Asynchronous DAC's explained (possibly).





For those of you using servers to listen to your music, you may well of heard of the term Asynchronous USB output and wondered what it was. Hopefully this will offer a simple explanation.

Computers, servers and the like can have Digital Coaxial, or Optical Digital outputs, but nowadays, the obvious interface to use, is USB. Although USB 1 is a little outdated for full lossless audio transfer, USB 2 and upwards is fine. Indeed, with a 24-Bit 192kHz stereo signal taking around 9.2Mbits per second of bandwidth, and USB 2 offering as much as 480Mbits per second, it’s obvious that there is ample room to spare. However, this is not the whole story in relation to getting the best audio signal down your USB lead.

USB supports as many as three different modes of data transmission. These are - Adaptive, Synchronous and Asynchronous. Of these three modes, the only that is completely compatible with high grade audio transmission is Asynchronous.

Many of you will be aware of another term when it comes to digital transmissions, and this is Jitter. Imagine that you look at digital signal, something like 1110101000100111010001010100010100100111100101010010101010. This signal has got to travel from your computer to the DAC (or indeed from any digital source like a CD transport etc), and this is where many people who are computer experts like to argue until they’re blue in the face that, a digital signal shouldn’t be effected by the cable it runs through, or by the device it come from. Digital is digital, ones and zeros are ones and zeros. If they get there, they get there. This is not true and this is because of (amongst other things) Jitter.

Jitter is the timing of the one’s and zeros. Imagine that instead of them arriving in a constant flow like this: - 1110101000100111010001010100010100100111100101010010101010

Instead, they more likely arrive like this: -
11 101 01000 100 1 11 010 0010 1010 001 0100100111 1001 0 1 0100 1010 1 010

So that’s the same lot of digits, but there are spaces in-between. The timing is in-correct caused by this Jitter. The job of the clock in a CD transport is to try and stop this happening. The job of your DAC is to make sense of this havoc and correct the Jitter (timing issues). The analogy is like a car suspension (The DAC), it’s very much easier for it to make the ride nice and comfy if the road you’re traveling on (The signal transportation) is smooth. If you drive on a cart track, the suspension will do its best, but it won’t be as good as it could have been. Computers are notoriously bad at this and many computers or audio servers (even expensive ones!) have lots of Jitter.

So, this is where Asynchronous comes in. Instead of the DAC having the information pushed to it by the computer or server, it actually pulls it. This allows the DAC to control the data flow which allows for very high precision and ultimate control of Jitter, or the lack there of.

Now this is a rather rudimentary explanation of the process, and doubtless there are other web pages out there with a much larger array of facts and figures, but the true audio person need only worry about the outcome and not the mechanics of how it was come by. And to this ilk, using your computer or server through an Asynchronous connection allows for considerably higher audio quality. Job done.