Chapter 9
Practical DSP Applications: Audio Coding and Audio Effects
Audio coding exploits unique features of audio signals to compress audio data for storage or transmission. Today, digital audio coding techniques are widely used in consumer electronics such as portable audio players. This chapter introduces basic concepts of audio coder and decoder (codec) based on moving pictures experts group (MPEG) layer 3 (MP3) and implements a more recent, license-and-royalty-free Ogg Vorbis decoder using the BF533/BF537 processors. In addition, two audio effects and their implementations are presented and used for experiments.
9.1 OVERVIEW OF AUDIO COMPRESSION
The overall structure of audio codec is illustrated in Figure 9.1. An audio codec is an algorithm or a device that compresses the original digital audio signal to a lower bit rate and decompresses the coded data (bit stream) to produce a perceptually similar version of the original audio signal. The goals of audio coding are to minimize the bit rate required for representing the signal, to maximize the perceived audio quality, and to reduce system cost in terms of memory and computational requirements of codec. In general, bit rate, quality, and cost are conflicting issues, and trade-offs must be resolved based on a given application.
The human ear can perceive frequencies of sound between 20 Hz and 20 kHz. According to the sampling theorem defined in Equation 2.2.5, we have to sample a sound (analog signal) at least twice the ...
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