Digital cable length- SPDIF vs AES/EBU- 1.5 meter for either?

Also, some comments on the Lessloss writeup that Steakster linked to, which I disagree with to some extent:

It makes no mention of the effects of length on the **timing** with which signal reflections arrive at the destination component (i.e., the DAC), and instead focuses mainly on the amplitude with which those reflections arrive. But in a home system application what is **far** more likely to be significant (for a given set of component and cable impedance values, within their respective +/- tolerances) is arrival time, as explained in Steve Nugent’s paper that was linked to earlier in the thread. Not arrival amplitude, which won’t differ greatly as a function of cable length, in home system applications.

In fact the Lessloss paper itself states that it provides a "reflection-attenuation network, built into the very cable itself, ... [which reduces] the level of the first reflection by 5.6 dB. This is equivalent to a silver digital line of this type of 117 meters in length."

5.6 db is a reduction of only about a factor of 2, in terms of voltage, which by their statement would occur without the special built-in network only if the cable were 117 meters long!

The reason timing is what matters is that what the DAC detects are the **transitions** between the high voltage and low voltage states (and vice versa) of the signal it receives. If the reflections arrive at times in between those transitions, or at times during those transitions that are not close to their mid-point, they will be ignored.

That said, I have no specific knowledge of how the reflection-attenuation network of the Lessloss cables may be designed, and no experience with or knowledge of the sonics Lessloss digital cables may provide in typical applications. In any event, though, it sounds like the inclusion of the special network in their cables means that the rationale for the usual 1.5 meter recommendation is inapplicable in their case.

Regards,
-- Al

Are you all saying any coax digital cable that is not 1.5m long should be tossed out?

Kalali, please read the posts by me and by Kijanki dated 1-25-2016. As you’ll see, there are numerous technical variables affecting what length will be optimal in a given application. Some of those variables are almost never specified (e.g., the risetimes and falltimes of the signal provided by the source), and some have little if any predictability (e.g., susceptibility of the two connected components to ground loop effects, which can contribute to jitter at the point of D/A conversion).

So as I said in concluding my post:

I would consider Steve’s recommendation of 1.5 meters to at the very least provide the best odds of being optimal.

This assumes, btw, that a very short length, such as say 6 inches, is not practicable. In circumstances where it is practicable, I suspect it is likely to be an even better choice than 1.5 meters.

Also, past threads here have provided anecdotal evidence that 1.5 meters is not always the best choice. Some members have reported making direct comparisons of 1.5 and 1 meter cables that are otherwise identical, and preferring the 1 meter length.

Regards,
-- Al

I would not be surprised if cable designers do in fact lack knowledge. There’s one well known cable company that sells a digital 110 ohm cable, terminated in RCAs, which is way off base because SPDIF protocol calls for a 75 ohm cable while a 110 ohm cable is an AES/EBU cable and there you use XLRs .

Yes, that could very well reflect lack of knowledge, but another possibility that wouldn't surprise me is that it was done intentionally, to make that cable sound as different as possible than the competition.  My perception has been that it is not uncommon among audiophiles for "different" to be perceived as "better," at least in the short term, even if it isn't.  And as Steve wrote in this paper regarding jitter, which would presumably be the main consequence of this kind of impedance mismatch:

Another interesting thing about audibility of jitter is it's ability to mask other sibilance in a system. Sometimes, when the jitter is reduced in a system, other component sibilance is now obvious and even more objectionable than the original jitter was. Removing the jitter is the right thing to do however, and then replace the objectionable component.  The end result will be much more enjoyable.

Jitter can even be euphonic in nature if it has the right frequency content....  It is fairly easy to become convinced that reducing jitter is not necessarily a positive step, however this is definitely going down the garden path and will ultimately limit your pursuit of audio nirvana.

  Best regards,
-- Al
 

I remember the original threads and article claiming this and had bounced it off a very sharp engineer friend of mine at the time and several others. In short, I don’t believe it is true; there are more than a few high-quality SPDIF and AES cables on the market which test out perfectly at 1.0 meter.

As is explained in Steve’s S/PDIF article to which Zavato provided a link, what length will be optimal is dependent on the risetimes and falltimes of the signal provided by the component which drives the cable. (Risetimes and falltimes referring essentially to the amount of time required for the signal to transition between its lower and higher voltage states, and vice versa). Those parameters are rarely if ever indicated in the specifications of digital audio components.

Several other factors are also involved, including the happenstance of how well the impedances of the cable and the two interconnected components match, the propagation velocity of the particular cable, the susceptibility of the particular components to ground loop issues, the jitter susceptibility of the particular DAC, etc.

So there is obviously some unpredictability that is involved. But as someone who has also on occasion been alleged to be a sharp engineer :-) I would consider Steve’s recommendation of 1.5 meters to at the very least provide the best odds of being optimal.

This assumes, btw, that a very short length, such as say 6 inches, is not practicable. In circumstances where it is practicable, I suspect it is likely to be an even better choice than 1.5 meters.

Regards,
-- Al

Edit: This post was written before seeing Kijanki’s post just above, with which of course I agree.