# Digital cable impedance question

I see that digital cables are rated at 75 ohms for RCA and 110 ohm for AES/EBU. How are these measurements taken? It seems to me that wire should have no resistance if it is a straight measurement from one end to the other. Anyone know how they got these figures? Thanks for reveiwing and replying.
4 responses
 01-10-2012 2:05amJust guessing, they must feed a high frequency the cable is designed to carry through it. How well it transfers the frequencies it will need to carry. Reflections in it, and stuff that's too complicated for me to describe. Signal loss, capacitance, propagation speed, plus others? An ohmmeter itself would just check resistance without a frequency, and no other necessary measurements would be done using a basic ohmmeter. Just my guess. 01-10-2012 2:50amI just looked on Google under "cable impedance measurement", and found some info. This may help for a starter. A lot of info on Google. [http://na.tm.agilent.com/8720/applicat/srlpaper.pdf] 01-10-2012 2:56amHifihvn is basically correct. The 75 ohms and 110 ohms in this case do not refer to resistance, which as you realize will be well under 1 ohm for reasonable cable lengths. They refer to what is called characteristic impedance, which to a close approximation is equal to (square root(inductance per unit length/capacitance per unit length)).At high frequencies such as the frequency components of digital signals, where the cable length becomes a significant fraction of the signal wavelength, the cable's characteristic impedance has to be a close match to the input impedance of the component that is being driven through the cable. Otherwise some fraction of the signal energy will reflect back to the source, instead of being absorbed by the load. If the output impedance of the source is also not closely matched to the cable's characteristic impedance, then some fraction of the reflection will re-reflect back to the original destination, resulting in waveform distortion that can cause jitter and other adverse effects.Regards,-- Al 01-10-2012 3:34amThanks Almarg. That was the part needed to make it complete, and a more appropriate link.