Why do digital coax cables matter?

Are there any engineers or physicists in the posts above?

Why is is that everyone thinks they are an expert?

Well, I'm an engineer, and I used to manufacture excellent digital and analog cables, so here are the reasons:

1) Losses that slow the risetime of the signals on the cable - this causes the receiving component to detect the edges with less certainty resulting in more jitter

2) Dielectric Absorption - this is also called "soakage" and is analogous to a sponge absorbing water. The dielectrics absorb some of the charge and then it is not discharged at a constant rate. Some cables eliminate this effect by putting a DC charge on the cable with a battery. Others minimize the effect by using air dielectrics or air-filled teflon etc.. The effect is that the energy required in the signal to make a rising or falling edge is not the same for each edge because of the charge in the dielectric. The signal must overcome this charge and it cannot, so some edges are displaced in time, causing jitter.

3) Impedance mismatches - The nominal impedance of a S/PDIF coax cable should be 75 ohms, but this varies all over the map with different cables and the connectors on the ends also affect this. Impedance discontinuities cause reflections on the cable when the signal is launched into it. These reflections can bounce from end to end until they finally dissipate with the cable losses. If they happen to hit the receiving end when it is detecting the signal edge, the edge may be pushed in time, creating jitter.

4) Metallurgical defects in the conductors - Low-jitter S/PDIF signals can have risetimes in the 1nsec range. When signals this fast are launched into a cable, the conductor metallurgy affects the signal propagation down the cable. If there are a lot of faults in the crystal lattice of the metal conductors, this causes small reflections. They are like small impedance discontinuities. These reflections can appear at the receiver at the time it is detecting the edge and cause the edge to be displaced in time, causing jitter. You can look at TDR plots of this effect on real conductors here:

http://www.empiricalaudio.com/computer-audio/technical-papers/direct-immersion-lno2-study

5) Length of the cable - All S/PDIF coax cables are imperfect and therefore cause some level of reflections, which can result in jitter if the timing of these reflections is unfortunate. By making the cable at least a certain length, one can avoid the effects of these unavoidable reflections, thereby avoiding the added jitter. This has been proven in double-blind tests by the magazine UHF in Canada. Here is a white-paper on the effect:

http://www.positive-feedback.com/Issue14/spdif.htm

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.

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

"Some manufacturers like Sonic Frontiers implemented an I2Se interface to avoid these issues."

I2S is available on Empirical Audio, PSAudio, Wired for Sound and other gear. Some SE and some differential.

Even I2S requires a good cable. Actually more-so than S/PDIF because the frequencies are a lot higher on I2S.

"I guess my message is, if you supposedly need a $500+ coax cable to get the job done then maybe you need to choose a better interface. Stack the deck in your favor at least, don't be a victim!"

Like what interface? They all need good cables.

Steve N.
Empirical Audio

Rower wrote:
"For digital, you just have to pass the eye-pattern level for proper digital signal retention. The more open the "eye" the fewer error bits, the lower the jitter. This should be the full channel, too. Cable and connectors both. You don't need expensive cables to virtually remove bit errors."

True, but we are not talking about bit errors here, we are talking about psecs of jitter. The cable matters, as does practically everything else.

Steve N.
Empirical Audio

"How user can possibly know that wire or system meets the specs?"

You can't. The best option for consumers is to read the reviews from a reputable reviewer and then try one.

"In noisy system (external or internal noise) it is better to get fast switching transport getting more of reflections but in very quiet system it might be better to get slower switching transport to minimize reflections."

Have you tried it? It sounds nice in theory, but usually does not work well.

"Are you saying that, assuming some impedance mismatch, 1.5m cable will be always better than 6" cable (that I used not long ago)?"

No, 6" cables are generally not commercially available. I'm saying that a 0.5m or 1m cable will not be as good as a 1.5m cable. In order to actually get 6" total, you would need probably a 3" cable since there is cable in the transmitting and receiving device.

Jitter measurements are a rat-hole IMO. Jitter has never been effectively correlated with SQ anyway, and based on my experience, it is very dependent on the spectral signature of the jitter. Single jitter measurements are useless to say the least.

Steve N.
Empirical Audio

Rower - if you are getting bit errors, the eye pattern is so bad that jitter is the least of your concerns.

"reclocking the data at the DAC is the key to superior reproduction"

You would think so, but unless you can synchronize the source to the DAC using word-clock, it is not the best solution. Very few sources have this capability.

You will achieve much lower jitter making the source jitter low and feeding it to a DAC without internal reclocking. This is because all of the techniques for asynchronous reclocking for jitter reduction in DACs are inferior, both PLL and ASRC.

Steve N.
Empirical Audio