Coaxial SPDIF cable? Options

[bennetng]

We were told only use 75 ohm cable for SPDIF purpose. So what is the impedance of typical analog RCA audio cable?

Another question is I have a 75 ohm TV antenna cable, is it suitable for SPDIF purpose?

Some people said that wrong impedance may not cause problem if the cable is short enough, if it is true, then what cable length will cause problem?

I ask these because I just made a SPDIF cable with two spare RCA plugs and a 75 ohm TV antenna cable (3 feet), I tested it using RMAA and found no problem (+- 0dB freq. response, -146dB noise level, 0% THD and so on) and I can't hear any problem as well. Does it mean that I don't need to spend money to buy a standard cable?

Thanks.

[washu]

I've found SPDIF rather resilient when run over cables that are not to spec. I've got a 30' run that is made of a standard 1/8" phono jack extension cable (because it's what I had) with adapters on either end. The far end has a splitter to feed it into an old set of cambridge soundworks 4.1 speakers that needs separate inputs for the front and back pairs. A very hackish solution and definitely not in spec, but it works fine.