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.

[Dynamic]

You should get away with a lot with S/PDIF because a lot of the design criteria really help. The Bipolar Manchester Encoding of ones and zeroes makes thresholding at the mid-point easy and clock recovery very robust. The limited bandwidth lets you low-pass filter to remove nearly all the ringing seen in those avtechpulse diagrams if you should mismatch. Any attempt to match impedance, how ever poor, is likely to be enough. And the 0.2V minimum voltage swing at the input should be well clear of the range where Bit Error Ratio becomes even measurable.