Component Video Wire (1 Viewer)

[Ken Smith]

I was talking to an electrician at work today and I asked him about the plenum jacket on the 1695A RG6 cable. He said that the plenum jacket is used to run inside walls because of it's fire resistance. When it melts it won't put out any toxic fumes either. I asked if this accounts for the big price difference between the 1694A and the 1695A. He said that a plenum jacket alone would make the cable that much more expensive.

I saw a thread on hometheaterspot.com where someone said that there is no such thing as a "true 75 ohm" connector. He said only the cable could be 75 ohm. Is this true? Anyone know how to test either the cable or connector to see what they actually are?

 

[brucek]

Ken,
Well, you could write a book on characteristic impedance, but very simply, connectors purchased for coaxial cable will "all" have a characteristic impedance and will be designed specifically to physically attach easily to a specific type of cable with that characteristic impedance. So, if I have a length of 50 ohm coaxial cable, I would look for a 50 ohm connector recommended to terminate it with. Not that I couldn't make a 75 ohm connector fit on the 50 ohm cable, but it may upset the characteristic impedance of that line. Canare has nice 75 ohm connectors. http://www.canare.com/connectormainframe.html
When a coaxial cable is specified as 75 ohms, this is not a DC resistance, but an AC impedance that states if I terminate this cable in its characteristic impedance with a 75 ohm load, then that impedance will be reflected at its input. The line will theoretically then appear as infinite, and will exhibit no standing waves or reflections from the load, with its ratio of voltage to current being constant over the entire length of the line. Now the terminating connector will have a small effect on this situation, and it's wise to use the correct value connectors for the given impedance of a piece of coax. This isn't something you can easily measure at home. You purchase the correct cable and connectors from manufacturers specifications.
Characteristic impedance is a culmination of the 4 standard distributed electrical constants that a transmission line will possess of (capacitance between conductors) and (inductance along its length) and (resistance of the wire) and (low level leakage conductance between the conductors). Actually the L and C are so dominant in an infinite line used to calculate the impedance value, that the formula for characteristic impedance derives down to the square root of L over C...... Well, so much for the technical stuff..
In reality, I wonder how tightly controlled, the output and input impedance of video circuits are, in our equipment. I suspect it isn't controlled that closely, since the manufacturers know that at these fairly low frequencies (particularly NTSC baseband video) and short distances it won't have much of an effect if the output impedance of their video circuit was say 74 ohms or 76 ohms. Then all our effort of getting an expensive 75 ohm cable with "true" 75 ohm connectors would be for not.....
Certainly, the fixation that some audiophiles seem to have with regard to a perfect 75 ohm cable for audio interconnects is unfounded. It's a very common engineering practice to ignore characteristic impedance and consider only the DC resistance of a cable (transmission line) when the line is short in comparison with the wavelength of the electrical energy that it conducts. Now, I would consider a one or two meter cable short in comparison to a 10 kilometer wavelength (and that's at 20Khz audio frequency). In fact, in very short cables, in relation to the wavelength of the signal, the resistance of the line is considered completely insignificant and the energy transferred is considered lossless. This would be the case for an audio interconnect with regard to impedance matching.
brucek

 

[Bob McElfresh]

Brian: You are correct, I was trying to describe how a long run of Component cable will result in a different signal strength at the end of each of the 3 cables, and I used a "Red/Green/Blue" description of the cables. I did not realize until later that this might confuse with the graph (because I could only see the URL).

I apologize for the confusion.

BruceK: Great post!

You almost caught me with the "fixation on a 75 ohm audio cable" comment. I'm one of the ones that DO fixate on this, but for the video cables.

I also caught the comment about consumer-grade input impedences. A few ohms off is fine IMHO, so long as the input impedence is ... say 10% of it's target value.

And if I were an engineer and could force the input impedence up/down a bit, I'd make all video inputs 70 ohms so the signal coming from a proper 75 ohm device/cable did not see a higher impedence in my circuit and cause a reflection.