JitterBugging

by Vance Hiner | October 27, 2015

'll just say it: I've never been a fan of music played over USB. The first time I connected a MacBook Pro to my DAC several years ago, while drinking my morning coffee, my immediate reaction was a classic spit-take, followed by "Jeeves! Get this thing out of here and fetch me a vinyl palette cleanser!" Even after years of chasing down jitter and finally getting a DAC that sounded a bit like real music instead of the usual hi-fi facsimile produced by many CD players, computer audio through USB is, as the kids like to say, not my jam.

Despite the ongoing surge in computer audio's popularity, apparently I'm not entirely alone in my disdain for the de facto computer interface. In fact, when I received AudioQuest's $49 JitterBug USB noise filter, I noticed these lines on the packaging:

". . . computers contribute a considerable amount of RFI and EMI pollution onto the signal paths -- all of which can easily find its way onto your USB cables and into your audio system. This noise and interference has many negative effects. Noise-compromised digital circuitry increases jitter and packet errors, resulting in distortion that causes a comparatively flat and irritating sound. Noise-compromised analog circuitry also damages the sound’s depth, warmth and resolution."

Yup. The key word there is "irritating." It really sums up my past experience with audio delivered via USB. Better cables helped a bit, but I was always physically relieved once I returned to a good old AES/EBU or S/PDIF connection. Even though I'm clearly biased when it comes to this particular transmission method, I'm never comfortable disliking something I don't quite understand. So, when my generous editor offered a chance to try out AudioQuest's attempt to improve USB, I was all in.

My PS Audio DirectStream is the only DAC I've ever heard that manages to equalize the performance of its various digital inputs. In other words, I can listen to the same source material using the same grade of cable via S/PDIF, AES/EBU or HDMI with little to no performance difference. However, the USB connection is another matter. It just doesn't sound as good as the others, even though I've used Shunyata Research's very fine Venom USB cable and Chord's much pricier Signature Tuned Array. At first, I thought what I was hearing might be the result of my bias, but the DirectStream's designer, Ted Smith, says that USB connections are a special challenge to DAC designers. According to Smith:

"USB has the additional complication that it runs the power and ground signals very close to the data signals over a long length and hence dirties up the power and ground noticeably. High-frequency noise from the high-frequency signal on a USB cable is very hard to filter out. Even without a connection of the [USB cable's] 5V line in the DS [DirectStream], we'd still have a radio signal coming from the USB connector that needs to be carefully dealt with in the power supplies and analog circuitry."

Smith goes on to explain that

"The [DirectStream] takes great care to filter the jitter better than most DACs out there, but even a perfect DAC that introduces no errors of its own, and is not affected by any errors (including jitter) on any of its inputs and radiates nothing outside of its case, will add multiple ground loops to the rest of the system with the digital, analog and power connections and further those connections (especially the digital ones) will add radiation that affects the rest of the system. This is especially a problem with USB since there's almost always a computer on the other end which has an almost infinite number of rude noise sources and (with good reason) is almost always plugged into different outlets than the audio system."

In a nutshell, the AudioQuest JitterBug tackles at least some of USB's plethora of challenges by employing two discreet noise-dissipation circuits that, AudioQuest claims, reduce "noise and ringing" present on both data and power lines of USB ports. AudioQuest also says that the JitterBug measurably reduces jitter and packet errors. One could argue that the degree to which such error correction improves sound quality depends upon how well a given DAC corrects such errors itself. While PS Audio's Smith had not yet used the JitterBug when I communicated with him, he told me he would not be surprised if it improved the DirectStream's USB performance.

I'm not an MIT-trained software engineer like Smith, so the JitterBug's performance in my system did surprise the heck out of me. As I streamed music files from my stock 2011 MacBook Pro with one JitterBug connected to the main USB port, the first thing that struck me was how much more relaxed I felt compared to previous computer audio listening sessions.

The warmth, color and energy of Allen Toussaint's Bright Mississippi [Nonesuch 7559799287] always makes me grin and hum along when I listen to the CD through my PerfectWave transport. By contrast, whenever I play that file through my computer, it sounds a bit flat, homogenized and usually devolves into background music. With one JitterBug in place, my grin was back when Touissant began to play. (If the grin test is not part of your evaluation method, feel free to stop reading now.) When I added a second JitterBug to a parallel port on the Mac, I started humming along and the time began to fly by. Why? I would say it's because transients were smoother and the background was blacker. That's the audiophile speaking. The music lover would say that everything just sounded more relaxed and natural. Take your pick. It's worth noting that I had precisely the same experience when I used the JitterBugs during my listening sessions with the much more affordable Marantz HD-DAC1. Consider this: Two JitterBugs working in unison were enough fun to get me thinking about how I might like a souped up Mac Mini or [gulp] even a NAS.

It would be going too far to say that the AudioQuest JitterBugs completely remove the USB of its various shortcomings. I still prefer AES/EBU, S/PDIF and HDMI. The music is simply less glassy and has a bit more weight when I use those other connections in my system. That caused me to wonder: what else is going on that might be degrading USB? Then I saw an earlier post by Smith on PS Audio's user forums. He wrote,

"Jitter is actually a minor effect in cables compared to the obvious issues of radiation (especially with the frequencies involved in digital as compared to those in audio) and ground loops. Radiation can cause problems anywhere in the system, e.g., it is folded down into the audio band by any non-linearities in the signal path. Ground loops add circular currents in a system that depend directly on the area of the loop and the flux thru the loop. This current is not insignificant and causes haze, etc. even if you aren't hearing any 60Hz hum."

Hmmm. Ground loops. Smith mentioned that before. So, I fired off an e-mail to Stephen Mejias at AudioQuest, and he put me in touch with Steve Silberman, who is Vice President of Development at AudioQuest and a major player in the JitterBug project. When I asked him about the issue of noise coming from ground loops, he replied,

". . . removing noise and preventing ground loops is a good thing and should invariably improve the sound. Removing noise from the ground and VBUS not only helps in situations where the DAC, or the DAC’s USB module, requires power from the computer, but it helps with the data lines as well. If there’s noise on the ground and the VBUS, that can couple with the differential data lines. Everything affects everything."

When I asked whether the JitterBug removes noise present on the ground, there was a long silence. Very long. Seems I'd taken this little fishing expedition a bit too far. AudioQuest is not comfortable revealing exactly how they make the JitterBugs actually do the jitterbug. And that's fair. If I had a runaway hit on my hands, I wouldn't be eager to tell everyone how to duplicate it either. But you can't blame a gumshoe for trying. So, I'll just keep the JitterBugs plugged into my MacBook and wait for the next surprise from Silberman and his fellow wizards.

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