Tone receiver for digital data communication systems

A tone receiver for use in a digital data communication system for detecting a presence, within a received signal, of a tone whose frequency lies outside the spectrum of a data signal, said receiver including:

• an analog-to-digital converter (ADC) for sampling the received signal and converting samples Xn thereof to digital form and outputting said converted samples;
• a first recursive digital bandpass filter having an input and an output and centered at the frequency of the tone signal, the input of which is connected to said ADC output;
• a limiter having an input and an output and serially inserted within the feedback loop of said first bandpass filter;
• an input feedforward loop having an input and an output and connected to said ADC output and providing a predetermined gain .gamma.;
• an adder having at least two inputs and an output said inputs being connected to the outputs of said first filter and of said feedforward loop;
• a hard limiter having an input and an output and connected to the output of said adder;
• a second digital filter connected to the output of said hard limiter and providing an input signal;
• and,
• means responsive to the signal provided by said second digital filter for measuring the energy V of the signal provided thereby and for indicating receipt of said tone when the energy V exceeds a predetermined threshold value.