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.

[Speedskater]

a] The longer the cable the more important the 75 Ohms is. At 3 feet it's not important, at 50 feet it's rather important.
b] But some poorly designed components are sensitive to cable differences and lengths.
c] Typical nice analog audio cables very well may be about 75 Ohms. It's the cheap sweet spot for cable manufacturing.
d] RG-6QS (Quad Shield) is designed for the very high frequencies of cable TV so it's not the best choice for digital or analog audio. Ordinary RG-6 is a better choice.
e] At 3 feet, I suspect almost any cable would work.

(Yes I have references for [b] & [c] )

[greynol]

I have references for [b] & [c]

I'd be interested in a reference for [d].

[Arnold B. Kruege...]

I have references for [b] & [c]

I'd be interested in a reference for [d].

Just to review:

d] RG-6QS (Quad Shield) is designed for the very high frequencies of cable TV so it's not the best choice for digital or analog audio. Ordinary RG-6 is a better choice.

I think that what is being referred to is the fact that most quad-shield RG-6 has what is known as a copperweld center conductor. This means that the wire is copper over steel (for strength)

This is, however not a 100% generality as I have seen quadshield RG6 that has a solid copper center conductor/

If you are putting a cable TV signal down the line, the lowest frequency is usually 50 MHz and skin effect is so strong that the fact that the center of the cable is steel which has only a fraction of the conductivity of copper, isn't an issue. Only the center is steel, and very little of the signal flows there. The signal flows mostly on the outside which is copper.

There are even more extreme cases of this sort of design of the larger cable TV cable (used for trunk lines) where the center conductor is an aluminum tube with silver plating. Again skin effect is so strong that the fact there is no conductor at all and just a hollow tube doesn't matter.