Why do digital coax cables matter?

Almarg wrote:
"Do you feel that the following may also be significant contributors to sonic differences between S/PDIF interconnects, at least in some systems?

6)Differences in noise-induced jitter, due to ground loop effects and/or RFI/EMI pickup, both of which may be sensitive to cable differences.

7)Differences in radiated RFI, that may affect circuitry in the system that is not directly related to the S/PDIF interface."

These are both potential contributors to jitter, although #6 is not directly related to cable quality, and # 7 is mostly a function of the receiving device IMO.

As for cable pricing, I have found that in general cables below the $500 mark for 1.5m length sound about the same. Significant improvements are not realized until one spends more than $500. This is when you start to get the more exotic constructions, conductors and dielectrics, as well as better shielding. Just my experience.

Steve N.
Empirical Audio

Mapman wrote:
"on the several occasions where I have compared different digital cables going into my DAC(s), if there was a difference, it was not enough for me to take clear notice or even care. I know that in theory different levels of jitter is the result and that jitter level matters. But does it really in practice?"

Perhaps you didn't test the right cables, or your preamp creates enough distortion, noise and compression that you dont hear the benefits because they are masked. This is fairly common when using an active preamp. I dont use a preamp, so I dont experience this masking anymore. It's a system after all, so every component and cable matters.

Steve N.

"Characteristic impedance different than 75 ohm can be very good, as Al mentioned, if it is better match for given system."

Sure, but I would sell that system and get one that meets the specs so I dont have to try to find a wacked-out cable that matches it.

"Same for slowing down the edges. Uncertainty of threshold is not caused by long transitions but by the noise."

Noise will certainly cause jitter (signal integrity or ground-bounce), but slow edges by themselves will also cause jitter and usually worse based on my experience. The problem is the voltage reference which sets the switching threshold at the receiver. This reference is usually noisy due to the system voltages and ground-bounce. Very difficult to make it noise free.

"With very little noise present longer edges might reduce impedance mismatch caused reflections, reducing jitter in effect. "

It sounds like common sense, but it doesnt work in practice. Faster edges and precise matching works a LOT better.

"Making cable "at least certain length" is not precise since cable is not even considered transmission line when propagation time (one way) is shorter than 1/8 of transition time being about 0.6m for typical 25ns transitions (assuming 5ns/m)"

I know this "rule of thumb", but really low jitter systems have risetimes of 3ns or less, not 25nsec. Even at 25nsec, the cable length helps however. the A/BX testing proves it.

Steve N.
Empirical Audio

Jitter is measurable, correct?

certainly

"Will a cable of some determinate length not add some measurable, repeatable, non-arbitrary amount of jitter within a particular range of measurement, regardless of any jitter coming from the source component?"

Yes, assuming the signal is repeatable.

"Are there any cable manufacturers that measure and publish jitter specifications for each of their different cable products and cable lengths?"

I cannot think of any cable manufacturers that can afford a $150K analyzer from Agilent that it takes to measure this. Even JA of Stereophile with his latest and greatest AP system cannot measure it.

The other thing you must understand is that a lot of the jitter problem in cables is caused by imperfect receivers in the DAC, not the cable itself. If you put an analyzer at the end of the cable instead of the DAC receiver, everything changes. You lose half of the effects.

Steve N.

Ones and zeroes doe not exist in real life; it is all analogue signals, interpreted as numbers.
You would like thee signals to be the only one on the cable, and you want them unaltered, so interpretation is flawless.
If the cable picks up or rejcts other signals, noise, it could upset f.i. feedback loops in amp stages, giving distortion.
It the signals that represent the numers themselves are distorted, interpretation and timind can suffer; this is jitter.
So a cable could do a few things wrong, and then there is impedance match which I frankly do not quite understand.