Why do digital cables sound different?

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.

I would have to agree with the impedance argument, if a digital connection says 75ohm you need a 75ohm capable cable that is purpose designed to sound good doing what you want to use it for. I would suggest looking at Canare's RCAP true 75ohm RCA connectors $3-$5 each and some of the canare LV-77S cable I think it is $2 a foot or so. That combination will outperform $60-$100 digital interconnects. Canare takes cables very very seriously and they are highly regarded by the profesional audio world.

Ooop's, I was commenting on speaker cables and interconnects and missed the word digital.