Computer Audio Asylum: RE: PC + Asynchronous USB DAC Question by John Swenson

Asynchronous USB is not a guarantee that a DAC is immune to all outside influences. What it does is give the possibility of better immunity IF the implementation is done very well. As others have mentioned the quality of that implementation varies greatly from DAC to DAC. Some designers think that since they are async they don't have to work as hard on the design.

Even if the implementation is done very well there is STILL possibilities for outside world to influence what is happening in the DAC. I wrote a detailed post on this a while back (you can track it down if you want the details). Basically it comes down to what I call first order and second order effects. The first order are thing that add gross levels of jitter and noise such as PLLs, frequency sysnthesizers etc. These are the things that async mode is supposed to get rid of. But that is not all there is to it. The second order effects are caused by noise on the groundplane and powersupply system in the DAC. These are still significantly affected by the input signal. When a USB receiver receives a packet the decoders generates a lot of noise on the groundplane, this noise can increase jitter at the DAC chip. Thus the timing of the packet from the computer IS going to affect the jitter at the DAC chip. Thus all kinds of things related to the timing of the packets can affect the jitter at the DAC chip.

A few designers understand this and have taken steps to minimize this, but many DAC designers do not know about this and have not done anything to minimize its affects. To my knowledge nobody has yet come up with a DAC that is fully immune to these affects. Some DACs are very sensitive to it and others are much less sensitive, its going to be very design specific.

An interesting example is the HRT music streamerII, it IS asynchronous USB, but it is still very sensitive to what happens in the computer. On the SAME COMPUTER I can make simple changes to OS scheduling parameters and get a huge difference in the sound. At one end of the scale its dull, lifeless, no depth at all, at the other end of the scale it is wonderfully alive, dynamic, huge depth etc. And all this by just changing a few numbers, NO change in hardware at all.

There are other DACs that sound very good no matter what settings you use, but there is still a difference, its just that at its best its not all that much better than the worst. With everything out there you will be able to get improvements with tweaks to the computer, its just that with some the range is much greater than others.

Another aspect to this is that what optimizes a computer for best results for a USB DAC is not necessarily the best for an S/PDIF DAC or an internal soundcard. So a specific recipe such as from CICS may not be the most optimal for a USB DAC.

Everything I have come across shows that you get best results with a dedicated computer that you can tweak for audio purposes, you will have to make compromises if you want to support other activities.

John S.