Digital cable length

Posted by: naimUnT on 08 November 2011

I need some advice on the maximum length for a 75 ohm digital cable. I have a 8 foot HDMI cable from my CABLE TV to my LCD TV and from the TV I propose to run a 8 foot 75 ohm (phono one end and BNC other end) digital cable to my NDX DAC input. Would this be okay or is the length going to be a problem? Would greatly appreciate your advice!

Posted on: 08 November 2011 by james n

That won't be a problem. It's only TV sound anyway so nothing to get too hung up about. Any reason why you can't run Toslink ?

James

Posted on: 08 November 2011 by pcstockton

how about digi straight from your STB to the NDX? Why send it to the TV first?

Posted on: 08 November 2011 by KRM

Toslink is generally not as good as electrical, but copes better over longer runs. Eight feet is not very far and, as James says, it's only the telly :-)

Keith

Posted on: 08 November 2011 by pcstockton

Originally Posted by KRM:

Toslink is generally not as good as electrical,

why is that?

Posted on: 08 November 2011 by rich46

poor quality opto domestic cables maybe poor but with high quality fibre , you wouldnt tell the difference, its also better over distance and isolates any unwanted noise

too much codswollop from expensive cable manufacturers

Posted on: 08 November 2011 by Simon-in-Suffolk

Hi if your coax is a regular industry standard RF based RG6 or RG7 then your loss over 100 feet is less than 1 or 2 dB at the frequencies used for the top SPDIF rates. Be aware of mumbo jumbo snake oil cables that are marketed to audiophiles that could be a lot less efficient.
It's worth remembering at radio frequencies, losses are mostly down to central conductor size in coax and sizing and quality of dielectric. (the material between core and shield).
Simon

Posted on: 08 November 2011 by Guido Fawkes

Originally Posted by pcstockton:

Originally Posted by KRM:

Toslink is generally not as good as electrical,

why is that?

Good answer > codswollop from expensive cable manufacturers

I prefer Glass Toslink to Electrical when a computer is connected through to a DAC.

With the W4S Sonos it doesn't make much odds with Electrical sounding as good, but certainly not better than a good Glass Optical.

The Wireworld Glass Optical and the no longer made TCI Glass Optical are excellent cables. I'd avoid using more than 1m if possible, but I can't see why a longer length would be so bad.

I'm not keen on plastic ones, especially ones that are plastic and expensive.

Don't pay a fortune - high price Coax is not worth it IMHO.

Posted on: 08 November 2011 by pcstockton

In my last job I sold VERY high-end diamond abrasives (3M Microfinishing Films) to a premier boutique manufacturer of fiber optic cables and terminations. Trust me that that the material used has little to no effect on transmission. The glass cables are easier to polish. That said, they are not appropriate for certain applications given the likelihood of cracking.

When I asked him why one would use one over the other they said glass is cheaper for them so money would be the overarching reason for selecting glass. But it all depends on the spec for the terminals and sheathing. Also how many fibers are bundled into a single cable etc.... It is all spec'd by engineers smarter than any of us.

Even in glass cables there is a plastic core. Otherwise the light would not transit well. It needs to reflect off of the dissimilar surfaces.

Basically I was told that in a single toslink cable used for a single stream of audio, it matters not what the material is. What DOES matter though is the quality of the polishing of the cable and getting the core perfectly centered and the light scattering consistent etc...

In a vacuum, glass vs all-plastic as a substrate doesn't matter. Unless of course you need tight bends, then plastic is better.

Whether or not cable mfrs use plastic, and maybe shoddy polishing, on their lower end cables has nothing to do with the material used.

-Patrick

Posted on: 08 November 2011 by Guido Fawkes

Thanks Patrick - I upgraded from a plastic cable to a TCI Glass Optical and it did sound a lot better, but it may be the quality of the polishing, connectors and sheathing rather than merely its use of glass, as all of these were different. The TCI cost £70 and is the best digital connection I've found. The Wireworld Super Nova is very good, but is double the price. Unfortunately, TCI no longer makes the one I have.

All the best, Guy

Posted on: 09 November 2011 by naimUnT

Thanks guys for your helpful advice! Yes, I could use a Toslink from the Cable TV to the NDX, it's a lot shorter but problem is I have 2 other devices which use HDMI and I've run out of inputs on the NDX. As I understand, HDMI uses digital for audio and video which I route to my TV and then coaxial digital out from TV to NDX. I know it sounds silly having to let the bits travel all that distance but I really have no other choice unless I buy a nDAC!

That would be another black box to my increasing box count! From all of your feedback, it seems I should be fine with the coaxial length I have in mind. The other problem has to do with lip sync. If I take the short route, the lips don't match the speech! There is some sort of lag. Ah, I wish other devices had Naim's PRAT and never miss a beat. The long route seems to cure the sound lag! Strange!

Posted on: 10 November 2011 by Simon-in-Suffolk

Patrick, I wouldnt presume that some of us here arent smart engineers ;-)
Any way as i said the big gotcha with fibre optic for SPDIF is he micro phonic induced jitter, so me hanical decoupling and lossy sheaths are the order of the day.
Simon

Posted on: 10 November 2011 by pcstockton

Originally Posted by Simon-in-Suffolk:

Patrick, I wouldnt presume that some of us here arent smart engineers ;-)

I do presume that some here are not smart engineers, myself included. sorry.