Detection of single-frequency signals

Apparatus for detecting a single-frequency signal having a predetermined frequency or range of frequencies in a non-differentially encoded signal, which signal comprises:

• converting means, responsive to the non-differentially encoded signal, for converting the non-differentially encoded signal into a differentially encoded digital signal;
• and
• detection means, responsive to the differnntially encoded digital signal, for detecting the single-frequency signal having the predetermined frequency.

Apparatus for detecting a single-frequency signal having a predetermined frequency or range of frequencies in a differentially encoded signal which comprises:

• means, responsive to the differentially encoded signal, for detecting a predetermined number of cycles of the differentially encoded signal;
• means for determining the length or the frequency of the cycles, for determining whether or not the length or frequency is within predetermined bounds and for keeping count of the number of such detected cycles which are within the predetermined bounds and the number of such detected cycles which are outside the predetermined bounds;
• and means for determining whether the count of detected cycles within predetermined bounds has reached a first predetermined amount, whereby a single-frequency signal having the predetermined frequency has been detected, and for determining whether the count of detected cycles which are outside the predetermined bounds has reached a second predetermined amount, whereby a single-frequency signal having the predetermined frequency has not been detected.