AEGIS
From The RadioReference Wiki
This is MA/Com / Ericsson's original implementation of a Digital voice solution. Very few agencies today are using AEGIS Digital Voice solutions, since they are 1st generation digital voice solutions and considered dated. Voice quality on AEGIS systems are very poor.
EDACS AEGIS systems use the MA/Com / GE EDACS Control Channel.
There is NO scanner available today that can monitor EDACS AEGIS Digital voice.
This system in the Washington, DC area uses AEGIS for some of it's voice traffic.
Here is a 1994 patent describing some aspects of the AEGIS vocoder and encryption.