Why do digital cables sound different?

Hi,

Digital cables can make a HUGE difference. But all those fancy multi-megabuck cables with exotic materials won't buy you much, unless they also solve the key technical problems. At high frequencies, these cables MUST be impedance matched. Ie ...Coax connections should be 75Ohm. If not, reflections inducing jitter related distortions will occur, and this IS what differentiates these cables. This along with dialectric absorption and inproper shielding. It is my feeling that cables that sound sharper, cleaner, more resolved, are simply reducing the jitter component compared to others. If your DAC reclocks than these differences are less apparant. If it does not, as my EVS doesnt, you will be blown away.

When I went from a rather expensive "fancy" cable to an RG6U(Quad Shielded 75Ohm) design cheapo cable(email me if your interested in what this was)...The differences were as big as the difference between various DACs...not insignificant. I love it when something cheap does the same job of something much more expensive, and sometimes better.

Good luck

Jt, rather than clog up your inbox and have to make several individual replies, how about posting your comments here ? I for one would like to know what you have gotten such good results with, especially since both I and my Brother along with several others here are running EVS DAC's.

Before you comment, you should be aware that there is NO such thing as a 75 ohm RCA connection. This is true regardless of what Canare states. Nor is the nominal impedance of the wiring from the digital input to the circuit board inside the EVS 75 ohms. None the less, you may have stumbled onto something in terms of increased power transfer / minimized loss / reduced standing waves. The last part ( reduced standing waves ) is directly related to a reduction in jitter. Sean
>

Zilla - it is impossible for a digital cable to act as a "tone control". What happens is that the jitter caused by the cable losses results in frequency modulation of the analog signal. This can cause lack of clarity and focus. In general, the better clarity and focus, the better the digital cable.

Jt25741 - You are dead-on when you say that jitter is the main issue with digital cables. However, with some cables, you get what you pay for. Every cable manufacturer has a digital cable and many of them are not even close to 75 ohms characteristic impedance. They need to be 75 ohms. As for the connector, the best you can do with an RCA is to get the impedance right up to the entry point of the jack. After that there will be a discontinuity. Jacks are never 75 ohms. Some manufacturers do this. I do.

Sean - Transmission-line effects are the main concern with digital cables. Characteristic impedance matching is a big part of this. However, dispersion of the digital signal is also caused by dielectric absorption, which can cause jitter, so just matching to 75 ohms is not sufficient to minimize jitter.

"Shouldn't a cable that has a higher velocity factor be less prone to signal deterioration / absorption due to the signal spending less time in the cable ?"

There will generally be less absorption in a high-velocity cable because in order to get high-velocity, you need a low dielectric constant. Low dielectric constant results in lower capacitance and lower dielectric absorption. The time that the signal transits the cable (propagation time) is really of little consequence itself. This will obviously change depending on the length of the cable. The rise-time of a SP/DIF signal is on the order of 20 nsec, so you would have to have 100 feet to equal the risetime. Technically, this makes impedance a non-issue for a 6-foot SP/DIF cable. However, in practice, impedance discontinuities do impact the sound, particularly the image focus and detail, by adding to the jitter.

As for dielectrics, PVC is at the bottom, getting progressively better with foamed poly, solid Teflon, foamed Teflon, expanded Teflon and finally air. I use Expanded Teflon in my Digital cable. It is hard to put a percentage on the improvement without measuring it. I have plans to purchase a Tek CSA803 communications analyzer, which will measure jitter accurately to a few picoseconds, so I will eventually be able to measure this.