Siren

• Format: 0x28E
• Company: Microsoft

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 encoder has been reverse engineered and is available for download at : https://svn.sourceforge.net/svnroot/amsn/trunk/amsn/utils/tcl_siren/src/ (excluding the tcl_siren* files). The decoder has not yet been written, but should be fairly easy to do considering the encoder part is done.