Question about cables (1 Viewer)

[Evan H]

When a cable is listed as being 50 or 75 ohms, what exactly does that mean?
I connected a video composite cable to an ohmmeter today to see what it's impedance would be registered as. I expected to read somewhere between 50 and 100 ohms... but the meter would vary between .6 and 1.3 ohms. This made no logical sense to me, since I expected the cable to register 75 for line impedance matching purposes, but if the cable doesn't have that impedance, what is the actual difference in the cables?

 

[Gifford L]

75 Ohm are usually for video and 50 for line level audio. You cannot measure impedence with an ohmmeter either as it will only measure the DC resistance of the cable which should be as low as possible.

 

[Bob McElfresh]

Gillford: nice reply! Welcome to HTF.
Evan: Impedance is "resistance to a changing signal".
Your ohm meter only measures DC resistance, not impedence. And guess what? The impedence is different for different frequencies.
Here is the trick: When you try and shove a signal out of one device to another, the "most efficient" wire is one that matches the input and output impedence of the two devices.
If the wire has a LOWER impedence than the destination device, the signal will see the resistance. Some part of the signal may reflect back to the source. (This happens more and more as the frequency goes up). This is called "ringing" or "reflection". You see it as a "ghostly" outline on your TV.
If the wire has a HIGHER impedence, it will shrink the signal. Not a problem in theory (the destination just has to be forgiving of the voltage swings on it's input). But your wire has some "noise" on it. And it is not affected by the higher impedence wire. So the destination sees the shrunk signal, with lots of noise.
For years, the standard input/output impedence for VIDEO devices has been 75 ohms. This is why 75 ohm coax is always used for video, CATV and even the coaxial-digital connection on your DVD player.
You can find coax that is 50, 75, 110, and 300 ohms. Many cable sets with L/R/Video cables are all made with 75 ohm coax. (You just buy the bundled Component video cable and slap RCA plugs with different colors on the ends. Works great.)
Hope this helps.

 

[Evan H]

Thanks for the responses