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.

[mzil]

Besides the wire itself, the plug used on the end partly dictates the overall impedance based on its geometry, including the center core's diameter and distance (the ratio) from the outer shield. F-pin and BNC connectors would technically be the proper termination to maintain the 75 Ohm rating that S/PDIF is looking for, however in real world use, and looking at the jack panel on the back of most consumer gear, RCA works just fine, at least for the short distances we might use in a home.

Properly terminating coaxial wire such as RG59 or RG6 (often used for TV/cable/satellite wiring) should work just fine with RCA plugs for your application, in fact it might be even better (in practice) than commercially purchased RCA wires that claim they are "75 Ohm". [Non truly are, the RCA's geometry simply wont allow it, as they were never designed from the get go to be 75 Ohm, but they are close enough for most situations, so they work.]

The wire you have prepared should be fine.

I don't think there is an answer to your question "At what distance should I start to worry" because it is very gear dependent. In one situation a 4 meter wire might have problems, yet in another situation a 6 M wire works fine. If all your wires stay within the same rack and don't exceed 3 to 4 meters tops, I doubt you'll have any problem, with most gear. If you have to run the wire across the room or to other rooms of your house, there could be issues.