The basic idea of a 4-2-4 processing system is to derive four streams of information from two streams of information. Essentially, the four streams of information are:
- The information in stream A
- The information in stream B
- The information that is the same in stream A and stream B
- The difference between the information in stream A and stream B
The first two channels are fairly straightforward. The A stream feeds the left speaker, and the B stream feeds the right speaker. But the "same" and "difference" channels are a little more complex. To understand how this works, you need to know a little bit about how speakers produce sound.
A basic speaker is built around an electromagnet, a metal cylinder with a wire coiled around it. The electromagnet is surrounded by a permanent natural magnet. When you send an electrical current through the electromagnet, it becomes magnetized and acts like a natural magnet, with a north pole and a south pole. The coiled wire in the electromagnet is connected to the (+) speaker wire on one end and the (-) speaker wire on the other end. The audio amplifier is constantly changing the direction of the current so that the orientation of the poles keeps switching.
Changing the orientation of the poles shifts the attraction between the electromagnet and the surrounding natural magnet. This causes the electromagnet to move back and forth. As the electromagnet moves, it pushes and pulls a speaker cone, which rapidly pushes out air and then pulls back in. This movement of air particles produces the sounds we hear. (For more information, see How Speakers Work.)
An audio signal, then, is just a fluctuating electrical current. When the current fluctuates one way, the speaker cone moves in; when it fluctuates the other way, the cone moves out. This signal can be represented as an oscillating wave. The particular sound produced depends on how rapidly and how far the cone moves, which is dictated by the fluctuation pattern in the electrical current.
In a surround-sound setup, the signal for the center channel is recorded on both the A stream and the B stream. The center signals on both streams are identical in amplitude and frequency, and they are synchronized exactly.