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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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Speedskater
post Nov 25 2012, 17:41
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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] )


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greynol
post Nov 25 2012, 18:27
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QUOTE (Speedskater @ Nov 25 2012, 08:41) *
I have references for [b] & [c]

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


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Arnold B. Kruege...
post Nov 26 2012, 02:53
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QUOTE (greynol @ Nov 25 2012, 12:27) *
QUOTE (Speedskater @ Nov 25 2012, 08:41) *
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.
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Soap
post Nov 26 2012, 03:00
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QUOTE (Arnold B. Krueger @ Nov 25 2012, 20:53) *
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 done for economic reasons only. I have in front of me copperweld RG6 and RG6QS. I also have solid copper RG6 and RG6QS.

So if you mean "most inexpensive cable" instead of "most" I'll agree, but I still want to see any evidence than the preponderance of RG6QS being copper-over-steel is higher than that of RG6.

This post has been edited by Soap: Nov 26 2012, 03:01


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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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