An AM/FM modulation scheme is presented for optimizing communications
between a remote control device held by a user and a receiving unit
installed in a vehicle. Historically, these systems communicated with
amplitude modulated signals having good range but poor immunity to
ambient random radio frequency noise. Frequency modulated systems provide
improved immunity to noise however at the expense of range. The object of
this invention is to use two or more modulation modes to combine their
advantages. A scheme has been devised whereby circuitry capable of
alternately operating in at least two modes is used. The same information
is therefore transmitted using two or more modulation techniques in
succession. The receiving unit also has circuitry and has listening
cycles ensuring eventual synchronization with the transmitting unit. If
one mode fails due to distance or ambient noise, the other can then
attempt to establish a link.