Appendix V defines a coding scheme for mid-side (MS) stereo using Recommendation ITU-T G.722 Annex B (G.722-SWB). By introducing the mid-side stereo coding into stereo terminals, interoperability with the monaural devices could be obtained in very low complexity.

The basic coding scheme is as follows;

a)         Two channels of the left-right (LR) stereo are converted to those of the mid-side stereo and then the signals of each channel are independently encoded using ITU-T G.722 Annex B,

b)         At the decoder side, the mid-side channels of the bitstream from the encoder are decoded respectively and then the decoded signals of the mid-side channels are reversed to those of the LR channels.

The LR-MS conversion and its inverse are conducted in a conventional way. On the encoder side, two additional arithmetic operations per sample are required for the LR-MS conversion and one operator for the MS-LR conversion in the decoder. In an STL2009 [b‑ITU‑T G.191] basic operator implementation, the conversion complexity amounts to about 0.2 WMOPS in total. The coding algorithm for each channel is identical to the one in Recommendation ITU-T G.722 Annex B.