Length of digital cable?

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.

"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

Thanks Paul!

I've read that 1.5 meters is the optimal length to minimize the impact of reflections in RCA terminated digital coax cables. Is this true? Or is shorter always better?

Hi Camb,

Thank you, it all depends on the cable design, termination, and transmission and receiver circuits. In our tests, the audible results are superior with a specific design over short lengths, which also minimises potential interference from all the mass of power cables etc usually found behind a system. What would be the shortest you could use?

Rgds,
Paul.

The 1.5m cable length is a good guideline. Shorter cables can be problematic depending on the signal risetime. The 1.5m length insures that any jitter causing reflections occur too late to have any impact. Really short, say 6 inches can also be good, but between 1 foot and 1.5m, you are taking a risk IMO. Dont spare the change.

Adapters do make a difference, but again depending on the signal risetime. The faster the risetime, the better the jitter result. Here are some plots showing jitter with different cables and adapters:

http://www.audioasylum.com/forums/pcaudio/messages/13/132743.html

I have been doing this a LONG time and I know what I'm talking about.

Steve N.
Empirical Audio