Computer Audio Reviews

If an audiophile visiting an audio show in 1991 were to have been transported two decades into the future, at first he would not be aware of any difference: A two-channel system would be playing in a hotel room. But on closer inspection, he would notice that the CD player, the ubiquitous source 20 years ago, would be conspicuous by its absence. Yes, there might be a turntable—"Good to see that people are playing LPs in the future," he would think—but why is there a PC in the room?

I don't remember where I was when the Berlin Wall came down, and I already don't remember what I was doing when Liz Taylor died. (I suppose I was busy not thinking about Liz Taylor.) But I do remember when USB-based computer audio became a serious medium: That was when Gordon Rankin, of Wavelength Audio, introduced asynchronous data streaming, with his proprietary Streamlength software. After that, things picked up speed.

A computer is not optimized for the uninterrupted streaming of audio data. It has rapidly become established wisdom, therefore, that the optimal means of extracting audio data from a computer's USB port is to operate that port in what is called "asynchronous isochronous" mode. This lets the receiving device, such as a digital-to-analog converter (DAC), control the flow of data from the PC. In theory, asynchronous USB operation (not to be confused with the asynchronous sample-rate conversion used in some DACs) reduces jitter to unmeasurable levels, depending on the accuracy of the receiver's fixed-frequency oscillator, which is used to clock the data to the DAC. By contrast, in the alternative and almost ubiquitous USB operating mode, called "adaptive isochronous," while the sample rate of the output data, averaged over a longish period, will indeed be the specified 44.1 or 48kHz, there will be short-term fluctuations, or jitter, due to the oscillator having to change its frequency every millisecond to match the uncertain rate of data flow from the PC.