Spatial Audio Processing: MPEG Surround and Other Applications

2.3 Audio coding

Generally speaking, audio coding is a process for changing the representation of an audio signal to make it more suitable for transmission or storage. Although high-capacity channels, networks, and storage systems have become more easily accessible, audio coding has retained its importance. Motivations for reducing the bitrate necessary for representing audio signals are the need to minimize transmission costs or to provide cost-efficient storage, the demand to transmit over channels with limited capacity such as mobile radio channels, and to support variable-rate coding in packet oriented networks. In this section, representations and coding techniques which are of relevance to spatial audio are reviewed.

2.3.1 Audio signal representation

Audio signals are usually available as discrete time sampled signals. For example, a compact disc (CD) stores stereo audio signals as two separate audio channels each sampled with a sampling frequency of 44.1 kHz. Each sample is represented as a 16-bit signed integer value, resulting in a bitrate of 2 × 44.1 × 16 = 1411 kb/s. For multi-channel audio signals or signals sampled at a higher sampling frequency the bitrate scales accordingly with the number of channels, bit depth, and sampling frequency.

2.3.2 Lossless audio coding

Ideally, an audio coder reduces the bitrate without degrading the quality of the audio signals. So-called lossless audio coders [51, 55, 95, 213, 223] can achieve this by reducing the bitrate while being ...