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Difference between coaxial/composite cables


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3 replies to this topic

#1 of 4 OFFLINE   Peter Doten

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Posted January 04 2005 - 04:03 PM

I know this is a little bit of a rehash of stuff that has been in a few other threads recently but I am rewiring my HT tomorrow and I am trying to make sure I am using the best cables that I have.

Coaxial video cables are 75ohms as are digital coax. But Audio cables can vary in their impeadance (sp?) correct? Would a lower or higher impeadance be better? I would assume lower but I don't really know. I have a DMM handy and I figured this wouldn't be a bad way to test the cables. Would a cable that deviated from the 75ohm standard be better in certain applications (ie a lower impeadance the better for audio but not video)

TIA,
Pete

#2 of 4 OFFLINE   Wayne A. Pflughaupt

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Posted January 05 2005 - 05:44 AM

Quote:
But Audio cables can vary in their impeadance (sp?) correct? Would a lower or higher impeadance be better?
I think 50 ohms is typical for audio cables.

Quote:
I have a DMM handy and I figured this wouldn't be a bad way to test the cables.
I doubt a multi-meter will tell you what you’re looking for. They typically measure resistance, not impedance.

Just make sure for video and digital that you use specifically designated cables (actually, they are interchangeable). Audio is less critical.

Regards,
Wayne A. Pflughaupt

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#3 of 4 OFFLINE   Bob McElfresh

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Posted January 05 2005 - 04:23 PM

Quote:
But Audio cables can vary in their impeadance

Well... any given cable set will have both wires with the exact same impedence. They can be made with 50/75/110 or even 300 ohm coax.

Quote:
a lower impeadance the better for audio but not video

Here is the issue: the audio input jack has electronics behind it. This electronics has some "impedence". The best cable to use will have the exact same impedence so the signals flowing wont see a sudden change.

So you dont care if the cable is lower or higher impedence. You want to "impedence match" the cable to your electronics.

AUDIO vs VIDEO

Think of a small speed bump in a road. How harmful is it to hit that speed bump at 3 mph? How about 75 mph? The results are very different depending on how fast you are traveling.

Audio signals are like the 3 mph case. They really dont care about the speed bump. It does not bother them at all.

This is why electronic manufacturers DONT even have a convention about what impedence cable to use - it just does not matter.

Video - a different story. All the video input jacks on your television are designed to have a "75 ohm" impedence. By convention, all video cables are made with "75 ohm" coax so that the much faster video signals wont see a difference at the junction.

So does this make sense?

#4 of 4 OFFLINE   KurtBJC

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Posted January 06 2005 - 04:52 AM

For unbalanced analog audio, impedance, as such, doesn't really matter. Most of the time in unbalanced audio circuitry the input and output impedances aren't even matched, and a lot of devices are outputting low impedance into high impedance inputs; there's actually a saying about this, which is "low into high, you'll get by..." (there's also something about "high into low" being bad but I can't actually recall that part).

Why doesn't impedance match matter? Well, that's because at short lengths, an interconnect doesn't behave as a transmission line and can be treated, for most purposes, as equivalent to a direct connection. What's a short distance? It's dependent on the wavelength of the signal in question, and wavelength is the inverse of frequency (specifically, wavelength in meters times frequency in Megahertz equals about 300). When interconnects are short in relation to wavelength, their impedance matters relatively little, but as we lengthen the interconnect, and it starts to approach a significant fraction of the wavelength, impedance becomes of increasing importance.

So, say we're dealing with a high audio note--15 kHz. Wavelength is about 20 kilometers. If we say that impedance matching starts to have a substantial effect at, say, 1/10 of wavelength, we'd need our audio interconnect to be 2 kilometers long. The phone company actually has to worry about stuff of this sort, but the rest of us can ignore it--a good thing, since our audio devices aren't impedance-matched anyway and so we can't match the cable to both ends of the line.

In practice, high impedance interconnects tend to be better than low, but this has nothing to do with impedance as such. Here's the deal: impedance is a function of inductance and capacitance. Inductance, in a low-current, high-impedance application like unbalanced analog audio, is basically a nonfactor because current flows are so small. But capacitance, if excessive, will smear and roll off high frequencies. The lower the capacitance, other factors being equal, the higher the impedance. In practice, it's not that easy to get capacitance to super-low levels, however, because of other considerations: flexibility and shielding. To lower capacitance, we need to increase the distance between the shield and ground, but as we do so, the cable gets thicker and thicker and becomes less manageable. Alternatively, we can dispense with a shield and run well-separated parallel conductors; for example, ham radio operators occasionally use what's called "ladder line," a pair of conductors separated at intervals by ceramic insulators, with an impedance of about 450 ohms. Capacitance is very low because of the use of air dielectric, but the line is huge--inches across--and worse, it offers no shielding. That's not a problem when you're running the balanced output of a radio transmitter into it, but it's a big problem if you're running line level signals through it and exposing the line to EMI/RFI.

Now, video, of course, is another matter. Frequencies are higher, wavelengths are shorter, and impedance match ranges from being marginally important to critical. A rusty coathanger will carry almost anything a foot, but for significant lengths in critical applications, you really want your impedances matched at 75 ohms.


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