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Length of digital cable?

Picking up where I left off a year ago, somewhere on the net I ran into a recommended length of 18 ft of digital cable between transport and D/A converter. Various lengths were discussed and one source was very confident of the 18 ft length to successfully resolve jitter. And the Beldon 1694A was recommended. For $30+ or whatever I gave it a shot and was surprised by the quality of sound, and I swear on a stack of owners' manuals that this length with this brand of cable resolves jitter. The sound is rock solid gorgeous.

The Beldon lies behind the system mostly in a coil, and I think coils are probably not ideal. I'm wondering if anyone has found a shorter length that works. Al suggested 8", but I was unable to make it work because the components aren't close enough. I know that 1 ft" doesn't work and that 1 meter doesn't work.

Part 2: Does length of ethernet from computer to avr also affect jitter? My sister who is a computer person and not an audio person set up my avr with computer using cat 5. The sound quality is bad. I'd like to get good sound quality with computer as source down the road but need advice on how to get it.
arnettpartners
camb
144 posts
 
Steve, I understand one can't make a true 75 ohm digital cables with RCA connectors. That's the advantage of using BNC connectors, right - that is the BNC connectors allow for a true 75 ohm cable? So if you need to use RCA adaptors on the BNC cable, doesn't that negate the benefits of a true 75 ohm BNC to BNC cable?
audioengr
3,282 posts
 
Camb - Its true that RCA connectors are more like 45 ohms, but if you use good 75 ohm BNC adapters like I do, the effect is negligible. If you have BNC on either the output or input device, you can use the native BNC which is better.

The benefit is really the cable itself and how it is terminated to the BNC and shielded. It cannot be just any 75 ohm BNC. The BNC must be designed specifically for that particular cable to avoid an impedance change at the cable termination.

If you terminate to an RCA, it is very difficult to avoid a big impedance discontinuity because it is never a good match for the cable.

Steve N.
Empirical Audio
camb
144 posts
 
Thanks Steve,

I wish more manufacturers offered a BNC option. RCA still seems to be the norm, unfortunately.

Cam
ps68
214 posts
 
Although BNC would theoretically be preferable to RCA connection, there is actually next to no audible difference between an spdif using either. Steve mentions that the BNC must be designed for a particular cable, which frankly is a dubious statement. The connectors 'are what they are', and the cable that exists to connect them together should offer as close to the 75ohm Zo as is possible, which is not difficult. If using RCA connectors, the shorter the better, i.e. Furutech FP126, and designing the cable to terminate at 75ohms exactly can make a perfect match. Using adaptors also makes little difference, both on a scope and audibly. Saying that, I do agree that more equipment manufacturers could do with offering them.
Cable length should not be an issue if the said cable is an exact impedance match for the output and input, as refections should not occur. Cable length is an issue in mismatched impedances though. As I mentioned earlier, I use 40cm of my own design, which offers one of the cleanest, detailed and most spacious and unfatiguing sounds we have heard from many digital cables, which are in fact just high speed analog signals really anyway. It is most likely that there may be system limitations that would mask the subtleties of the true sonic differences between connection options.

Best Rgds,
Paul.
audioengr
3,282 posts
 
"Although BNC would theoretically be preferable to RCA connection, there is actually next to no audible difference between an spdif using either. Steve mentions that the BNC must be designed for a particular cable, which frankly is a dubious statement. The connectors 'are what they are', and the cable that exists to connect them together should offer as close to the 75ohm Zo as is possible, which is not difficult."

Whether you hear differences will depend on your source and your system. If the source is truly low-jitter and has fast risetimes to insure that the low-jitter is maintained through the receiver, the BNC will make a BIG difference.

Termination of the coax to the RIGHT BNC connector is critical. Even a millimeter difference in diameter inside the connector will change the impedance. Many so-called 75 ohm connectors are also not 75 ohms exactly.

"If using RCA connectors, the shorter the better, i.e. Furutech FP126, and designing the cable to terminate at 75ohms exactly can make a perfect match. Using adaptors also makes little difference, both on a scope and audibly. Saying that, I do agree that more equipment manufacturers could do with offering them.
Cable length should not be an issue if the said cable is an exact impedance match for the output and input, as refections should not occur."

Also untrue. There will ALWAYS be reflections, even in the best of cables and connectors. The reason to have a 1.5m minimum length as recommended in my paper is to insure that when those reflections get back to the receiver, that it is not when the receiver is detecting the edge. Its just common-sense.

You obviously did not read my white-paper, which has been the standard for S/DIF cable length for many years now.

Steve N.
Empirical Audio
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