What sets this frontend for ffmpeg apart from other stereo simulators is it creates an illusion of actual stereo separation. More importantly, it produces none of the weird phasing, and/or time delay artifacts. And very little, if any of the tone discoloration, when those others aren’t meticulously set up just right. All this is accomplished by using the ffmpeg crossover audio filter to split the sound into 8 frequency bands. The split points are based on center frequencies of a typical 1/3 octave equalizer. Those 8 bands are then panned in varying degrees to left and right. The varying width of each band is set to achieve the best balance between the left and right channels.

sourceforge.net/projects/ultimate-mono2stereo