Apart from all the marketing hype, asynchronous USB
transfer mode simply
means that a "downstream" audio device, the Audiophilleo in this case, controls when the computer sends data through its USB interface
and how much at a time. The Audiophilleo becomes the bus "master", and no longer
has to rely upon noisy clocks generated by the computer or its USB interface logic.
Products using synchronous or adaptive USB
transfer are inherently "slaved" to the computer; clocking information has to be
extracted from USB packet
and frame timing signals, which are seldom accurate enough for high-end audio
Advantages of using asynchronous USB
(together with the Audiophilleo's regenerative
power supply) include:
output quality is now completely independent of USB interface jitter.
The particular computer used as a media server, or type of USB cable will no longer effect sound quality in a significant way. Be sure to try your
cables, but differences between them and those supplied with each Audiophilleo are likely to be small.
Fixed Frequency Clocks can be used
because the master clock circuitry can now be in the audio device. Much
results, because it's not necessary to employ phase locked loops or similar
approaches for syncing with "upstream" clocks.
The disadvantages of using computer or USB clocks include:
- Dependency on a particular computer or USB
cable to get the best sound. As we all know, in the past, some computers and USB cables
have definitely sounded better than others.
- Variable-frequency clocks must be usedto "track" the computer or USB clocks.
Such clocks are inherently more jittery than fixed-frequency oscillators. The
widespread use, until recently, of adaptive or synchronous USB resulted in
rather poor jitter performance, to the end that
manufacturers were reluctant to publish thir data.
- Many DACS have built-in USB functionality,
but if they don't use asynchronous mode transfers, the sound quality is
likely to be less, compared to that of their
inputs. Connecting the Audiophilleo directly to any DAC will deliver the
best sound possible.