What is stereo widening?

Widening of the stereo image can be achieved by manipulating the relationship of the side signal S and the center signal C: C = (L + R)(1 / 2);S = (L − R)(1 / 2). A positive part of the side signal S is now fed into the left channel and a part with its phase inverted to the right channel. Some boom boxes feature such a process.

Another way of looking at this same effect, withtout extrapolating a center and side signal from the left and right signals, is to simply add the left signal, slightly attenuated and phase inverted, into the right channel and vice-versa. Taking this a step further, a small delay (20-100ms) can be added to the inverted signal before mixing it back in to the original for output, adding a slight reverberation to the effect.