| This work addresses the problem of designing and implementing a digital audio amplifier that exhibits both high fidelity and high efficiency. The traditional analog audio amplifier can be very linear but suffers from poor energy efficiency. High efficiency is crucial to minimize power supply and heat sink size, and to maximize battery life in portable applications. Efficient amplification necessarily involves switching power stages, which in turn requires discrete-valued modulation. Given the digital nature of such modulation, and the present tendency towards a higher penetration of digital audio sources, it makes sense to pursue what can be called an all-digital audio amplifier.; The approach presented in these pages processes the audio information digitally and generates a two-level pulse-width modulation (PWM) signal that is amplified by a switching power stage and finally low-pass filtered before it is sent to a speaker. The purpose of the digital signal processing (DSP) portion of the system is to transform the original audio information into a feasible, low-level PWM signal, with minimum possible degradation. The digital audio information is first upsampled, then converted from uniform to natural sampling to minimize distortion, and finally noise shaped to reduce the time resolution needed by the PWM edges without compromising the signal-to-noise (SNR) ratio. A PWM generator converts samples into pulses, and a switching stage creates a high-power replica of the PWM signal. Switching minimizes energy losses and introduces minimum extra degradation. The output low-pass filter extracts the audio information from the PWM signal and delivers it to the speaker. Each of the functional blocks required by this approach has been justified, implemented, and evaluated. |
