Electrical filter and signal discriminating apparatus employing the same

A signal discriminating apparatus for determining if the frequency of a signal received thereby has either a first or a second value, said apparatus comprising:

• a filter;
• means for applying said received signal to said filter;
• said filter having a central frequency lying between said first and second values, said central frequency being chosen such that when said frequency of said received signal is equal to said first value, said filter will cause a phase shift in said received signal of more than a predetermined value and such that when said frequency of said received signal is equal to said second value, said filter will cause a phase shift in said received signal of less than said predetermined value;
• first determining means for determining whether the phase difference between said received and said phase shifted signal appearing at an output of said filter is greater than said predetermined value, said first determining means producing an output having one value when said phase difference is approximately zero degrees and producing an output having a second, different value when said phase difference is approximately 180 degrees;
• should determining means for determining whether a signal is being received by said apparatus;
• and
• means for actuating a first or a second external device responsive to the detection of the presence of a signal whose frequency has said first or said second value, respectively.