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Coaxial SPDIF cable?
bennetng
post Nov 25 2012, 15:19
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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.
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mzil
post Nov 25 2012, 18:14
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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.
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pdq
post Nov 25 2012, 21:52
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QUOTE (mzil @ Nov 25 2012, 12:14) *
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?
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Posts in this topic
- bennetng   Coaxial SPDIF cable?   Nov 25 2012, 15:19
- - Speedskater   a] The longer the cable the more important the 75 ...   Nov 25 2012, 17:41
|- - greynol   QUOTE (Speedskater @ Nov 25 2012, 08:41) ...   Nov 25 2012, 18:27
||- - Arnold B. Krueger   QUOTE (greynol @ Nov 25 2012, 12:27) QUOT...   Nov 26 2012, 02:53
||- - Soap   QUOTE (Arnold B. Krueger @ Nov 25 2012, 20...   Nov 26 2012, 03:00
||- - Arnold B. Krueger   QUOTE (Soap @ Nov 25 2012, 21:00) QUOTE (...   Nov 26 2012, 14:31
||- - Soap   FWIW the fraction of my interior wiring done by th...   Nov 26 2012, 14:37
|- - Soap   QUOTE (Speedskater @ Nov 25 2012, 11:41) ...   Nov 25 2012, 18:36
- - pdq   Analog audio cables may not be closely matched to ...   Nov 25 2012, 18:00
- - mzil   Besides the wire itself, the plug used on the end ...   Nov 25 2012, 18:14
|- - pdq   QUOTE (mzil @ Nov 25 2012, 12:14) F-pin a...   Nov 25 2012, 21:52
|- - mzil   Both exist: http://en.wikipedia.org/wiki/BNC_conn...   Nov 25 2012, 22:49
|- - bennetng   QUOTE RG6 and RG59 look the same in photos, but RG...   Nov 26 2012, 03:51
||- - mzil   Your cable is marked with a JIS (Japanese Industri...   Nov 26 2012, 05:40
|- - 2Bdecided   QUOTE (mzil @ Nov 25 2012, 21:49) Both ex...   Nov 26 2012, 13:52
- - Glenn Gundlach   QUOTE (bennetng @ Nov 25 2012, 06:19) We ...   Nov 25 2012, 18:17
- - bennetng   Thanks all. Although I don't have enough knowl...   Nov 25 2012, 18:45
- - washu   I've found SPDIF rather resilient when run ove...   Nov 25 2012, 19:03
|- - Arnold B. Krueger   QUOTE (washu @ Nov 25 2012, 13:03) I...   Nov 26 2012, 03:01
- - greynol   Are you sure your speakers aren't being fed by...   Nov 25 2012, 19:08
|- - washu   QUOTE (greynol @ Nov 25 2012, 13:08) Are ...   Nov 26 2012, 03:28
- - bennetng   Thanks mzil!   Nov 26 2012, 08:56
- - Dynamic   You should get away with a lot with S/PDIF because...   Nov 26 2012, 20:39


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