Get to Know Coax Snake Cable

You say you never heard of coax snake? Yes you have. You might even be using some on a fancy computer monitor.
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You say you never heard of coax snake? Yes you have. You might even be using some on a fancy computer monitor. It's also called “RGB” or “VGA” cable. This usually comes in three, four or five coaxes in a bundle, although you can also get six, eight, 10, 12, even 16 coaxes in some styles.

Besides all that video stuff, this cable offers a number of interesting audio uses.

First is unbalanced audio. Many of these cables take RCA connectors, since many televisions now use that for RGB inputs. Of course this is overkill for analog audio, but it looks nice; it is low capacitance; and you can even get six-channel cable to do 5.1 surround!

If you are a musician in a rock band, the eight-coax unbalanced snake could be just the thing for multiple instruments, or multiple recording feeds to that inexpensive mixer you bought.

Digital

Another application is digital audio, starting with S/PDIF consumer digital audio.

Again, we're talking RCA connectors. I suppose it's rare that any home theater system would need to send multiple S/PDIF signals from place to place. But S/PDIF stuff is getting pretty good and so it is showing up with alarming regularity in the professional audio or broadcast world. This is all 44.1 kHz sampling (5.6448 MHz bandwidth), and two channels per coax.

If you feed true AES digital audio (converted to unbalanced) it will easily feed a S/PDIF device. You might have to kluge a BNC-to-RCA adaptor or buy one at the local electronics store. You might want to check levels, since S/PDIF runs at 0.5 volts, and true AES runs at around 5 volts (think of 5 V supply rails). Old AES devices could run as high as 7 volts. Overdriving digital devices is not like analog. There is no such thing as “headroom” in digital, just bit errors and path failure.

Manufacturers who make those AES converter/baluns for digital audio (XLR to BNC) often make versions with pads in them that “pad” the signal from 5 volts to 1 volts. ETS (www.etslan.com) makes an especially high-performance, yet cost-effective, version. This will, at least, get you closer to the S/PDIF requirement. You can't just put on a voltage-divider pad, like you could for analog audio. You have to stay at the correct impedance, 110 ohms on the balanced side, 75 ohms on the unbalanced side. Doesn't matter which side has the pad.

In the professional digital audio world, we have AES3-id, professional digital audio on coax. Then it would be common to carry multiple channels, so the high coax count cables might be just what you need. Just like the twisted pairs, the gauge size rules the distance. The chart shows various “sizes” of coax cable and the distance they will carry digital audio.


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And this brings up another point about S/PDIF.

The source voltage of S/PDIF is 0.5 volts (not the 2 volt test signal of true AES). And the minimum receive voltage is still 0.2 volts (200 mV). So, for home installs, you will have to divide the distances above by four. So the small, common RGB stuff will only go 500 feet with S/PDIF.

Five hundred feet? How big a home do you have? I don't think even Bill Gate's home is 500 feet.

Even the RG-6 is available in bundles of 10. And just look at the distance! More than a kilometer. So if you need to send 20 channels of digital to the next town … Well, maybe I'd be using fiber optic cable by then.

That 10-pack RG-6 is huge and expensive. But just a three-foot sample, under the seat of your car in a dead-end alley on a dark night could come in handy.

Of course, the smaller stuff, the type you are probably using right now, is a lot cheaper and easier to use. And you'll notice that at 2,000 feet, it's no slouch for distance either. So you can use the same cable for RGB (your HD monitor), VGA (your computer monitor), unbalanced audio (your 5.1 surround sound), your digital audio (S/PDIF). Wow!

There's also a new style of coax snake that is “jacketless,” which my employer Belden has patented under the name Banana Peel. All the coaxes are extruded and stuck to a central core. They can then be individually “peeled” off. This technique uses no glue in the process, so there is no residue after you've peeled it off, and no extra plastic you have to shave off with a razor blade. Just three, four, five or six coaxes, in mini and RG-59 sizes. Six is the maximum that this technique can join. Sound appealing?

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