Voxengo's Spatifier is a new plugin with a simple purpose: to turn mono tracks into "spatially-enhanced" stereo ones.
Said to work well on guitar, synth, piano, organ, backing vocal and other parts, Spatifier "decorrelates" the signal in the left and right channels while retaining mono compatibility. This decorrelation process is said to create a wide surround effect, and can be used in place of other techniques such as double tracking.
Spatifier works by first designing an impulse response, which can be adjusted with equalizer-style controls. Using these, you can set the spatial effect for each frequency band individually. The impulse response is then used by the internal convolution processor.
You can find out more about Spatifier on the Voxengo website. It's available now for PC and Mac in VST/AU formats priced at $60 (regular price is $70). There's a demo version, too.
