Decoder for frequency-modulated tones of two different lengths

Claim 1

A decoder for coded frequency-modulated control signals of two different lengths used in alternating sequence of signal length, comprising a frequency recognition circuit, a switchable timing circuit responsive to the output of the frequency recognition circuit, a threshold switch circuit responsive to said timing circuit, and a bistable circuit responsive to said switch circuit and having a first output for controlling the switchable timing of said timing circuit and a second output for operating a control function for the operation of which said control signals are useful, and further comprising the improvement wherein: said frequency recognition circuit (4) comprises two phase locked loops (5,6) and an AND-gate (7), each of said phase locked loops having a voltage controlled oscillator and a phase comparator; the voltage controlled oscillator of the first phase-locked loop (5) is tuned to the carrier frequency of said FM control signals and the voltage controlled oscillator of the second phase locked loop (6) is tuned to the frequency-modulation frequency of said FM control signals; a connection is provided from a point in said first phase locked loop (5) following its phase comparator to an input of the phase comparator of the second phase locked loop (6) for supplying frequency modulation detected by said first phase locked loop to said second phase locked loop, and each of said phase locked loops has outputs (19,31) from whch a signal showing a locking-in of the oscillator to an input frequency provided to the loop, each of said outputs being connected to an input of said AND-gate (7), of which the output is the output of the frequency recognition circuit to which said switchable timing circuit is responsive.