Impedance is like resistance, except for AC instead of DC. It doesn't matter if the cable is a metre long, or 20 metres long, alternating currents are still going to 'see' a 75R impedance. Resistance & DC currents are different, because the longer the wire, the higher the resistance becomes.
However, RCA connectors are not 75R, more like 35R. To get a true 75R cable, you need to use something like BNC connectors. At RF frequencies (such as SPDIF), a change of impedance means signal reflections.
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