Audio Signal Processing and Coding

5.1 INTRODUCTION

The field of psychoacoustics [Flet40] [Gree61] [Zwis65] [Scha70] [Hell72] [Zwic90] [Zwic91] has made significant progress toward characterizing human auditory perception and particularly the time-frequency analysis capabilities of the inner ear. Although applying perceptual rules to signal coding is not a new idea [Schr79], most current audio coders achieve compression by exploiting the fact that “irrelevant” signal information is not detectable by even a well-trained or sensitive listener. Irrelevant information is identified during signal analysis by incorporating into the coder several psychoacoustic principles, including absolute hearing thresholds, critical band frequency analysis, simultaneous masking, the spread of masking along the basilar membrane, and temporal masking. Combining these psychoacoustic notions with basic properties of signal quantization has also led to the theory of perceptual entropy [John88b], a quantitative estimate of the fundamental limit of transparent audio signal compression.

This chapter reviews psychoacoustic fundamentals and perceptual entropy and then gives as an application example some details of the ISO/MPEG psychoacoustic model 1. Before proceeding, however, it is necessary to define the sound pressure level (SPL), a standard metric that quantifies the intensity of an acoustical stimulus [Zwic90]. Nearly all of the auditory psychophysical phenomena addressed in this book are treated in terms of SPL. The SPL gives the level ...