Can't get SPDIF through 50 ft of wire! - diyAudio
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Old 8th March 2003, 10:24 PM   #1
Luke M. is offline Luke M.  United States
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Default Can't get SPDIF through 50 ft of wire!

Hi everyone,

I'm a bit new to the forums so nice to meet all of you.

I want to run digital audio from my PC to home stereo. I've just upgraded to an SBLive! soundcard with digital out. Previously I'd been using the analog front and left, routed through a thin telephone-like three conductor wire to my stereo, about 50ft away from the PC. It worked fine.

When I put in the SB card the analog worked fine there as well. So I cut off the phono plugs and put RCA's on this same cable and ran it from the SPDIF out on my Sound Blaster to the digital in of my receiver: Nothing.

I checked the cable, and there's no short or any such. Thinking the digital signal may be too weak to travel that distance on such a small wire, I ran a 16 gauge speaker wire the entire distance, and still, not a peep from the receiver.

Finally, I lugged my entire PC into the living room, and hooked them together with a two foot RCA cable. It worked and sounded perfect, so I know there's not a problem with my soundcard or receiver.

Unless someone else has any theories, my guess is that the power output on the SB card's digital out is rather low. If that's so, I guess I need to wire up a signal amplifier.

On the suggestion of another I tried one of those 3-volt Radio Shack headphone amps. Still didn't work, I'm thinking the amplifying transistors in the circuitry aren't made to switch at the high speeds needed for a digital signal.

What do you all think?

Does anyone have experience with this? Surely others want to hear digital music in a different room.

Well, thanks in advance for any help!
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Old 9th March 2003, 12:31 AM   #2
OliverD is offline OliverD  Germany
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A perfect solution to your problem might be a "customized" link similar to the one Elso Kwak suggested in the "digital transformer" thread some time ago.

You would pick up the S/PDIF signal at TTL level somewhere inside your soundcard, "boost" it with 74xx logic and drive a twisted pair cable balanced. At the receiving end, a proper line termination followed by a fast comparator can convert the signal back to TTL level.
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Old 9th March 2003, 12:35 AM   #3
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Don't be surprised that some jitter will be introduced with 50 feet of unshielded telephone-cable
It's only audio. Member of the non modular PCB design committee
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Old 9th March 2003, 12:52 AM   #4
OliverD is offline OliverD  Germany
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After re-reading your post, one more hint: Your entire connection should be a 75 Ohm coaxial connection. Speaker cable or telephone wire will NOT work with a standard S/PDIF interface. Impedance matching matters more the longer the cable is. A standard S/PDIF connection was not designed for 50 ft cable length AFAIK, but it might work anyway. If you don't get it to work, a customized interface (as mentioned above) is the way to go.
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Old 9th March 2003, 07:09 AM   #5
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Lightbulb Long Distance SPDIF

Hi Luke,
I tested my interface in the digital transformer thread with 13m (that is about 42 feet) common 75 Ohm coax. I did not hear any quality loss.
For best results use double shielded coax and 75 Ohm BNC connector system.
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Old 9th March 2003, 03:37 PM   #6
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Default SPDIF in the long run.....

Impedance matching matters more the longer the cable is.
Actualy is the oposite see the caracteristics of the standard IEC958 Digital Audio Interface.

-Cable: 75ohm +/-5% (l<10m) or 75ohm +/-35% (l>10m)

As stated with more than 10 meters runs the inpedance of the cable can have a tolerance of 35%.

For more information(for the fews that do the home work)see:


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Old 9th March 2003, 04:21 PM   #7
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I don't know if this will help you, but I used a 30Ft run of 75ohm sattelite co-ax cable between my sblaster card and receiver.

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Old 9th March 2003, 04:23 PM   #8
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When dealing with digital audio and all flavors of video: Match your impedences. All along the path.

For S/PDIF: Use a 75 coax ohm cable. Belden cable model 1694 is excellent, but just about any 75 ohm cable should work.

If the connectors are BNC type, make sure you are using 75 ohm BNC connectors.

If the connections are RCA/Phone types, use a good quality connector. I do not know of any RCA/Phono connectors rated at 75 ohm.

If you have some restrictions that you are forced to use existing twisted pair cabling, you can use 75 ohm to 110 ohm transformers on the output, go through the twisted pair, then go 110 ohm to 75 ohm at your destination.

The transformers will cost more than 50 ft of 75 ohm coax plus connectors, so if you can re-run your cable, that is the way to go.

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Old 9th March 2003, 09:10 PM   #9
Luke M. is offline Luke M.  United States
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Default It works!


I tried the coax and voila! Though the cable is connecting to RCA's on both ends, Radio Snack has nice screw-on adaptors from BNC to RCA. Turns out, the best solution was also the easiest one. I was even able to get white coax, you can't hardly see it along the wall.

Thanks everyone for the advice, these forums are great!
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Old 2nd April 2005, 04:03 PM   #10
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Default Transformer

I first discovered this thread after starting anew one. Look what I did....

Different ways to transport SPDIF Audio signals
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