Digital Cables compared – AES/EBU, Coax,Toslink and ST Glass
Do digital cables really make a difference in sound? And if so, what are the differences?
Sceptics and die-hard technicians claim that differences in sound are impossible with digital cables, as long as the receiving device reclocks the signal. Well, I have experimented with many different DA converters that have reclocking, resampling, extra RAM or a combination of techniques and I haven’t come across one DAC that is completely independent of source or cable. You can always tell the difference between one cable and another. Even with top-notch equipment.
I haven’t based this on only a few digital components but have tested many: Mark Levinson no.36, no.360S and 390S, Wadia 12, 25 ,27ix GNS, 861, dCS Delius/Purcell, Krell KPS20i, Weiss DAC2, Ayre QB9, Arcam rDAC, DAD AX24, Meridian, Audiomeca, Audio Aero and many more. All these DACs and CD players showed comparable differences between digital cables used. I have also tested many digital cables, too many even to mention here. So please, rest assured that I have thoroughly researched this phenomenon!
Firstly, I have to mention that normal people (not audiophiles) would probably categorize the differences between cables I am talking about as nuances. I do feel however that these nuances can mean a lot in a well-balanced highend setup. Even if you’re not looking to squeeze the last drop of quality from your system it may still be interesting to note that differences do in fact exist even though some flat-earth-technicians may claim that it is impossible. I cannot explain why but that these differences exist is very clear. This has been shown in many listening tests in my own setup as well as in other people’s setup.
WHAT ARE THE AUDIBLE DIFFERENCES?
The most prominent differences are known as PRAT. PRAT stands for pace, rhythm and timing. In layman’s terms this means temporal changes rather than tonal. I have also noted tonal differences between digital cables but for these differences you need to audition two very different cables. For example an Apogee Wyde Eye and Belden RG59. In that case you’re comparing solid core in a tight jacket to litze in a very flexible jacket. If you compare cables that are more alike, as is usually the case, the cables more or less resemble each other in tonality. And then it is easy to say that the cables sound alike. But listen more closely and you’ll find that one cable can sound slower and more rounded than another that is sounding faster and more dynamic.
The more you listen for differences in cables the better you learn to listen beyond the generic “sound” of a cable. There will appear to be more subtle changes such as fluidity in the treble, fullness of bass or lack thereof, overall warmth, decay of subtle sounds and tightness of attack in transients. The more careful you listen, the more you’ll hear.
three of these cables are of the RG-59 type, and so should adhere to strict specs, yet they all sound different. The fourth cable (with the tinned shielding) is a special double shielded variant for higher spec video use over long distances or in signal unfriendly environments.
This may sound incredible, and I know that many technicians would declare me crazy but I’ve listened to every RG59 coaxial cable I could lay hands on and I can tell you that there are large differences between the various brands. Even going from one Belden type to the next, there are big differences. The differences in sound are not usually timbral in nature and hardly concern detailing but can best be described as temporal. One cable can be very fast and dynamic sounding while another can sound slow and dynamically compressed.
WHAT ABOUT AES/EBU?
AES/EBU is a professional standard and although it can run over a coax cable, and the AES flag can be either on or off, it really is meant to run over a balanced cable because the signal itself is also balanced. This is because it is meant for applications in recording studios and post production facilities where there can be enormous lengths of cable between any two components. 100 meters is not uncommon. And this is where the balanced signal comes into play. Just like with balanced analog signals, in the digital form there is a plus and minus side which are in opposite phase tpo each other. Any disturbance picked up by the cable is likely to be present in both positive and negative conductors. A phase comparator in the component at the receiving end subtracts both signals and consequently gets rid of the disturbance. This is possible because the disturbance will be in phase on both conductors while the music signals are in counterphase.
For this to work, a DAC needs to present the digital data to the output in balanced form. A lot of dacs can do this and some actually make use of it but in a lot of cases the signal is unbalanced by nature and made balanced by an extra outputstage. You can imagine that any extra processing going on can have a negative effect on the sound. But whether it’s this processing or merely the fact that a comparison is made, which in itself is also a form of processing, there’s something to AES/EBU that makes it sound different from SPDIF. And that’s not only the cable. I’ve tested many, and have also used AES inputs and outputs with coax cable in balanced and unbalanced form. In all cases the AES input sounded different from the SPDIF coax input.
WHAT ARE THE DIFFERENCES IN SOUND BETWEEN S/PDIF AND AES/EBU?
Under most circumstances where I tested this, I find AES/EBU to sound overly controlled or overly clean. And perhaps because of its very tidy sound, AES/EBU often fails to excite me. The bass is clean and articulated but via SPDIF it is so much bigger and more physical. Of course opinions vary and there are people that like AES/EBU better than coaxial.
