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.

[pdq]

F-pin and BNC connectors would technically be the proper termination to maintain the 75 Ohm rating that S/PDIF is looking for

I thought that BNC connectors were matched to 50 ohms?

[mzil]

Both exist:

http://en.wikipedia.org/wiki/BNC_connector
---

I think my cable is RG-6 after a photo search.

RG6 and RG59 look the same in photos, but RG6 is slightly larger and stiffer, which makes it harder to work with in a tight stereo cabinet. If the center core wire diameter is around 1.02 mm, then you have RG6, if it is .64 mm then you have RG59. Both should work well but be careful not to bend them too tightly; in best practice never more than 6 inch diameter circles should ever be attempted. Many people understand the danger of snapping the inner core, but what a lot of people don't realize is you also potentially can compromise the performance of the wire if it is bent more tightly than this, even if you don't snap or kink the center core.

This post has been edited by mzil: Nov 25 2012, 23:36

[2Bdecided]

Both exist:

http://en.wikipedia.org/wiki/BNC_connector

Beware that, in some industries, no one really cares about the difference.

Some connectors will cross-connect, and others won't. There's quite a debate about it...
http://www.diyaudio.com/forums/parts/18200...-connector.html
http://www.vintage-radio.net/forum/showthread.php?t=47357
I have connectors that defy every piece of information in those threads, and others that match what's said, so...

A general point is that, up to 10MHz, it doesn't really matter. On short runs, it doesn't really matter.


RCA/phono connectors were never designed to work at 6MHz, and often exactly the same cable is used for audio and video connected to phono connectors. On short runs, it's usually good enough for SD analogue composite video, and more than good enough for SPDIF digital audio. Cables designed to work with analogue HD video (or shall I say, cables that work really well with analogue HD video) are probably some of the best consumer cables out there. If they make great HD video cables, they'll make exceptional SPDIF cables. Though they normally come in triplets!

(I'm not claiming any audible difference from using better cables for SPDIF. I doubt there any detectable differences even in the SPDIF raw signal over short runs, never mind the audio that's decoded from it. On longer runs, you might reach a point where differences became detectable.)

Cheers,
David.