what i think you meant to ask is whether you can use a standard analog (red/white) rca audio cable as a digital coaxial (spdif) cable. (digital optical cables have a different type of connector). as kal notes, an analog rca cable has a different impedance and won't work well as a digital coaxial cable. however, a standard analog (yellow) composite video cable is 75ohm and should also work as digital coaxial cable.
RCA vs optical cable?
Are there any diferences between a regular RCA interconect cable and an aoptical cable? Can I use a regular RCA interconect cable to hoop up a transport and a dac? I understand that both cables have a 75 ohms resistance, so I suppose there is no difference between these cables. I am completely wrong? Any comments. Thanks in advance for your help.