An improved decoder for providing a decoder output signal when a tone signal contains frequencies that are sequentially within {TAIL ~ a predetermined passband} surrounding {HEAD ~ first and second detection frequencies} f1, f2 
An improved decoder for providing a decoder output signal when a tone signal contains frequencies that are sequentially within { TAIL ~ a predetermined passband } surrounding { HEAD ~ first and second detection frequencies } f1 , f2
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH