A lot of video and digital cables have an impedance of 75 Ohm. What does this mean?
The 75 Ohm impedance is a world-wide accepted value for all kinds of coaxial high frequency signal connections. Impedance is another word for the value of electrical resistance for alternating current: An alternating voltage applied at one cable end results into an alternating current flowing into the cable. The voltage to current ratio is the impedance. When you measure a coaxial 75 Ohm cable between its shield and core with an Ohm meter you will however measure an open circuit and not 75 Ohm. This because your meter measures using direct current, i.e. measures DC resistance, not AC resistance.
A coaxial cable’s AC resistance (=impedance) value arises from the electrical capacitance between the centre conductor and screen as well as from the electrical series inductance of the centre conductor and screen. This capacitance and inductance are distributed along the entire cable length and at higher frequencies their interaction allows for the electrical signal energy to travel along the cable in the form of an electromagnetic wave – just like radio waves or waves in a pond, but very effectively channelled within the cable.
The cable’s capacitance and inductance values per meter constitute the typical construction parameters of a coaxial cable. This capacitance and inductance must exhibit low electrical losses and be very stable along the entire cable length to achieve a good 75 Ohm impedance cable which allows maximum signal energy transfer over a wide frequency spectrum. This maximum energy transfer is much like with an amplifier and speakers; your power amplifier for instance produces the highest output power (voltage * current) at 8 Ohm.
Are balanced cables also 75 Ohm?
Regular balanced cables have a higher impedance, like the standard 110 Ohm AES/EBU type for professional digital audio transfer. In special applications balanced 75 Ohm cables can be applied, but the layout of the design is different and the capacitance between the two conductors is somewhat higher.