Designing modern USB audio systems

Kendall Castor-Perry, Cypress Semiconductor Corp.
EETimes (10/27/2010 2:39 PM EDT)

USB audio is a ubiquitous interface supported by all but the most ancient personal computer hardware and operating systems. With its robust connection and data rate, one might believe that delivering high quality audio over this interface is simple. However, today's successful USB-based audio products are the result of a lot of chip- and system-level attention to solving the thorny problem of clock recovery.

In essence, the problem is that the final output device that delivers audio to the speakers, headphones or line-out socket needs a 'master clock' to pace the audio conversion cleanly. This master clock needs to have two independent attributes:

1. it must be at exactly the correct multiple of the underlying audio sample rate (so that you never have to lose or duplicate an audio sample through timing failure) and
2. it must have low enough jitter (or, equivalently, phase noise) that the performance of the digital-to-analogue process isn't compromised.

The challenge lies in meeting both these requirements simultaneously.

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