Stereo image widening is the process of changing the perceived panoramic width of an audio signal. It can be used to make a signal sound wider, as well as narrower.

This process is based on mid-side processing. As part of the process of encoding and decoding, the relative amplitude of the “mid” and “side” channels are adjusted.

To make a signal sound wider, the amplitude of the “side” channel is increased compared to the “mid” channel. To make a signal sound narrower, the amplitude of the “mid”channel” is increased compared to the “side” channel.

A variable called $width$ can be created to narrow or widen the perceived stereo image. This variable has a range from 0 to +2.

When $width = 1$, the stereo image is unchanged. When it is less than 1, the stereo image is narrowed. When it is greater than 1, the stereo image is widened.

The following equations can be used to process the ‘mid’ and ‘sides’ channels to create the effect:

$width \in [0 , 2]$

$newSides = width \cdot origSides$

$newMid = (2 - width) * origMid$

Notice how the $width$ variable changes the amplitude of the ‘mid’ and ‘sides’ in a complementary way. As the amplitude of one increases, the other always decreases.

After the amplitude has been scaled, mid-side decoding is performed to recover the ‘left’ and ‘right’ channels of a conventional stereo signal.