The good thing is that the differences in cables are a little less clear with AES/EBU than with SPDIF and even very standard cables can do very well.
As always, opinions vary, as do setups. Who knows, AES/EBU may well sound very good in your system. Just be aware that it needn’t be the better standard. Please just experiment and decide for yourself!
Update 2017: with the Aurender N10 connected to a Wadia 521 and many other DACs I now find that coax and AES/EBU are much closer to each other in sound than I have heard before.
ST GLASS VERSUS TOSLINK
This is (in the audiophile world) an old standard by now. It comes from the telecom world and is still used there extensively. Unfortunately not many manufacturers of audio equipment today use this standard. An exception is M2Tech with its HiFace EVO USB interface. This little unit actually allowed me to compare a 25 meter long Belden RG59 coaxial cable to a same length ST glass cable directly into the Levinson no.360S DAC. ST Glass transmission is optically but don’t confuse it with Toslink. First there is a laser, not a LED in the transmitter, second, the conductor itself is made from very thin glass, not plastic. Lastly, the wavelength is very much higher and is wholly appropriate, even oversized, for consumer digital audio whereas Toslink is actually underspecified. Toslink connections tend to soften transients and lessen dynamic behaviour compared to an electrical connection. This isn’t the case with ST glass. Instead, musical portrayal is very much as dynamic and speedy as with RG59 but devoid of the added sharpness and “zing” of the latter cable. At the same time ST Glass is more detailed and refined than RG59. Do note that in certain system setups, RG59 might sound subjectively better just because of its more aggressive character. ST Glass is so much so devoid of artificial sharpness that it may appear to “rock” less. In my setup (Jeff Rowland and Magnepan speakers) I prefer ST glass, especially since I need to use a very long 25 meter cable. Whether or not ST glass is better for you too depends on your setup and your taste. Toslink however is always the lesser choice, unless you have a setup that is perhaps somewhat edgy and overagressive, in which case Toslink might soften things up a bit. But this will be at the expense of some dynamics and speed. The other good thing about ST glass is that it is designed for long distances. And by that I mean hundreds of meters. I have yet to do the actual comparison (and will do of course) but in theory a length of 25 meters sounds the same a a length of 1 meter. From what I gather in my 25 meter cable comparison, I have no reason to doubt this. The ST Glass cable I tested was a standard no-name cable that cost less than 40 usdollars for 25 meters. That’s even cheaper than Belden RG59!
BNC to cinch adapters are cheap and handy and seem to do their job just fine. However, when you start comparing, it becomes obvious that these adapters do add some edge to the sound and depending on your system’s balance and your own sensitivity to this, are best avoided. For example when using equipment having both BNC and cinch inputs and outputs, Belden RG59 with crimped on BNC connectors, has a nicely neutral, yet dynamic and lively character with excellent focus, drive and attack. When comparing to the same cable with cinch connectors and BNC adapters, you notice that the resultant sound is more “impressive”, with more presence in the treble and seemingly more attack. But you quickly realise that this also makes the cable sound a bit agressive and unnatural. In overly smooth setups however this can give some nice extra “bite” to the sound.
BNC VERSUS COAX
Some components offer both Cinch and BNC connections for SPDIF. In the cases where I have compared these outputs, usually BNC sounds cleaner with sharper focus. This makes sense because it is technically also the best connection. However, depending on circumstances you may also prefer the cinch outputs because these tend to sound beefier in the bass and arguably a little more colourful.
USB AND FIREWIRE
Here’s something some readers will probably frown upon… there were several occasions where I noticed that two USB cables sounded different from each other and the same happened with Firewire. I’ve heard it with various DACs, for example with the Weiss DAC2 and with the Mytek 192DSD. A thicker, very supple cable will usually also sound more supple, and mostly less articulate, but this is by no means a rule.
I seem to see parallels across more tests I did, in which USB seems to be on one side of the camp and firewire in the other. For example, with the Mytek 192DSD I compared USB to Firewire and noticed a difference so large that I think it would be audible on most systems. At least, it was on my Magnepan MG3.6R Magnetostatics with their pure ribbon tweeter. Anyway, the difference isn’t in the detail department, it’s more of an emotional response. While Firewire sounded more precise and articulate, USB (2.0) sounded more fluid and polished. I found Firewire to sound a little dry compared to USB but you could also say that USB is less accurate.
Update 2017: this matter is also very much dependent one the implementation of the USB circuit at sender and receiver side, and so is very relative.