Android - What is the Virtualizer sound effect in music players and how does it work?
In the description of PlayerPro in Google Play this was mentioned:
STEREO WIDENING effect (virtualizer)
and a quick search on Wikipedia about what Stereo Widening is:
Widening of the stereo image can be achieved by manipulating the relationship of the center signal M and the side signal S: M = (L + R)/2; S = (L - R)/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 boomboxes feature such a process.
Another way of looking at this same effect, without 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.