Receivers that detect stereophonic/monophonic signals are incorporated into a vast number of devices used in everyday life. For example, such receivers are used in automobile radios, a variety of communication systems such as cellular telephones, and even in children's toys. Unfortunately, many modern receiver systems suffer from performance shortfalls, such as frequent switchover between monophonic and stereophonic modes due to noisy channel environments and false detection of stereophonic signals as monophonic due to rated maximum system deviation (RMSD) mismatch.
In order to receive FM audio signals, be they music or any other type of information, a receiver must be robust enough to handle changes in the channel wherein the transmission could become very noisy and/or must overcome interference. Generally, a pilot tone is transmitted as part of the baseband signal that is used to modulate an FM carrier signal in order to indicate the nature of the transmission to be stereophonic. The energy of the pilot tone may fluctuate significantly in a harsh channel scenario. Simply comparing the pilot tone energy, estimated at the receiver, against a predetermined threshold may cause the receiver to switch between monophonic and stereophonic mode too frequently and degrade the entertainment quality of the audio program delivered to the consumer.
In addition, the receiver structure and the accompanying algorithms must also be flexible enough to handle a situation where the transmitted FM signal RMSD is not known beforehand. Generally, the allowed RMSD values are 75 kHz and 50 kHz. Hence, a mono/stereo transmission may be utilizing either one of them. However, the receiver may be set to operate at a different RMSD than the received signal RMSD. If the received signal RMSD and the RMSD of the receiver are not matched, a situation may occur wherein a stereophonic signal may be falsely interpreted as monophonic by the receiver. This results in the listener being denied the stereophonic quality of the program that the service provider is transmitting on the airwaves.