Hi kastro68, The different AAC encoders already have bitrate adjustment. As you said like " Joint stereo encoding ", AAC uses M/S matrixing and intensity stereo. Just a snap of two methods: M/S matrixing: (Mid/Side stereo) It is like joint stereo applied to front and surround channels.As we have seperate front/rear L & R, a matrix is applied to calculate relative sum and difference of original channels. Then this information is processed for creating final AAC file.Advantage, only the difference is encoded to seperate channels, saving bitrates. Intensity stereo Intensity stereo coding is a method that achieves a saving in bitrate by replacing left/right signals by a single representing signal plus directional information. For more information on internal audio compression technology, Refer