Mixed signal decoder for audio frequency FSK applications with transmission rate comparable to the carrier frequencies

Frequency shift keying (FSK) finds application in low frequency transmission over band limited audio frequency channels, such as calling number identification, a service currently provided by telephone service providers. The conventional approaches to FSK demodulation work well when the transmitted frequencies are significantly higher than the baud rate. However, when the baud rate is comparable to the transmission frequencies, they become costly to implement. This paper describes a mixed signal approach to FSK demodulation suitable in particular to the decoding of this type of signal, based on a finite state machine (FSM). The proposed circuit consists of an analog interface followed by a threshold device, which is itself followed by the FSM.