Gsm is an implementation of the final draft GSM 06.10
standard for full-rate speech transcoding, a lossy
speech compression algorithm.
The compressed form involves 76 variables with different numbers
of significant bits packed into 33 bytes.
If you are interested in investigating the details of this
coding scheme, gsm_print() can be used to dump the contents
of individual gsm_frames to a file pointer provided by
the application.
RETURN VALUE
gsm_print() returns -1 if the frame is invalid, else 0.