Why do digital cables sound different?

Ar_t...One interesting experience in my career as a systems engineer on Submarine-Launched Ballistic Missiles was a visit aboard a 20 year old boat to evaluate the ability of the existing wiring from tube to fire control (as much as 300 feet) to transmit digital data at the higher bandwidth to be used by a new system. We injected pulses at one end and looked at them with a scope at the other end. My God! Were they ever distorted. All kinds of spikes and overshoots. But, and here is the point, the information transfer over the wires using those sorry-looking pulses was flawlwss.

You have described how the digital waveforms are distorted by improper impedance, stub terminations, etc. but it is still unclear how an analog wareform reconstructed from digital information could be affected by the shape of the digital pulses.

It has to do with the way the clock is extracted from the SPDIF signal. There is a high degree of correlation in it. This leads to a great deal of data-related artifacts in the recovered clock.

(If someone was to hook up some sort of listening doo-hickey to the point in the circuit where the PLL filter is, you will hear a very distorted version of the programme material.)

Any reflections in the data stream manifest themselves into a change in the data-dependent jitter. Not so much in the actual amount, but the frequency distribution. Absolute jitter numbers by themselves are of little good without the corresponding spectral distribution. Close-in jitter, say <10 Hz, is more detrimental than jitter at 1 kHz. So, as the reflections alter the decision point, they alter the spectral distribution.

I know.....a lot of technical mumbo-jumbo, but that is it in a nutshell.

If the clock and data were sent via separate cables, this sort of problem would not occur. Which is why one-box solutions will always be better.

What Eldartford describes is basically a Time Domain Reflectometer. I hope to have some pictures of different cables soon. (I need to construct a hood for my camera, so that I can photograph the screen. The TDR I use was made in '63. Back before they had a data port on the back to pull out the data in a form you can make a JPEG with.)

Ar_t...The system I described does have a separate "clock" line. Actually there are three lines, One, Zero, Strobe.

1, 0, 1 is a one.
0, 1, 1 is a zero
Any other set is invalid.
.

Yes, and that is why it does not have the problems that SPDIF has.

I've just read this thread with great interest.

I did some tests on my MBL transport and DAC (1621A/1611F) which I find interesting. It has multiple inputs and outputs, so I can run several cables in parallel and switch the input on the DAC via the remote, thereby making A/B very easy. It switches instantly too so there is no interruption to the music, making it even easier to detect any audible differences.

I ran a total of 4 cables in parallel, each on its own interface: 1 AES/EBU (either Synergistic Research Precision Reference (yeah, a regular XLR cable) or Audio Metallurgy GA-0 (a digital cable)), 1 RCA (I tried Gabriel Gold Revelation, Zu Varial, cheapo $1 plastic cable - all three regular ICs, and one digital $100 Monster cable), 1 Toslink (cheap Monster cable from the mid 90s) and 1 BNC (DIY copper, specced as a digital 75ohm cable).

Playing around with these in various configs and switching inputs back and forth (both blind and active, I used some fellow audiophile friends) - there is NO difference whatsoever between ANY of these cables.

I find this pretty mysterious to be honest. I've played with a lot of cables in the past and found many worthwhile improvements. But with this MBL duo, it's all the same.

Can someone perhaps shed some light on what is happening here? What have the Germans done inside the boxes to pull this off? I have no idea, and I'm not qualified to guess either - science isn't my forte. :)