The invention relates to a device for processing a stereo audio signal comprising a first channel (L) and a second channel (R). The stereo signal is analysed (12) for obtaining a measure for a bit quantity, whereby said quantity is required by a coder for coding the stereo audio signal using a coding algorithm. The first and the second channel are subsequently modified (14) when the measure for the bit quantity is greater than a predetermined value. Modification is carried out in such a way that the energy of a sum signal of the first and second modified channel (L', R') bears a predetermined ratio in relation to the energy of a sum signal of the first and second channel and that a difference signal of the first and second modified channel is muffled in relation to the difference signal of the first and second channel. The side channel is muffled, especially for audio coders that require a constant output bit rate, when the coding of stereo audio signals cannot observe the output bit rate of the coder. Stereo channel separation is thus abandoned in favour of an increased audio bandwidth or a reduction of quantisation interference.