Signaling system

1. 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.

2. The apparatus of claim 1 wherein said authorized communication line-signals have different frequencies in a frequency shift keyed modulation system.

3. The apparatus of claim 2 wherein said authorized communication line signals have two frequencies in a frequency shift keyed modulation system.

4. The apparatus of claim 1 wherein said periodic original signal has, at a give time, one of a plurality of known base frequencies, and said one base frequency of said original signal at each said given time is known to said bandpass filter.

5. The apparatus of claim 1 wherein said authorized communication signals represent a bit stream, said authorized communication signals have two periodic components, and said periodic components are generated by binary frequency shift keying based on said bit stream.

6. The apparatus of claim 5 wherein said periodic components of said authorized communications signals are generated by dividing said periodic original signal by either of two integers (M and N) corresponding to "1" valued bits and "0" valued bits in said bit stream.

7. The apparatus of claim 1 wherein said multiplier comprises a harmonic generator for generating harmonics of said periodic components including Mth and Nth harmonics, and said bandpass filter filters the output of said harmonic generator in a narrow passband at frequency f.sub.o

8. The apparatus of claim 1 wherein said authorized communication signals are generated digitally from aid periodic original signal.

9. The apparatus of claim 1 wherein said communication line comprises a band limited channel, said communication line signals being within the passband of said channel, said periodic original signal being outside said passband.