a signal processor processes communication signals for a communication link by extracting amplitude information from the signal and converting this amplitude information into two signal tones . the frequency information of the communication system is transmitted as a constant amplitude signal with two narrowband fm control channels . each control channel is used to transmit one of the two tones . after the communication signal is received , the two tones are removed from the narrowband fm control channels and are used to generate control signals that will control a variable gain amplifier and transfer the amplitude information back to the constant amplitude signal thereby facilitating the reception of the communication system .