원문주소는
http://centrance.com/products/dacport/

아래 본문 주소는 (위 원문을 정리한)
http://www.head-fi.org/t/493152/low-jitter-usb-dan-lavry-michael-goodman-adaptive-asynchronous

비동기식이 jitter감소 등에 더 우월하다는 주장을 반박하는 내용입니다.
아주 간단히 요약하면

동기식이든 비동기식이든 (즉, clock master가 DAC이든 PC이든) PC로부터 나오는 불규칙성(irregularity)가 생겨서 jitter가 usb로 흘러가는 것은 피할 수 없으며,
역시 jitter-free solution은 없다는 것입니다.

그보다는 D=>A 전환이 이루어지는 시점에 jitter가 clean up 되기만 하는 되는데
이것은 어느 clocking scheme (즉, 비동기식/동기식)의 우월이 있을 수 없고
다만 DAC에 도달하기 직전에 비트를 어떻게 재조럽힐지에 대해서 잘 알기만 하면 되는 문제라는 것입니다.

Jitter Management - Debunking the Asynchronous Myth

Some manufacturers may lead you to believe that Asynchronous USB transfers are superior to Adaptive USB transfers and that therefore you must believe in the asynchronous solution. This no more true than saying that you "must" hold the fork in your left hand. In fact, if you know what you are doing, you will feed yourself with either hand. The issue is really about good engineering practices.


Background: There are two popular ways of sending digital data from the computer to a peripheral device. The argument comes down to jitter management and goes as follows: In Asynchronous mode the device is the clock "master", meaning that the device generates the clock and the computer follows it as a "slave". In Adaptive mode, the computer is the clock "master" and the device follows it as a "slave". The truth of the matter is that either method works fine, if correct design principles are followed. Jitter is the result of Irregularity of the data flow. If the data flow is irregular, then jitter is high, if the data flow is regular (well-controlled), then jitter is low.



Tricky Detail: Here is the tricky detail that often gets omitted in discussion: No matter which side is the source of the clock (Computer or DAC), the two devices are still joined together by the USB cable. The digital data on that USB cable is always irregular. It is irregular simply because the computer is involved. Computers do many things at once and end up sending data over USB in imprecise intervals, no matter who is the clock master on the bus. This irregularity causes jitter. So, since the computer is involved in the transmission there simply cannot be jitter-free traffic on the USB bus. Claiming to have a jitter-free solution is just like claiming to have a dust-free house. Irregularity always creeps in and needs to be actively managed. How well it is managed is the question.



And the Argument Breaks Down: Here is where the Asynchronous vs. Adaptive argument breaks down: In either of the two clocking schemes, jitter is present during the USB transmission. It's inevitable and also ok, if it is properly cleaned up prior to the D/A conversion, where it matters most. Neither clocking scheme is superior during transmission. Both schemes are capable of performing very well if you know how to reassemble the bits prior to Digital to Analog conversion. It doesn't matter that audio samples arrive from the computer in imprecise chunks of data, as long as they are straightened out before the DAC.

The most important part is to make sure that samples arriving at the DAC are clocked accurately.

DACport employs JitterGuard™, a proprietary two-stage clock management system that does just that - cleans up the jitter on the USB bus so that samples are virtually jitter-free at the D/A conversion point. The result is a natural responce, crisp definition, extra resolution and wide soundstage.