A USB audio link transports packets of sample data, but provides no associated sample clock. To reproduce samples at the right points in time, the two ends of the link must synchronize their sample rates without exchanging clocks. The paper reviews data transfer modes used in USB audio. In many of these modes an accurate master clock, on whose 'cleanliness' the audio SNR depends, must be generated at the receive end. The flexibility of recent programmable systems-on-chip (PSoC) delivers an effective clocking scheme usable for all modes and sample rates. The frequency synthesizer at its core is described in detail.
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