[S Clark]

You often see digital cables advertised as having 75 ohm resistance.  Why?  Does it matter?

[Speedskater]

It's a radio frequency "75 Ohm Characteristic Impedance".  A digital audio signal is in the radio frequency range , so it does matter.  The longer the cable the more it matters.  But most common co-ax cables are at or near 75 Ohms anyway.

Note:
An analog audio interconnect cable does not have a "Characteristic Impedance".

[randytsuch]

A digital audio signal is a "traveling wave".

As such, if there is anyplace in the signal path that is not 75 ohms, there will be a reflection of the signal.  The more places this happens, and the farther the impedance is from 75 ohms, the greater the reflection.

The reflections cause two problems, they reduce the level of the original, and they look like noise riding on the signal.

BTW, this is also why you should use BNC's instead of RCA's for digital signals, because RCA's are not 75 ohms.

Randy