Digital communications equipment using differential quaternary frequency shift keying

A communications receiver for receiving a digital bit stream, encoded by forming multi-bit symbols and differentially encoding successive pairs of said multi-bit symbols to produce differentially encoded symbols, and frequency modulated so as to represent each differentially encoded symbol by one of a plurality of frequencies related in number to a number of bits of said multi-bit symbols, comprising:

• means for producing in response to an analog radio frequency input signal a demodulated FM signal in which different signal levels correspond to different ones of said plurality of frequencies;
• means responsive to said demodulated FM signal for adaptively determining a plurality of signal level thresholds corresponding to different ones of said plurality of frequencies and for comparing said demodulated FM signal to said thresholds to determine what particular symbol is being received and to produce a signal indicative thereof;
• means responsive to said signal for differentially decoding said signal into a stream of multi-bit symbols;
• and
• means responsive to said stream of multi-bit symbols for producing said digital bit stream.

A communications transceiver for transmitting and receiving digital information represented as a digital bit stream, comprising:

• means for converting a digital bit stream to be transmitted to a stream of multi-bit symbols;
• means for differentially encoding said stream of multi-bit symbols to produce a stream of differentially encoded symbols;
• means for frequency modulating a carrier signal in accordance with said differentially encoded symbols so as to represent each differentially encoded symbol by one of a plurality of frequencies related in number to a number of bits in said multi-bits symbols;
• means for producing in response to an analog radio frequency input signal a demodulated FM signal, in which different signal levels correspond to different ones of said plurality of frequencies;
• means responsive to said demodulated FM signal for adaptively determining a plurality of signal level thresholds corresponding to different ones of said plurality of frequencies, and for comparing said demodulated FM signal to said thresholds to determine what particular symbol is being received and to produce a signal indicative thereof;
• means responsive to said signal for differentially decoding said signal into a stream of multi-bit symbols;
• and
• means responsive to said stream of multi-bit symbols for producing a received bit stream.