High Resolution

Sigma-delta modulation (SDM) and pulse-width modulation (PWM) are compared as a means of structuring power digital-to-analogue converters (PDAC) designed specifically for wide-band audio, low power loss and direct loudspeaker drive. Recent innovations in SDM coding and output-stage topologies using pulse shaping techniques are discussed with emphasis on achieving stable and low distortion operation especially under high-level signal excitation having a modulation index circa 0.7 necessary to process peak transients. A simplified variant of predictive SDM with step back is introduced that offers both low latency and probability of instability and structures for both analogue and digital input data reviewed. 0 Introduction There is a growing awareness of the role for high-efficiency topologies in power amplifier applications and especially their strategic significance in the application to a wide range of audio products [1,2]. This is already evident in the home-theatre markets, where ever smaller size yet higher performance products are emerging. There is also the opportunity to lower overall power dissipation which when mapped into volume production is a most critical issue in terms of environmental factors. Key advantages stem from reduced size and heat loss; however there are also more fundamental philosophical reasons for adopting digital technology in the amplification process. Switching amplifiers configured specifically for use with digital signals take on the mantle of a power digital-to-analogue converter (PDAC). As such there is reduced analogue processing as the digital signals are, in a figurative sense, brought into closer proximity with the loudspeaker. Consequently, there is opportunity to maintain better signal integrity and to achieve a more transparent overall performance, commensurate with appropriate design and physical implementation. The reduction in analogue-related artifacts such as dynamic modulation of the closed loop transfer function through device non linearity including active device transconductance and internal capacitance modulation, implies less amplifier dependent signal