No digital cable with RCA termination will exhibit true 75 ohm (nominal) impedance. RCA connectors were designed for audio rather than radio frequency (RF) use. They do not exhibit a constant 75 ohm impedance at the RF frequencies used by the digital signal.
Ideally the digital cable, processor, and transport would use BNC connectors. Some manufacturers (i.e. Wadia) supply their gear with this connector in place of the RCA.
Ideally the digital cable, processor, and transport would use BNC connectors. Some manufacturers (i.e. Wadia) supply their gear with this connector in place of the RCA.