The widespread use of digital signal processor based modems over the HF channel has given rise to a need for more complex signal conditioning requirements. These requirements are usually in the form of fast automatic gain control (AGC) units and reliable real-time channel evaluation (RTCE) subsystems. The function of the AGC units is to overcome the severe signal fading conditions experienced over the HF channel. The RTCE subsystems are used in conjunction with channel coding and system management routines to improve the overall reliability of the system. The authors describe a new digital based AGC system (Mc-AGC) which performs the above two requirements in one algorithm. The Mc-AGC system has been developed to operate in conjunction with 4 tone MFSK modems.