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Roughly put, typical USB data transfer works as follows. Audio data is read from hard disk and stored in a temporary memory buffer. This gets tapped every millisecond to dispatch a data packet or so-called frame to the USB output whence the journey of the data to the DAC’s USB receiver commences. The rate of full-speed USB transfer thus operates at 1kHz and the control clock is embedded in the sender.

This clock isn’t immune to jitter. The send rhythm can fluctuate. This doesn’t yet talk of whether the data packets are properly filled, partially or completely empty or overflowing. Frames are sent regardless. Proper fill depends on whether the CPU is busy elsewhere with computer processes or programs that are not related to audio to pay any attention to the latter. Needless to say the USB receiver on the other end has its own memory buffer. This insures it neither over- nor under runs. This depends on synchronization with the PC. This protocol is guarded by the isochrone mode of which there are three flavours – synchronous, adaptive and asynchronous. The first hardly figures in modern DACs any longer. Here the converter clock is slaved to the 1kHz frame timing of the USB data stream with a max sampling frequency of 48kHz and word length of 16-bit. This scheme is considered highly susceptible to jitter.

Adaptive meanwhile is very common. Here the DAC attempts a form of frame timing recreation via a phase-lock loop which locks to an averaged data density condition per time period rather than directly to the actual frame speed. This recreation of the PC clock is dependent on the latter of course and that was never designed for maximum precision. In adaptive mode the PC thus remains master, the D/A converter slave.

In asynchronous transfer mode there is no clock recreation via PLL but an independent master clock gets embedded in the DAC which controls the timing via a feedback loop over the USB audio controller. This reverses the master/slave relationship for a claimed new independence from USB bus jitter. The feedback loop insures that the dispatched data packets arrive properly stuffed, i.e. neither empty nor overflowing. It sure sounds like perfection. On paper.

Async USB alone doesn’t a good DAC make however. As always implementation is king. Even Wavelength Audio’s Gordon Rankin who first popularised this transfer protocol for home audio admits that much depends on the jitter performance of the master clock. An asynchronous converter with run-of-the-mill clock can exhibit higher jitter than an adaptive DAC with superior clock. Again, North Star Design runs the Essensio Plus in async mode and galvanically isolated to prevent PC ‘electro smog’ from propagating into their circuitry. They also run with two master clocks for the 44.1/48kHz families of sampling frequencies respectively. Before our nerves now get jittery from tech talk, it's time to let the ears decide how it all translates.