Dual-modulation receiving apparatus

Claim 1

Receiving apparatus for a dual-modulation signal equivalent in waveform to a first carrier double-sideband amplitude-modulated by a first modulating wave, combined with a second carrier approximately equal in amplitude to, equal in frequency to, and spaced in phase by more than 10° and less than 90° from, said first carrier, double-sideband amplitude-modulated by a second modulating wave, producing a dual-modulation signal, which comprises: first sampling means which samples said dual-modulation signal at regularly-occurring intervals, for short periods substantially centered in time on instants of zero-crossings of said second carrier at said first sampling means, with a sampling frequency greater than the minimum sampling or Nyquist frequency for said first modulating wave and equal to said first carrier frequency divided by an integer, and second sampling means which samples said dual-modulation signal at regularly-occurring intervals, for short periods substantially centered in time on instants of zero-crossings of said first carrier at said second sampling means, with a sampling frequency equal to said sampling frequency of said first sampling means, and first filter means which has a pass-band the same as the frequency band of said first modulating frequency, which receives the output of said first sampling means and delivers the output of said first filter means to a first output circuit, and second filter means which has a pass-band the same as the frequency band of said second modulating wave, which receives the output of said second sampling means and delivers the output of said second filter means to a second output circuit, and pulse generator means comprising carrier frequency selection means, frequency changing means and pulse timing means, which receives said dual-modulation signal, and generates a first sequence of regularly-occurring pulses with short duration substantially centered in time on zero-crossings of said second carrier at said first sampling means, with a repetition frequency greater than the minimum sampling or Nyquist frequency of said first modulating wave, and equal to said first carrier frequency divided by an integer, and delivers said first sequence of pulses as gating pulses to said first sampling means, and generates a second sequence of regularly-occurring pulses with short duration, substantially centered in time on zero-crossings of said first carrier at said second sampling means, with a repetition frequency equal to the repetition frequency of said first sequence of pulses, and delivers said second sequence of pulses as gating pulses to said second sampling means, and dual-modulation indicator means which actuates signalling means, and which receives the output of said first sampling means at a first input and the output of said second sampling means at a second input, and which comprises logic means which compares the pulse amplitudes at each of said inputs with the normal pulse amplitudes, and disables said signalling means when said pulse amplitudes at either of said inputs are not substantially equal to the normal pulse amplitudes with a dual-modulation signal.