Home Automation

Levi Pearson levi at cold.org
Tue Feb 20 10:13:46 MST 2007


Scott Paul Robertson <spr at mahonri5.net> writes:
> A couple things to note about video cables and it's technology.
>
> Component is nice (better than S-Video and Composite (RCA)), but nothing
> compared to DVI/HDMI. Component provides up to 480p resolution, while
> DVI/HDMI can provide the signal needed for 1080p and higher.

Um, what?  I have a widescreen HD-capable monitor from a few years
ago.  It has '1080i' labeled on the front.  It has only component
inputs.  How do you explain this?  

> In standard def land component is the best. As for signals on a single
> wire, composite is the only one guilty of this. S-Video is better than
> composite because it is multiple wires in one cable, and component
> better still because it's one wire per cable (three cables). These are
> all analog.

So by this reasoning, the UTP wire we run our networks on is bad
because there are multiple wires in one?  No, this can actually be a
good thing.  Here's how the analog signal systems break down:

Coax: All audio and video information on a single coax wire.

Composite: Video on a single coax with RCA jack, left and right audio
channels separate.

S-Video: 4-wire video signal; two signal wires and two ground wires.
The Y (luminance) and C (modulated chrominance) signals are separated.
Removes the necessity for low-pass filtering, reduces color crosstalk,
removes dot crawl, etc.

Component: 3 separate coax cables with RCA jacks.  Separates the video
signal into Y, Pb, and Pr components.  Y is again luminance, and Pb
and Pr are the differenece between luminance and blue chrominance and
the difference between luminance and red chrominance, respectively.
This is the minimum necessary cabling for HD, and it supports up to
1080p.  A good component cable run can easily go 200ft without losing
significant signal quality.

> DVI/HDMI was designed for computer monitors. It is digital. The cables
> are designed to handle interference. You can get long distance cables
> that won't degrade the signal (though they might be expensive). Heck, I
> have a 20-ft VGA extension cable that doesn't degrade the signal, and
> DVI >> VGA.

DVI and HDMI use the same encoding mechanism, which uses R, G, B, and
clock signals.  The signal wires are paired with ground wires in the
cable in twisted pairs.  There are also provisions for a second set of
pairs to support a second channel, if the bandwidth needs exceed the
capacity of a single channel.  HDMI also carries audio data.

These cables have a much shorter range, which depends somewhat upon
the signal that you're passing through them.  A higher-frequency
signal will degrade faster than a low-frequency signal.  A site I
found (http://www.datapro.net/techinfo/dvi_info.html) mentioned
significant signal degradation at 12 meters on a DVI cable.

Also keep in mind that even when you run a digital cable, there will
still likely be signal conversion loss.  The digital format on the
wire is almost certainly not exactly the same as the original digital
data nor the digital data needed to display on the end device.  It's
nearly impossible to know exactly which conversions will happen with
different ports, so it's even possible for a component video
connection to look better than a DVI/HDMI video connection if the
component decoding circuitry works better.

                   --Levi




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