Generally most audio video cables are 75ohms. Mainly because it has become a standard cable type for audio and video systems.
These are as follows:-
• Analogue UHF (Ultra high frequencies) and VHF (Very high frequencies) Video cable
• Balanced and unbalanced audio cable
However digital cable would tend to us 110ohm cable for these situations: –
• AES-EBU which is also balanced
• CAT5 or CAT6 Ethernet.
• Digital video
It is important to use the correct impedance cable and with the same impedance connectors.
You really start to notice the effects of impedance with RGB video, were a seperate cable is used for each individual component R (Red) G (Green+sync) B (Blue), as Green sends a sub of both the Horizontal and Vertical sync data down the green the Green sync signal is very sensitive to impedance mismatch.
If you were to cut the Green cable 3cm shorter than the other two cables, then the impedance could becomes slightly lower than 75ohms (when compared to the other two cables) causing the sync to drop or become unstable.
(This only really effects long runs; however it is extremely important to keep all three cables the same length to help reduce this effect.