[vdr] Re: improving tosvcd 0.9 MPEG quality.

[Andrew Stevens <andrew.stevens@nexgo.de>]

HI,

Andrew Stevens here... (mpeg2enc author)

I've now got  DTV Card and started playing...
 is not so bad, at least on TV, but it depends on the material. mpeg2enc
> has some problems with dark movies

Do you have a short snippet of SVCD MPEG-2 that shows the Problem.
Also, which version of mpeg2enc are you using with tosvcd?

> > I used to create VCDs using an SGI O2 by capturing analog video
> > material via the O2's capture hardware and by compressing that
> > into MPEG-1 using SGI's dmconvert utility. The quality of these
> > VCDs was much, much better than the quality of the SVCDs created
> > by tosvcd. One would assume the opposite. Should not SVCDs be
> > better than VCDs?
>
> It depends on the compression and how many minutes you try to stuff on one
> CD.

Here here ;-)
Actually, if the original feed is a poor quality analog master that has then 
been enocded for DTV and you then transcode that you may get some Issues.
One Problem is that for SVCD you are scaling down to 480 pixels which means 
you're Macroblock boundaries fall smack in the middle of macroblocks in the 
SVCD image.   This produces big DCT coefficients and eats a lot of bits.

> > I assume that the encoder used by tosvcd (mpeg2enc) is the culprit.
>
> Yes

> > So my questions are:
> >
> > 1) Is there anything that can be tuned in the call parameters
> >     to mpeg2enc to create a better MPEG quality?
>
> Have a look at the mencvcd in the mplayer TOOLS/ dir, and make some
> Comparisons, look at the manpage of mpeg2enc to look what they are doing.
>
> > 2) Is there tuning work in progress in mpeg2enc that will
> >     produce better quality in the near future?
>
> Steven Andrews has begun to work again on mpeg2enc. One thing he is
> programming on is twopass mode , thats good for better size prediction and
> better usage of the bitrate. Lets wait which features he tries further to
> implement. (loog at the mjpegtools-ml-archiv)

Two pass mode won't make a whole heck of a lot of difference for SVCD.  The 
reason is that SVCD is so heavily bit-rate limited it is often running in 
more-or-less as a constant bitrate stream.    

What probably could make a big difference is a suitably tuned scaler for
producing the horizontally downscaled image for encoding. What scaler are you 
using in tosvcd?   Depending on the original source material you may find 
using a denoiser may help a lot too.   SVCD's peak bitrate is at the limit of 
what MPEG-2 can do with broadcast material...


	Andrew



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