1. An apparatus for generating a stereo side signal comprising a first side channel and a second side channel from a stereo input signal comprising a first input channel and a second input channel, comprising: a modification information generator for generating modification information, the modification information generator comprising a spectral weights generator for generating the modification information by generating a first spectral weighting factor using a mono mid signal and using a mono side signal of the stereo input signal, wherein the mono mid signal depends on a sum of the first input channel and the second input channel, and wherein the mono side signal depends on a difference of the first input channel and the second input channel, and a signal manipulator being adapted to manipulate the first input channel using the modification information to acquire the first side channel and being adapted to manipulate the second input channel using the modification information to acquire the second side channel.