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Quick question? Can you use any rca cable for digital coax?


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6 replies to this topic

#1 of 7 OFFLINE   MarkSchro

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Posted September 15 2006 - 10:38 AM

My friend asked if he could use any rca cable for a digtial coax signal.
I thought any rca cable would work and not to buy a $30.00 digital cable.
He was just going to connect from his cabel box to reciever for his digital
audio. Can anyone let me know if I am right. Thanks.
Mark

#2 of 7 OFFLINE   JeremyErwin

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Posted September 15 2006 - 11:48 AM

you can even use a coat hanger, if you wish. The standard practice, however, is to use 75 ohm video cable, or, if you want to avoid ground loops, toslink.

#3 of 7 OFFLINE   Bob McElfresh

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Posted September 15 2006 - 04:52 PM

Jeremy is correct. Coaxial-digital specifies a "75 ohm coaxial cable". Translation: a Video cable (usually with yellow bands on the RCA plugs).

Audio cables can be made with 50/75/110/300 ohm coax. And people have used audio cables by mistake and it appears to work. But then they notice something: occasional audio drop-outs. Sometimes only once every 15 minutes, sometimes several in a minute. (One member was about to return his new system because it was bugging him so much before we determined he was using a RCA cable from a L/R set.)

The coat-hanger trick - this does work. But only on a lab-bench with little or no other electrical devices nearby. To protect the weak, line-level signals you need a coaxial cable.

Hope this helps.

#4 of 7 OFFLINE   homthtr

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Posted September 16 2006 - 02:34 AM

A cheap way to do that is to use a good quality RG6 Coax, A short piece that has Compression (snap n seal) F type coax connectors. The same ones that the cable or sattelite company make up on site. Just get 2 f-rca adaptors for about $4-$5 and your good to go. The Rg6 Coax is a great sheilded coax Digital connection.

#5 of 7 OFFLINE   sarah99

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Posted September 16 2006 - 05:02 AM

Quote:
Originally Posted by Bob McElfresh
Jeremy is correct. Coaxial-digital specifies a "75 ohm coaxial cable". Translation: a Video cable (usually with yellow bands on the RCA plugs).

But as you all know, once a 50 ohm RCA plug is on each end, the 75 ohm cable has an effective impedance of its' lowest value component ........ 50 ohms. So why not use any (50 ohm) cable to start with.

(if you really needed 75 ohm impedance, the manufacturers would have fitted your equipment with BNC connectors)

#6 of 7 OFFLINE   ChrisWiggles

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Posted September 16 2006 - 07:37 AM

RCA plugs can approach 75ohm, though it is certainly true that BNC would be preferred.

Any 75ohm cables are appropriate. Thankfully 75ohm cables are cheap and readily available at any radioshack, hardware store, electronics store, the net etc.

If you want the very best pro grade stuff, that is also quit reasonable from someone like bluejeans.com.

#7 of 7 OFFLINE   Bob McElfresh

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Posted September 16 2006 - 11:11 AM

Quote:
So why not use any (50 ohm) cable to start with.

The issue is something called "impedence matching".

Think of the cable as a pipe carrying very high pressure water. The input to your receiver has a pipe fitted for 75. While you can hook a 50 pipe to it, the water will suddenly see a change in size of the pipe and not create a smooth flow into your receiver.

In electronic terms, this causes ringing or attenuation of the signal.

At low flow (like subwoofer or audio frequencies) impedence matching is not important. (Kind of like a speed bump or 'dip' at 5 mph). But that same bump at 30 or 60 mph - it causes problems. This is why higher frequency signals need cables that try to match the impedence of the electronics.

Quote:
if you really needed 75 ohm impedance, the manufacturers would have fitted your equipment with BNC connectors

I wish that were true. All analog video connections need to be 75 ohms. But consumer gear is stuck with the old friction-fit RCA plugs. Pro gear and some projectors do have BNC jacks. Even the cheap-as-dirt "F" connector on your CATV cable is a better connector for high-frequency signals than an RCA plug. But we are stuck with this.





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