10.2 Multi-channel stereo

Many innovations beyond two-channel stereo have failed because of cost, impracticability (e.g. number of loudspeakers), and last but not least a requirement for backwards compatibility. While 5.1 surround multi-channel audio systems [150, 228] are being adopted widely by consumers, this system is compromised in terms of number of loudspeakers and with a backwards compatibility restriction (the front left and right loudspeakers are located at the same angles as in two-channel stereo, i.e.±30°, resulting in a narrow frontal virtual sound stage).

It is a fact that by far most audio content is available in the two-channel stereo format. (In the following the term stereo is used for two-channel stereo.) For audio systems enhancing the sound experience beyond stereo, it is thus crucial that stereo audio content can be played back, desirably with an improved experience compared with the legacy systems.

It has long been realized that the use of more front loudspeakers improves the virtual sound stage, especially for listeners not exactly located in the sweet spot [169, 255]. Therefore, there has been a lot of attention on playing back stereo signals with an additional center loudspeaker [164, 246] (Reprinted in [74],) [9, 94]. Gerzon [94] also treats the more general case of up-mixing signals. However, the improvement of these techniques over conventional stereo playback has not been sufficient for mass adoption. The main limitations of these techniques are that they consider only localization and not explicitly other aspects such as ambience and listener envelopment. Further, the localization theory behind these techniques is based on a one-virtual-source scenario, limiting their performance when a number of sources are present at different directions simultaneously.

These weaknesses are resolved by the described technique using a perceptually motivated spatial decomposition of stereo audio signals. Given this decomposition, audio signals are rendered for an increased number of loudspeakers, loudspeaker line arrays, and wavefield synthesis systems [17, 264].