Oversampled and Noise-Shaped Pulse-Width Modulator for High-Fidelity Digital Audio Amplifier

The paper addresses the algorithmic and architectural design of a mixed-signal scheme, based on Pulse Width Modulation (PWM), for power audio amplifiers with direct amplification of digital input sources. The digital circuitry is in charge of PCM to PWM conversion and feed forward correction of non-linearity and non-ideality of the modulation scheme and of the switching power output stage. A new digital modulator combining oversampling, noise shaping and cross-point estimation is proposed to reduce the audio signal distortion below a HI-FI target level of -54 dB. The complexity and power cost of the digital circuitry are kept as low as possible to make direct digital amplifiers competitive in the consumer market scenario and to maintain the high power efficiency of the switching approach. The final architecture is implemented by logic synthesis of the digital processing part in FPGA while off-chip components are used for the power output stage. The whole system directly amplifies PCM digital input sources featuring a power efficiency higher than 90% together with a HI-FI signal quality over the entire audio frequency band.