is pretty much 
device specific ie. if you wanna create a dvd, or need an svcd / vcd. 
you should only really use them if you have a specific reason to do so. 
that said, most of them quality options within them so you can make 
stuff better / worse looking if you need to. xvid / divx at around 800 - 
900 kbps looks about the same as an average vhs tape, while 1000 - 1200 
gives my analogue tv a run for it's money.

 > Xvid with multi-pass is defiantly the best but takes a very long time.

hey, you didn't go making test vids of them all did you? that's what i 
wrote this for:

http://mythtv.org/wiki/index.php/Nuvexport

it's got example images, comparisons, explanations and recomendations of 
most of the options in nuvexport. though yeah, xvid multi / denoise / 
deint is pretty much the best quality (bar dvd). i haven't actually been 
able to create example images of the different codecs because transcode 
won't give me the choice, and ffmpeg is doing some horrible corruption 
stuff with the vids.

if you need to speed things up, just answer no to vbr (drops the 
multipass and uses cbr) and it'll look much the same, just 2wice as 
quick. drop noise reduction too for extra speed, and see how it looks 
after that. shouldn't be too bad.

and if you haven't already figured it out, use Transcode with nuvexport 
instead of ffmpeg (nuvexport --transcode) as it should produce better 
looking results.


Christopher