Class D power amplifiers (PA) offer excellent power efficiency when compared to
other classes of PA. The output stage switches at high frequency between fully on
and fully off and can be controlled by a pulse width modulation (PWM) signal. The
audio signal is kept in the digital domain right up until the final PA stage. This avoids
degradation of the audio through analog stages, and also eliminates the additional
bill of materials cost of a high quality DAC.
The XS1-L1 provides an excellent platform for an all digital iPod dock solution. Digital
audio samples are acquired from the iPod across its USB interface and then used to
drive a class D audio PA. A single XS1-L1 device provides all of the processing and
interfacing requirements of the system: implementing the USB host to obtain the
audio samples; performing digital signal processing (DSP) on the samples; driving
the PWM to control the class D PA.
A general description of class D PAs is contained within a separate application
note Class D Audio Power Amplifier Overview. This document describes a specific
implementation of a class D PA integrated with the XMOS iPod Dock Reference
Design.