Digital coaxial output on a plain RCA audio cable ?

Can I use an ordinary audio (or video) RCA cable to send coaxial SPDIF digital from my soundcard to a coaxial SPDIF digital input on another device ? Do I really need to buy a special coaxial cable or will the ordinary audio cable work ?

Coax video cable will work fine - better than an audio one, as the specs of a composite video cable is similar to those for SPDIF, IIRC.

Whilst it's true that 75 ohm video coax is a better match, the chances of noticing any difference between this and an ordinary short audio RCA-to-RCA lead are minimal, because the characteristic impedance of it isn't that far different from video coax anyway. So over 5-6 feet or so it's not a problem. These cables tend to have relatively high capacitance though, so the attenuation losses mount up over longer lengths quite fast. And that means that the relatively high frequency signals deteriorate to the point that sync lock-up fails.

So ultimately how far you can send S/PDIF this way rather depends on your individual cable, but using ordinary audio cable over longer distances isn't advisable at all, even if it actually seems to work, because you stand a good chance of it not being reliable.

Whilst it's true that 75 ohm video coax is a better match, the chances of noticing any difference between this and an ordinary short audio RCA-to-RCA lead are minimal, because the characteristic impedance of it isn't that far different from video coax anyway. So over 5-6 feet or so it's not a problem.

FWIW I've always used plain RCA audio cables without problems.  They've usually been fairly short interconnects, but I do have one RCA audio cable that'd be a couple of metres long that I've drafted into service.  But this is merely to connect an M-Audio 2496 to a <spit>SB Live</spit>, so it wasn't for any serious application.  But it can be done.

If you are lucky, you can get further than that, but you need a sympathetic receiver. The most I've ever managed with manky audio cable was about 14ft down to the end of the workshop, where there is a convenient oscilloscope that shows you just how much of the risetime you've lost - quite a bit! But with a bit of care, this was still resolved into clean audio.

The way it's actually specified is that for run lengths over 10m, you need 75 ohm coax with a tolerance of +/- 5% or better, but for any run up to 10m long, you can loosen that tolerance to +/- 35%. The further from the characteristic impedance you get, the shorter the run you can have. But all of this assumes that the terminations are accurate too - and with some manufacturers, there's absolutely no guarantee of this.