Databases Reference
In-Depth Information
17
Audio Coding
17.1 Overview
Lossy compression schemes can be based on a source model, as in the case of
speech compression, or a user or sink model, as is somewhat the case in image
compression. In this chapter we look at audio compression approaches that are
explicitly based on the model of the user. We will look at audio compression
approaches in the context of audio compression standards. Principally, we will
examine the different MPEG standards for audio compression. These include MPEG Layer I,
Layer II, and Layer III (or MP3), and the Advanced Audio Coding Standard. As with other
standards described in this topic, the goal here is not to provide all the details required for
implementation. Rather the goal is to provide the reader with enough familiarity so that they
can then find it much easier to understand these standards.
17.2 Introduction
The various speech coding algorithms we studied earlier rely heavily on the speech pro-
duction model to identify structures in the speech signal that can be used for compression.
Audio compression systems have taken, in some sense, the opposite tack. Unlike speech
signals, audio signals can be generated using a large number of different mechanisms. Lack-
ing a unique model for audio production, the audio compression methods have focused on
the unique model for audio perception, a psychoacoustic model for hearing. At the heart
of the techniques described in this chapter is a psychoacoustic model of human perception.
