[mpeg2] Re: Recommended capture cards?

[Warren Young <warren@etr-usa.com>]

Lawrence Gold wrote:
> 
> I'm thinking about purchasing a relatively inexpensive capture card to use
> under Linux, such as a BT848/BT878-based board.

I have yet to see a cheap capture board with good color support.  They
tend to have a poor D-range, forcing you to either destroy the white
detail to get good blacks and good contrast, or to accept low contrast
in order to have detail in both the dark and the light areas.

This is probably not the fault of the BT8x8 chip, though, because some
of the most highly-regarded Bt8x8 capture boards -- Osprey's boards -- 
are based on these chips.  But, instead of paying $50 (like for the
WinTV Go) you pay $170+.  Most of their boards do work in Linux, FWIW. 
http://www.viewcast.com/viewcast.html

> I'm trying to encode a video at SVCD bitrates (~2.6MBps); 

At that bitrate you'd better be using SVCD frame sizes as well.  For
640x480 or full D1, I personally never go below 3 Mbit/s, and I only go
that low with a good encoder.  Visit the link in my signature for my
reviews of 10 different MPEG hardware and software encoders (not yet
including sampeg or bbmpeg, unfortunately).  Notice how poorly most of
them fare at 3 Mbit/s with challenging video.

> My
> understanding of sampeg is that being scene-adaptive, it should be able to
> insert more I-frames in the difficult sections.  

Inserting I-frames won't fix blockiness or artifacting.  In fact, it
will worsen the problem, because they take more space than P or B
frames.  Quite the opposite, you may find that increasing your GOP
length will help more.  As long as you do that with an encoder that can
close the GOP early and start a new one when necessary, there should be
no major problems using long GOPs.  Unfortunately, most encoders are
limited to 15-frame GOPs.

> Can kfir do anything like this?  

I examined two old clips I have here, one encoded with the Sigma DVR and
the other with the Hauppuage WinTV PVR and both used fixed 15-frame GOPs
exclusively, except at the end, of course.  I don't have either board in
a machine right now to check if there is an option you can set to enable
short GOPs.  It may be that there is such an option and I simply
neglected to enable it when I encoded these two files.

-- 
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