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Re: projectors



At 07:55 PM 4/18/01 -0700, Bernard Cleyet wrote:
P.s. The TV did not have S-Vid.


Then at 11:38 PM 4/18/01 -0400, Hugh Haskell wrote:

That's probably the problem. Without the S-video link, the connection
between the computer and the TV is the pits. I've seen this before.

Hugh's right.

To make a science lesson out of this, let's see if we can apply Shannon's
channel-capacity theorem. That is,
capacity = analog bandwidth * log(signal-to-noise ratio)
where for composite video the bandwidth is less than 6 MHz, and we can
estimate log(SNR) to be something like 10 bits.

Contrast this with the monitor I've got here, which is putting out
80 Hz refresh * 1280 horiz * 1024 vert * 3 colors * 8 bits apiece

Collecting results, we have
composite video: 6 * 10^6 bytes per second
computer monitor: 314 * 10^6 bytes per second
(i.e. 80 * 1280 * 1024 * 3)

At this point, we need not wonder why the composite video looks five or ten
times worse. Indeed a better question would be why it looks _only_ five or
ten times worse, not fifty times worse. Most of the answer has to do with
perceptual coding. There are certain things that the human eye cannot
perceive, such as high-spatial-frequency color changes. The composite
video signal is verrrry cleverly coded to exploit this, assigning its
precious bandwidth to the sort of things that are most perceptible in
"ordinary" scenes.

The invention and adoption of the NTSC composite video standard is quite a
saga; see e.g.
http://www3.ncsu.edu/ECE480/ntsc-fink.htm

For starters, note that the 6 MHz bandwidth was a constraint imposed by the
frequency allocations for broadcast TV. See e.g.
http://members.aol.com/jneuhaus/fccindex/30_mhz.html
Separated video (S-video) is not broadcast, so it has no such constraint.

For details on Shannon's channel capacity theorem see e.g.
http://scitec.uwichill.edu.bb/cmp/online/p31q/lecture13/lect13.htm


Next time, get hold of a video projector. All the ones I've seen take
the monitor output from either a Mac or a PC and put it on the screen
just like it was the computer monitor itself, and the resolution is
fine.

Right. (I've dealt with some old ones that were not so fine, but the new
ones are quite impressive.)