FOUR-CHANNEL SURROUND (3-1 STEREO)
Purpose of four-channel systems
The merits of three front channels have already been introduced in the previous section. In the 3-1 approach, an additional ‘effects’ channel or ‘surround’ channel is added to the three front channels, routed to a loudspeaker or loudspeakers located behind (and possibly to the sides) of listeners. It was developed first for cinema applications, enabling a greater degree of audience involvement in the viewing/listening experience by providing a channel for ‘wrap-around’ effects. This development is attributed to 20th Century Fox in the 1950s, along with wide-screen Cinemascope viewing, being intended to offer effective competition to the new television entertainment.
There is no specific intention in 3-1 stereo to use the effects channel as a means of enabling 360° image localization. In any case, this would be virtually impossible with most confi gurations as there is only a single audio channel feeding a larger number of surround loudspeakers, effectively in mono.
Figure 17.2 shows the typical loudspeaker configuration for this format. In the cinema there are usually a large number of surround loudspeakers fed from the single S channel (‘surround channel’, not to be confused with the ‘S’ channel in sum-and-difference stereo), in order to cover a wide audience area. This has the tendency to create a relatively diffuse or distributed reproduction of the effects signal. The surround speakers are sometimes electronically decorrelated to increase the degree of spaciousness or diffuseness of surround effects, in order that they are not specifi cally localized to the nearest loudspeaker or perceived inside the head.
In consumer systems reproducing 3-1 stereo, the mono surround channel is normally fed to two surround loudspeakers located in similar positions to the 3-2 format described below. The gain of the channel is usually reduced by 3 dB so that the summation of signals from the two speakers does not lead to a level mismatch between front and rear.
Limitations of four-channel reproduction
The mono surround channel is the main limitation in this format. Despite the use of multiple loudspeakers to reproduce the surround channel, it is still not possible to create a good sense of envelopment of spaciousness without using surround signals that are different on both sides of the listener. Most of the psychoacoustic research suggests that the ears need to be provided with decorrelated signals to create the best sense of envelopment and effects can be better spatialized using stereo surround channels.
Excerpt from Sound and Recording: Applications and Theory, 7th Edition by Francis Rumsey and Tim McCormick © 2014 Taylor & Francis Group. All Rights Reserved.