Designing modern USB audio systems

Fixed-function microcontrollers just can't meet the tough performance requirements on this clock generation process. Their inflexible clock generation systems can't be adjusted to be both exactly correct and low in jitter, and they usually fall back on a crude "add/lose samples" approach. This might be workable for telephony but is completely unacceptable for high quality audio.

Meanwhile, dedicated USB audio interface devices (necessarily another entry on the BOM, in addition to the control microprocessor) can't simultaneously manage the critical bidirectional control protocol traffic that delivers innovative new functionality in these latest media players.

Programmable SoCs can also provide advanced functionality in a cost-effective manner. For example, additional value is provided by some family members of Cypress's PSoC3 family in the form of an embedded audio filtering engine (the Digital Filter Block). This DFB can post-process the recovered USB audio, for instance, for response equalization and crossover filtering. Sufficient performance is available to render additional digital processing devices redundant; at least ten second-order biquad filters can be implemented on each channel of a stereo pair, giving very fine control over frequency response.

The versatility of a highly configurable solution inevitably helps designers to 'mop up' most of the glue logic and analogue housekeeping circuitry. Direct LCD drive lowers display cost, and capacitative button sensing ensures both an elegant industrial design and a contemporary human interface.

An example of the result is shown in figure 2: a single device that can form the heart of a new generation of mobile device audio accessory or consumer audio appliances that deliver all the benefits of fully digital audio and data exchange. The flexibility of the programmable device means that the required functions and interfaces can be integrated on a mix-and match basis.

Eventually fixed function devices will catch up to programmable SoCs. But by adopting a design architecture based around highly configurable SoC architectures, developers can keep at least one step ahead and be able to act when the next "How do we do that?" shock arrives.

About the author:
Kendall Castor-Perry is a Principal Architect at Cypress Semiconductor,doing mixed-signal system analysis and design for the new PSoC platform. Kendall uses decades of experience in analog engineering, filtering and signal processing to capture signals across many domains, extract the information from them and do something useful with it.

Related links:
Signal Chain Basics (Part 36): Digital Audio Interfaces -- Part 2
USB Audio Class Tutorial
USB and audio convergence enabled through analog switches