USB AUDIO - SYNCHRONOUS/ASYNCHRONOUS DATA TRANSFER
There are a couple of methods for sending audiodata over an usb cable. Here's an explanation of the principles behind them and the advantages of one over another.
Synchronous USB connections use a one way digital connection for music replay and are considered the worst type of connection for audio purposes
Adaptive mode is a little smarter. It asesses the amount of data in the frame and adjusts that dac's clock-timing to it.
Asynchronous mode is technically most advanced in that it has a feedback loop so that the amount of data in the frame can be controlled.
Conventional USB connections use a one way digital connection for music replay and use the computer's bus frame rate as source for the clock which is less stable than a fixed one. A computer sadly cannot maintain perfect timing of the data sent via USB. For adaptive mode, the receiving chip adapts to this drifting signal by re-adjusting its own frequency every milisecond to match the incoming frequency. This is already considered better. The downside of both methods is that they tend to cause high levels of jitter. On top of this, the computer's clock is not as stable (powerline fluctuations/RF pollution) as you would like for highend audio purposes.
Asynchronous USB (not to be confused with asynchronous samplerate conversion) uses a clock housed near the dac (usually in the external dac's casing) and allows it to drive the converter directly, thereby not relying on the instable computer's clock. Well, someone who is technically more adept at this matter than me informed me that this is still not entirely true as the interface or dac is still somewhat dependent upon the stability of the PC's bus clock*. It is called asychronous because the dac's master clock isn't synchronized directly to any clocks within the computer. Instead, the dac is controlled by a (potentially high-precision) fixed-frequency clock. This clock controls the datastream from the computer to a buffer near the DA converter.
The thing is: Asynchronous USB data transfer doesn't guarantee perfect sound**. Well, technically speaking, it does. But bit-perfect transfer doesn't guarantee enjoyable sound. When using a PC as source, having very precise, very correct sound can be too much of a good thing if the source computer already sounds thin or when the rest of your system tends to the overanalytical. Having all the bits in place is a good starting point, but many tests I've done have shown that extremely precise dacs can be very unforgiving and too-controlled sounding, while other dacs can be extremely musical. But what constitutes musical? This is something that everyone has to decide for him or herself but my point is that what you find sounding good doesn't neccesarily have to measure well. This sounds controversial, I know. Of course it is best to use accurate applications on the computer and also try to keep the data transfer as integer as possible. But this would lead to the best sound only if everything else in the system is carefully matched. For example: on my full size PC, I like to use kernel streaming. It provides the best soundquality in my system. But when I use the Macbook, I prefer iTunes without additions such as Amarra, simply because the Macbook, for whatever reason, sounds very grey, flat, undimensional and technical. In my opinion, using bit-perfect transfer (for example by means of Amarra) pushes this already lean and mean sound over the edge.
Compare it with the analog, physical world. If you were to assemble a system based solely on specs, chances are that it will end up sounding very mediocre. Use your ears instead, swap some cables, try a different rack, add component feet etcetera and you will effectively have coloured the system to match your taste or compensate for its weaknesses. In the computer world it is not much different.
The bottom line? Use your ears, not only your mind, and listen to the computer/transport/interface/dac of your choice and don't let the technical mumbo jumbo get too much in the way!
*Well, someone who is technically more adept at this matter than me informed me that this is still not entirely true as the interface or dac is still somewhat dependent upon the stability of the PC's bus clock.
Gordon: This is totally false! We don't care about the PC's bus clock at all.
**Asynchronous USB data transfer doesn't guarantee perfect sound.
Gordon: Many of the less expensive units do not use a fixed oscillator and instead use a frequency synthesizer and therefore have worse jitter than a dedicated fixed oscillator. Many companies don't even know how to power fixed oscillators. If you go to say Crystek and ask for a really good oscillator and then power it with a 3 terminal regulator the results will be less than staggering. For high end you need a discrete regulator with ultra low noise to establish a really good low jitter Master Clock.