Newsgroups: rec.audio.tech
Subject: Re: what is 75 Ohm cable?
References: <[email protected]>
[email protected] (Eric Meijer) writes:

> I understood that for transfer of digital signals you need 75 Ohm coax
> cables instead of `audio grade' coax cables.  This is because the
> the digital signal has a much higher frequency than audio signals.  Now
> I wonder what exactly is 75 Ohm about these cables?  

I have written a small document on cable impendance and it can be found at
http://www.hut.fi/Misc/Electronics/docs/wiring/cable_impedance.html

> I'm quite sure it is not the DC resistance.  

It is not DC resistance. The impedance described how the cable
behaves with high frequency signals.

> What is it then, and how is it better for high frequency signals?

At high frequencies it is necessary that transmission lines
are terminated with the same impedance than the cable has on both
ends (transmitting and receiving) to avoid problems caused by
signal reflections. If the the termination is not correct, the signal
will not transfer nicely to the cable and out of it and the signal
reflections caused at those points are the problematic
(distort the signal).

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web pages at http://www.hut.fi/Misc/Electronics/