An example from my day yesterday...
I have two 'cheap boxes', one from nation wide chain store (who sells
things other than high tech and appliances, a wall to wall mart if you
will) and one from a local Austin vendor. The behavior was checked
against multiple instances of boxes so we know it isn't a single bad box.
Each machine has all the normal stuff, 1G of RAM, 2x80G HD. They also
include an AGP video card, PCI video capture card (no tuner, commercial
quality board), and a PCI 10/100M Ethernet. Running under Linux using
various Open Source tools. We used different AGP and network cards to
verify brand dependence. Different brand board for AGP or network made no
difference. We couldn't try different video capture boards due to cost,
we had a single board.
One of the boxes works fine. The other drops frames if the network traffic
gets too high or you really push the video board. It doesn't drop much,
down to about 27fps from 30fps, but it drops. And at the same time you
get digital aliasing [1], which is the real killer. Pull the AGP or
network (usually the network because we like the pretty pictures) and
all is fine (though w/o network it's a little annoying).
Why?
The interrupt controller on the slower box isn't up to it.
I have a similar project under Plan 9 where I'm trying to take four of
these cheap-ass television cards in a cheap-ass box and export them into a
namespace so you could at least in principle watch television from just
about anywhere. The video frame is limited to 320x200 (for several
reasons I won't go into here). And those babies drop frames for this
same reason. I will grant that video support on Plan 9 is down right
pre-historic so some improvement may be gleaned from re-doing that (I
hope so or else we'll drop this as infeasible at this level of tech).
So, no, setting up a -quality- video capture system isn't easy or mundane
on expensive systems and certainly not cheap boxes. But then again, you
may not even notice the aliasing or dropped frames. That you don't notice
the jitter or blocky display speaks to you, not the technology.
Which is -not- to say it can't be done, I see from 3-5 of these sorts of
systems a month built that work fine. But it does take time and effort, it
is -not- plug and play. Also be prepared to tweak the television drivers
for Linux since they are seldom optimal.
[1] This is that 'blockish' effect you will see on a lot of television
shows now because a lot of them are moving to non-linear video editor
suites, it occurs when the conversion process stalls a bit in frame. It
comes from the machine not being able to keep up and update the field
completely so you'll get the even or odd field but not both. Clouds are a
really good place to look for this effect.
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We don't see things as they are, [EMAIL PROTECTED]
we see them as we are. www.ssz.com
[EMAIL PROTECTED]
Anais Nin www.open-forge.org
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