Jitter and 75ohm cable length

Magfan, CD data stream is asynchronous. It is also jittery because of less than perfect CD printing and reading plus quality of the transport and system noise. What is needed to reduce jitter is either to create stable clock for D/A converter based on average datastream rate locking both with PLL (Phase Lock Loop) - solution used in most CDP or ignore completely datastream rate and reclock it with fixed stable clock in Asynchronous Rate Converter (Benchmark DAC1).

We can add to this jitter introduced in A/D process, that cannot be removed no matter what you do. At the very beginning a lot of analog recordings got digitized with less than perfect (jittery) clock and the only way out is to digitize it again if analog master tapes still exist.

The cable length of 0.6m without transmission line effect, that I calculated, applies to 25ns transition time assuming that driver delivers constant slew rate. There are drivers that do that but very often leading "knee" has higher slew rate. Because of that I would perhaps limit such cable to half of that (0.3m). Above that careful matching of characteristic impedance is recommended. This characteristic impedance has very strange definition. It is impedance of infinite cable or finite cable terminated with its own characteristic impedance - which sounds a little like Catch22. For all practical purpose it is simply SQRT(L/C) implying particular geometry.

I'm not sure what happens with balanced cables. Impedance is 110 ohm and voltage levels are much higher but at the same time slew rate is likely higher and reflection induced jitter taking over noise induced jitter. Maybe Al can help here?

I'm not sure what happens with balanced cables. Impedance is 110 ohm and voltage levels are much higher but at the same time slew rate is likely higher and reflection induced jitter taking over noise induced jitter. Maybe Al can help here?

I'm not sure either, mainly because I don't have any specific knowledge of what risetimes/falltimes/slew rates tend to be for typical AES/EBU outputs. My suspicion is the same as yours, though, that those parameters are likely to be faster than for typical S/PDIF outputs.

Also, while on the one hand the higher AES/EBU voltage levels (assuming the particular equipment in fact conforms to the AES/EBU voltage standards) and the balanced operation would seem likely to help with respect to noise-induced jitter, on the other hand I would expect that in many or most cases balanced digital cables will provide less accurate control of characteristic impedance than a good 75 ohm coax will typically provide.

Excellent elaboration in your posts, btw, on the distinction between noise-induced jitter and reflection-induced jitter, and the competing tradeoffs that result.

Best regards,
-- Al

Thank you Al, and thanks for the links especially the second one. I suspect that balanced cable might be superior in noise rejection simply because of common mode rejection on the receiver side but also because wires are twisted what might be superior to any shielding. Twisting wires exposes them evenly to interference (capacitive or electromagnetic) leading to cancellation while shielding have serious limitations. I mentioned it before, but non magnetic shielding does not protect against EMI (magnetic in nature) but fortunately induced noise travels (to ground) on the outside of the cable - shield, because of skin effect. This beneficial skin effect is good at high frequencies but less than perfect at the lower frequencies where cable is still long enough to become effective antenna (antenna is practically ineffective below 1/10 of wavelength). Understanding of this should lead to understanding that shortest cable is the best cable. Statement that digital cable should be at least 1.5m is not complete - it should state instead "as short as possible but not shorter than 1.5m". In ICs (or speaker cables) twice shorter means twice better. Sales people often recommend 1m IC vs 0.5m IC because that's what they have in stock - absolutely no other reason.

Can someone here give me a hand with the math and it's implications with respect to two different cable lengths from the exact same manufacturer. Much of the math in this forum is in meters, the typical standard. One of my cables is 3' in length and the other 6' in length and strangely they sound different. Specifically, the 3' cable sounds very fast slightly bright (or hyper detailed) where the 6' cable sounds a bit warmer, maybe slightly rolling off the top, or maybe just cleaner at the top. I've talked about these cables earlier choosing the 3' cable as the champ, but now I'm finding the 6' cable easier to listen to. How can this be explained and may the 6' rout look better on paper and on the ears - thanks!
Rpg

Rpg, 1 meter is about 39.37 inches. And of course 12 inches = 1 foot.

Therefore 3' is about (12 x 3)/39.37 = 0.91 meters, which DOES NOT conform to the length recommendation.

6' is about (12 x 6)/39.37 = 1.83 meters, which DOES conform to the length recommendation.

I've talked about these cables earlier choosing the 3' cable as the champ, but now I'm finding the 6' cable easier to listen to. How can this be explained and may the 6' rout look better on paper and on the ears.

Chances are that less jitter is present with the 6' length, but as explained earlier that is not a certainty. Also, as explained in the section entitled "Jitter Correlation to Audibility" in the Steve Nugent paper I linked to a few posts above, depending on its spectral (frequency) characteristics jitter can sometimes be euphonic in character, and/or mask or compensate for inaccuracies elsewhere in the system.

So the bottom line obviously is to try to decide based on listening, but I would say that if the cables sound different but neither can be determined with confidence to be "better," chances are it would be best to go with the longer one.

Regards,
-- Al