Qsound

Qsound is a technology intended to reproduce surround sound with only two speakers. The results can be rather spooky. Sonic images appear in 360˚ patterns around the listening position. That's quite remarkable given their two-channel source. The classic example of Qsound is Roger Water's Amused To Death. If you've never heard this work on a decent stereo system it's worth the purchase just to experience surround sound from your stereo. There's a catch. Qsound only works for one person at a time. That person must be seated dead center between the speakers to appreciate the surround effects. Move off-center even just a bit and we get standard stereo again. According to the Wikipedia article: "QSound is essentially a filtering algorithm. It manipulates timing, amplitude and frequency response to produce a binaural image. Systems like QSound rely on the fact that a sound arriving from one side of the listener will reach one ear before the other and that when it reaches the furthest ear, it is lower in amplitude and spectrally altered due to obstruction by the head. However, the ideal algorithm was arrived at empirically, with parameters adjusted according to the outcomes of many listening tests." That's correct but a much simpler way to understand what's going on would be to focus on phase differences, which is what Mathew Polk and his team at Polk Audio did years before in their SDA products. Using cross-connected stereo speakers and an additional set of midrange drivers, a bit of out of phase left was fed into the right channel's "dimensional" drivers, and vice versa—the stereo width was extended beyond the room boundaries because the left ear got a slightly delayed out of phase signal from the right channel, and so on. The same technique called Interaural Crosstalk has been applied in multiple ways: Ambiophonics, Carver's Sonic Hologram generator, among the many. Essentially, all these systems were precursors to what today we use to direct sound in big PA systems so every seat gets the same image and at the same amplitude. There's a lot more than just two dimensions possible with 2-channels of audio. Just ask any audiophile about depth, width, and height for starters.