Q: How does USB sound?
A: I regret having to report that, at this stage in its development, USB does not rise to the level of an audiophile-quality interface. With carefully chosen equipment and accessories, USB can sound perfectly pleasant. At the other end of the quality spectrum, it is nearly unlistenable. To one degree or another, though, the interface consistently exhibits a distinct sonic signature characterized by three negative attributes.
First, USB sounds synthetic. Instruments are cardboard-flat, lack substance, and have a distinct “plastic” quality. In this respect, the interface is remarkably reminiscent of TosLink. USB’s second primary attribute is sloppy timing. Accordingly, would-be steady beats are only approximated, and rhythmic propulsion never materializes, all of which takes a major toll on musical engagement. Finally, USB tonality is pale and washed out, robbing instruments of both richness and distinctiveness. Under optimal circumstances, USB’s undernourished instruments can sound blandly agreeable; but they are never convincing. Strings suffer disproportionately; through this interface they are invariably shrill.
On track after track, device after device, these particular characteristics, though varying in degree of severity, proved constant. Nor is USB strong in other areas—imaging, bottom and top-end extension, dynamics—that could help compensate for its failings. Upon first hearing USB, you may well find its sound encouragingly acceptable. But listen longer. Over time, the interface’s artificiality, rhythmic imprecision, and lack of sonic substance become increasingly evident and unsatisfying.
Q: How does USB compare to S/PDIF?
A: There is no comparison. USB sounds much worse. All four of the DACs I tested support both USB and S/PDIF, making differentiating the interfaces a cinch. For each DAC, I compared the S/PDIF output of a good CD transport—as well as that of a budget CD player—to the USB output of a laptop, with both sources sending the same bits at the same rate (44.1/16) to the same DAC. In theory, if USB was at least as good an interface as coax S/PDIF, the PC would sound better thanks to its hard drive. Yet regardless of which DAC I used, that was never the case. USB was always a pale shadow of S/PDIF, due to the former’s tell-tale synthetic glaze, washed-out timbres, lackadaisical rhythms, and screechy strings. In contrast, S/PDIF—even when anchored by modest gear—is quite capable of a measured pace, faithful tonality, and fleshed-out instruments. (For a more detailed comparison between USB and S/PDIF, with examples, see my review of the Bel Canto and the Focusrite format converters by clicking here).
Q: Does the equation change at higher resolutions?
A: No. To answer this question, I used the custom-built PC since it natively supports both USB and S/PDIF at high bit-rates. Opposite the PC sat the Benchmark DAC1 Pre, the only DAC on hand that also supports high-resolution rates over both interfaces. The Benchmark was also a salutary choice because it incorporates the same top-quality USB input module as the Bel Canto USB Link 24/96, thus allowing USB to put its best foot forward. With this setup, I was once again able to play the same bits—this time more of them—from the same PC to the same DAC, varying only the interface. And once again I heard the identical differences between S/PDIF and USB that I had heard at lower resolution levels; that is, high-res USB was inferior to high-res coax S/PDIF, and in precisely the same ways.
Q: Do all USB DACs sound the same when playing USB?