Signaling system

Apparatus for use at the receiver end of a communication line to distinguish between authorized and unauthorized signals on the communication line,

• where it is known in advance at the receiver that authorized signals on the communication line are generated by dividing a periodic original signal having a frequency f.sub.o by at least one value so that (i) subsequent multiplication of authorized communication line signals by said at least one value will generate signals that are phase coherent with one another and have a common frequency f.sub.o, but (ii) subsequent multiplication of unauthorized communication line signals by said at least one value will generate signals that are not phase coherent with one another, and where
• it is NOT known in advance at the receiver whether any given data on the communication line is authorized or unauthorized,
• the apparatus comprising
• a multiplier for stepping up the frequency of the communication line signals by multiplying them by said at least one value to generate output signals which have frequency f.sub.o and are phase coherent with each other when the communication line signals are authorized, but are phase incoherent when the communication line signals are not authorized,
• a bandpass filter at frequency f.sub.o for filtering the output signals of the multiplier to build up, as an output signal, a single spectral line at f? , the phase coherence of the output signals of the multiplier permitting more of the spectral energy of the output signals of the multiplier to fall within the passband of the filter in the case of authorized communication line signals than for unauthorized signals,
• an analyzer for detecting the spectral energy in the spectral line of frequency f.sub.o in the output signal of the bandpass filter and for generating a signal indicative of the phase coherence or incoherence of the communication line signals relative to each other based on the spectral energy, and
• an information processor for determining whether the line signals are authorized or unauthorized based on the phase coherence or incoherence of the line signals relative to each other as indicated by the signal issued by the analyzer.