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I wrote yesterday I enjoyed the improvements wrought by the Jitterbug when playing music over USB. And I promised today I would tell you of another experience I had, this time with the Uptone Regen. But that will have to wait. I have received inordinate amounts of mail on yesterday's post: accusations of favoritism, using this forum improperly, fears DirectStream isn't living up to ts promise of low jitter performance. And all because I mentioned I liked what the Jitterbug did. Funny, but when I spoke about my favorite USB cable, JCAT, not one peep was heard. But when I claim an accessory does something–howls of indignancy permeate. Interesting. I thought it instructive, then, to first cover a little technical ground on why some of these USB devices make a difference. They are not jitter reducers, per se. Instead, they reduce other types of problems that, themselves, can cause jitter. For answers I turn to our resident guru, Ted Smith, designer of DirectStream. "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 5V line in the DS 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.

The DS takes great care to filter the jitter better than most DACs out there, but even a perfect DAC that introduces no errors of it's own, is not affected by any errors (including jitter) on any of its inputs and radiates nothing outside of it's 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, a product like Jitterbug is a line conditioner for the Vbus (5 volts) and a filter for electrical noises on the datastream. Not all that complicated, it does work - and it works not as a reclocker, lowering jitter, but as a filter removing noise. And, as Ted suggests, higher noise levels, both radiated and transmitted, result in compromised performance in any DAC.

Tomorrow I will report on the Uptone Regen USB fixer upper.

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Paul McGowan

Founder & CEO

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