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Video Cables - Are they important ?

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I have been using 3 lead servo wire to send video from my Camera to the different video conponents in my airplane. The servo wire is superrior to regualr video cable for vibration resistance and also much better than RCA connectors for not making noise with vibration. I am also using 3 lead servo wire from my video receiver to my video amplifier... Is there disadvantage to using non shielded 3 lead servo wire for 6 runs of video ???

JettPilot

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When I first started this game I used shielded cable but soon got lazy and used standard servo leads and never had a problem. I guess for long runs it may be worth while but not for what I do. The question is have you seen any problems with it?

Terry

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If cables are longer than 15-20cm use shielded ones - for shorter can be twisted normal , exept if you see any picture problems but then very probably is not only cables problem .

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I've always used servo cable evreywhere up to lenghts of about 2m. Never tried more as I have no plane that is big enough, but never had a problem ^_^

In other conditions I have however used stupid 2-wire, unshielded, untwisted cable over 40m with no problem, other than maybe a normal slight loss of image sharpness :)

Edited by Kilrah

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I remember reading that it's irrelevant what type of wiring you use if you're under 1/10 the wavelength of the maximum frequency you're using. In other words, losses due to impedance mismatches are minimal when the wire length (regardless of the type) is under 1/10 the wavelength.

Assuming a frequency of 10 MHz for composite video, the wavelength would be ~ 300/10 meters (or 30 meters), or a little under 100 feet. So, based on this, you would be able to use any wire up to 10 feet or 3 meters with virtually no loss. Is this correct?

Something that has confused me in the past is why people talk about the importance of having proper connections. More specifically, I've heard that RCA connectors don't have a proper 75 ohm impedance, so BNC is much better. But based on the above, the signal is carried over such a short distance over the connector that it would seem to make virtually no difference what connector you use or how many connections you have to make to get the signal from point A to point B, at least for composite video. So, I'm a little confused about this.

Just to add to what Kilrah said, I ran some experiments with composite video and wire length, and I was able to go well beyond 10 feet with just magnet wire (very small caliber and unshielded) with seemingly no loss (at least when viewed on my TV).

p.s. Let me add that the wavelength of 30 meters (for 10 MHz) is through a vacuum. The wavelength when the signal is carried over wire is about 20 meters. But my calculations above are still valid because some people use 1/4 wavelength as the cutoff instead of 1/10 wavelength. So, you could conceivably say that 5 meters instead of 3 is the cutoff for composite video transmission. These are very gross calculations.

Edited by ginger_marianne

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It is not the length of wire that will cause for issues, it is the fact that unshielded wire with sources of interference nearby (ESC, switched BEC, transmitter, etc.) will couple directly into an unschielded cable and cause for lines, noise, etc.

Cheers,

Sander.

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Exactly - shield is not for prevent signal lost , it is even no problem if any lost will . Shield is shelding against external glitches and helping to not watch on screen how ESC works and other "electronic" events in the air :) .

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Exactly - shield is not for prevent signal lost , it is even no problem if any lost will . Shield is shelding against external glitches and helping to not watch on screen how ESC works and other "electronic" events in the air :) .

True. But on the ground, going from receiver to monitor/recorder, you probably have no need for shielding. It depends on the application.

And when shielding is used, it probably doesn't have to be in the form of a thick cable with 75 ohm characteristic impedance, as long as the run is short enough. My understanding of video cables (limited as it is) is that cables are generally shielded to maintain characteristic impedance within tight tolerances (and not to prevent interference from outside sources). Correct me if I'm wrong.

Edited by ginger_marianne

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At video frequencies as at audio frequencies, a shield is used to keep outside interference from reaching the signal wires. Impedence really has no importance for our applications unless, as previously stated, real long cables are used when it "might" be helpful...

Edited by W3FJW-Ron

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I guess what I would say to the OP is if your cable/wire is around 10 feet (or maybe a little higher), don't even factor impedance into the equation. Also, you probably don't have to worry about shielding unless you actually notice interference. Impedance mismatches generally subtract from the image (i.e. the full signal is not being realized because of losses). Interference adds to your image. If you need shielding for your cable (due to interference), you probably don't have to worry about the characteristic impedance of the cable. With shielded cables (e.g. coaxial), I'm not sure how much of the bulk of the cable is actually used to maintain tight tolerances for the characteristic impedance. There are also other ways to cancel out interference, such as twisted pairs.

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I think you need to be a little more specific.

1. In an environment with little or no causes for interference (i.e. the receiver on the ground) shielding isn't that important.

2. In an environment with multiple causes for interference (i.e. the tightly packed cable mess inside a plane's fuselage) shielding is important.

Cable impedance only factors in when you look at the attenuation per length of wire. Hence for composite video you need not worry as the frequency is low enough, you can use long runs of cable without significantly attenuating the signal. What you will see in cables of significant length is reflections due to an impedance mismatch, i.e. a double image.

Cheers,

Sander.

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