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


Some have postulated, with Steve Nugent at the forefront, that a digital cable between source and DAC should be 1.5 meters. The articles I've read nominally speak of 75 ohm SPDIF cables. Does the same length reccomendation hold true for a 110 ohm AES/EBU cable? 
Convert?fit=crop&h=128&rotate=exif&w=128zavato

Yes, same for AES/EBU.


Steve N.

Empirical Audio

Thanks, Steve- makes sense
There are very very expensive 1.0 meter digital cables on the market. Either those designers lack the knowledge underpinning the 1.5 meter premise, they designed around it, or it's simply not true. Otherwise it would make sense for the lengths offered to start at 1.5 meters.
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 .

I had had another cable drop shipped to me direct from the manufacturer that simply did not work- no continuity in the center pin.

Another cable - an XLR, from yet another manufacturer, developed an intermittent short. Upon closer inspection, it was clear the workmanship in connecting the XLR was quite substandard.

I’ve experienced RCAs without any real strain relief eventually leading to a broken connection.

I would rather ther consider the opinion of an EE doing digital work vs someone who buys reels of Mogami or whatever cable, puts on some techflex, gets some heat shrink with a company name printed on it, and does in house terminations, because I’ve come to think many cable companies are no more than just that, just repackages. At least Blue Jeans cables is utterly transparent about that. 
So why do companies manufacturer very, very expensive 1.0 meter digital cables? Simple, people buy them. I totally agree they should be at least 1.5 meters as I have been involved in comparing different lengths of the same cable on a high resolution system.

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
 
Can someone direct me where I can learn why the 1.5 meter length is important? 
http://www.positive-feedback.com/Issue14/spdif.htm
I got a 50 feet AES cable run (using 4 out of 8 channels), it’s freaking perfection, it's the best audio interface by miles until wireless Dante becomes more available.

Pro audio equipment rocks. (Only downside is I’m still selling off my high end audiophile equipments to this day...)
Thank you and indeed it seems reasonable as a minimum length.

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.
It depends on the transition time.  1m might be perfectly fine with transports that has 15ns transition times.  In addition, reflections appear only on impedance boundaries.  With a perfect match (rare) there will be no reflections.  Very short cables, under a foot, will also work fine.  Rule of >1.5m is just for general case of typical transport.

Jitter is basically an added noise. Music free of noise might sound clinical and sterile at first. One person even posted, after listening to jitter supressing DAC, that he preferred all instruments "together" (sound blob) instead of hearing individual instruments.  Also, music with added noise can sound more dynamic - like distorted guitar vs clean Jazz guitar.  I believe that jitter destroys everything, but we tolerate it since we got used to it.  
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.
Speaking of digital, several years ago a buddy came over to hear my system , listened a while and got up and pulled the power-cord from my internet modem/router . Sound went from very good to live music !
Also put a heavy blanket over my TV which was not near the system. Not as big a deal but a slight difference .
I also read a thread here a bunch of years ago, where Charles Hansen of Ayre, recommended unplugging TV and DVD player before listening to music...makes a difference in my system 
Thanks for your post jl35 . Nice to know I'm not the only one .
 In my system the difference is NOT subtle ! Might be a function of living
 in a condo where the whole place is on same wires, but lot of people are in that situation ,
I'm in a house, in a small town, but for "really serious listening" I unplug TV, DVD and internet, and the difference is also not subtle...