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Article about HD over Cat 5

3072 Views 0 Replies 1 Participant Last post by  King244
I don't want to start any craziness here, at least not add to it, but check out this article and tell what u think. Here is an excerpt from the article and then a link at the end to the full article.

"We're often asked why that's so bad. After all, CAT 5 cable can run high-speed data from point to point very reliably--why can't one count on twisted-pair cable to do a good job with digital video signals as well? And what makes coax so great for that type of application?

First, it's important to understand that a lot of other protocols which run over twisted-pair wire are two-way communications with error correction. A packet that doesn't arrive on a computer network connection can be re-sent; an HDMI or DVI signal is a real-time, one-way stream of pixels that doesn't stop, doesn't error-check, and doesn't repair its mistakes--it just runs and runs, regardless of what's happening at the other end of the signal chain1.

Second, HDMI runs fast--at 1080p, the rate is around 150 Megapixels/second. CAT5, by contrast, is rated at 100 megabits per second--and that's bits, not pixels.

Third, HDMI runs parallel, not serially. There are three color signals riding on three pairs, with a clock circuit running on the fourth. These signals can't fall out of time with one another, or with the clock, without trouble--and the faster the bitrate, the shorter the bits are, and consequently the tighter the time window becomes for each bit to be registered.

Consider, by contrast, what the broadcast world did when it needed to route digital video from point to point. The result was HD-SDI, high-definition serial digital interface. One coaxial cable can route an HD SDI signal hundreds of feet without errors, with no repeater hardware or EQs in the line. Had the consumer industry opted for a coaxial-based standard, we'd be able to do the same in our homes. Admittedly, few of us need to make 300-foot runs; but the ability to run 300 feet without problems would be accompanied by rock-solid certainty of being able to do 50, or 75, without any worry at all.

But why is there such a big difference between twisted pairs and coax? It all has to do with the electrical properties of the two methods of routing signal from one place to another: balanced, through twisted pair, and unbalanced, through coax.

We tend to assume, when thinking about wire, that when we apply a signal to one end of a wire, it arrives instantaneously at the other end of that wire, unaltered. If you've ever spent any time studying basic DC circuit theory, that's exactly the assumption you're accustomed to making. That assumption works pretty well if we're talking about low-frequency signals and modest distances, but wire and electricity behave in strange and counterintuitive ways over distance, and at high frequencies. Nothing in this universe--not even light--travels instantaneously from point to point, and when we apply a voltage to a wire, we start a wave of energy propagating down that wire which takes time to get where it's going, and which arrives in a different condition from that in which it left. This isn't important if you're turning on a reading lamp, but it's very important in high-speed digital signaling. There are a few considerations that start to cause real trouble:"

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