Max length for 75ohm coax run (terminated in faceplate at either end) for digital

DT79

Well-known Member
I have 75 ohm coax cables installed running from behind where the hi-fi sits across to the other side of the room where the speakers are. When we had the lounge decorated I had these and some balanced cables installed for future proofing in case I ever wanted to switch to active speakers in the future. The idea was if ever needed, the coax would have been terminated in RCA sockets at either end and used for single ended analogue signals. I went with 75 ohm coax based on a prior recommendation that was the best stuff to use for long runs on account of being well shielded.

Anyway I’m thinking of locating my headphones over that side of the room and was wondering if I could use that coax run to send a digital signal from the digital out of my amp across the room to a DAC and on to the headphone amp. What is giving me pause is 1) the length of the cable (5-7m; not 100% sure how it’s routed under the floor) and 2) the continuity breaks with the faceplates.

The whole chain would be: amp dig out > 0.75m 75ohm coax > faceplate/~6m 75ohm coax/faceplate > 1m 75ohm coax > DAC > 0.5m analogue RCA > Headphone amp.

Before I go to the trouble of getting the faceplates and soldering up the wires....is this likely to successfully transmit the digital signal?

Thanks.
 

larkone

Distinguished Member
No reason it should not work but will depend on how much loss you introduce which will be a combination of the quality of the cable, the connection plates and plugs and your soldering etc.

I have used a 15m coax cable to transmit a digital audio signal without problems over a single uninterrupted run. TV signals over 75ohm coax are tiny voltages and very long runs - aerial to TV
 

DT79

Well-known Member
No reason it should not work but will depend on how much loss you introduce which will be a combination of the quality of the cable, the connection plates and plugs and your soldering etc.

I have used a 15m coax cable to transmit a digital audio signal without problems over a single uninterrupted run. TV signals over 75ohm coax are tiny voltages and very long runs - aerial to TV
Thanks. The installed cable is Van Damme pro stuff (as will be the ones connected to either end) so probably not going to get any better than that. I’ve no doubt the limiting factor will be my soldering.

I’ll give it a go then!
 

Mark.Yudkin

Distinguished Member
An RG6 coaxial cable will be fine up to 50m. Should you need a greater distance - that is lower attenuation - use RG11 cabling, it is readily available up to 150m. I doubt if your installed cable will cause any problems. Since you only cited a brand name, not the cable type, I can't promise adequacy.

The wallplates, if properly installed, will also not be an issue. That said, I have had issues with TV signal quality, resolved by having the wallplates replaced by a qualified service personnel from the cable TV company. Audio signals are of a much lower frequency than CATV, so are less fussy.

There are neat "how to"'s at Wiring up plugs, aerials and wall plates - A.T.V. Poles, Brackets, Clamps & Aerials.
 

noiseboy72

Distinguished Member
Digital audio has a much lower bandwidth than RF digital TV signals, but is not as robust in terms of handling poor signals.

6M of cable will not present any difficulties, but as Mark says, make sure termination and socket type is not creating an impedance bridge - as this will increase the amount of signal that gets reflected back down the cable. This reflection can increase error rates - which the DAC will attempt to correct by adding in "guessed" audio values for missing or corrupt samples.

I really don't think you will have any issue however.
 

DT79

Well-known Member
An RG6 coaxial cable will be fine up to 50m. Should you need a greater distance - that is lower attenuation - use RG11 cabling, it is readily available up to 150m. I doubt if your installed cable will cause any problems. Since you only cited a brand name, not the cable type, I can't promise adequacy.

The wallplates, if properly installed, will also not be an issue. That said, I have had issues with TV signal quality, resolved by having the wallplates replaced by a qualified service personnel from the cable TV company. Audio signals are of a much lower frequency than CATV, so are less fussy.

There are neat "how to"'s at Wiring up plugs, aerials and wall plates - A.T.V. Poles, Brackets, Clamps & Aerials.

Thanks for the ‘how to’, that will be handy.

This is the stuff.


Sounds like it should be fine as long as I do a decent job of the soldering!
 

oscroft

Member
Thanks for the ‘how to’, that will be handy.

This is the stuff.


Sounds like it should be fine as long as I do a decent job of the soldering!
At that link it says "Suitable for SP Dif digital audio, word clock and 360 Mb/s SD digital video up to 138 metres", so yep, it sounds fine.
 

Mark.Yudkin

Distinguished Member
Question on PC Sound Card SPDIF Out to DAC Cable lengths. This claims that the SPDIF standard has a maximum distance of 10m. But normally specs like that are conservative, so you can get longer distance.
The S/PDIF specification is for short runs and inexpensive cables. The full (expensive) book explains that "Longer cable lengths and higher frame rates may be used with better cables."

Commonly used XLR-3 microphone cables have various impedance ratings (30 Ω to 90 Ω typical) and exhibit poor digital transmission performance. The result is signal drop out and reduced cable lengths due to severe impedance mismatching (VSWR) between AES/EBU 110 Ω equipment. AES/EBU signal transmission work for few tens of meters with a good cable.
See S/PDIF - HwB.

The OP's cable will be fine. He will need to take care that his connectors and wall-plate are adequate and correctly installed.
 

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