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This audio codec is used by MSN Messenger for sending/receiving voice clips. It is also one of the available codecs for the 'Computer Call' feature (audio conference). It is based on the G722.1 codec and only has a few differences to it. For example, it uses a 2 byte big-endian format for storing the encoded data instead of little-endian, and it also adds a few bits (mostly 2) as a header to each frame to specify the sampling rate, and a 4 bits footer which contains a checksum of the frame (but keeping the size of the block to 40 bytes per frame).

The codec has been reverse engineered and is available for download at : (excluding the tcl_siren* files).

Possibly related to Vivo Siren.