Introduction

Audio functionality plays a critical role in embedded media processing. While audio takes less processing power in general than video processing, it should be considered equally important.

In this chapter, we will begin with a discussion of sound and audio signals, and then explore how data is presented to the processor from a variety of audio converters. We will also describe the formats in which audio data is stored and processed.

Additionally, we'll discuss some software building blocks for embedded audio systems. Efficient data movement is essential, so we will examine data buffering as it applies to audio algorithms. Finally, we'll cover some fundamental algorithms and finish with a discussion of audio and speech compression.

What Is Sound?

Sound is a longitudinal displacement wave that propagates through air or some other medium. Sound waves are defined using two attributes: amplitude and frequency.

The amplitude of a sound wave is a gauge of pressure change, measured in decibels (dB). The lowest sound amplitude that the human ear can perceive is called the "threshold of hearing," denoted by 0 dBSPL. On this SPL (sound pressure level) scale, the reference pressure is defined as 20 micropascals (20 ?Pa). The general equation for dBSPL, given a pressure change x, is

Table 5.1 shows decibel levels for typical sounds. These are all relative to the threshold of hearing (0 dBSPL).

Table 5.1: Decibel (dBSPL) values for various typical sounds

Source (distance)	dBSPL
Threshold of hearing	0
Normal conversation (3-5 feet	60-70
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