Method and system for encoding and decoding frequency shift keying signals

A method for encoding and decoding a frequency-shift keyed data signal for transmission over a non-ideal communications channel, comprising the steps of;

• a) encoding an arbitrary stream of data characters into a transmit sequence {(S(n)} of positive numbers enumerated by intergers n of an index set, such that each data character of said stream is represented by a subsequence (S(i+1), . . . , S(i+p)) of length p, and such that the S(n) can only assume a finite number of preselected values;
• b) generating a frequency-shift keyed data signal by phase-coherently combining halfwave sinewave segments of frequencies 1/(2S(n)) such that all halfwaves start at a peak value or at a zero crossing;
• c) transmitting the frequency-shift keyed data signal over the non-ideal communications channel;
• d) receiving a perturbed frequency-shift keyed data signal from the non-ideal communications channel;
• e) generating a receive sequence {E(n)} of positive numbers by measuring the time intervals between consecutive peak values or consecutive zero crossings of the received perturbed frequency-shift keyed data signal;
• f) comparing subsequences (E(i+1), . . . , E(i+q)) of length q of said sequence {E(n)} with each of a set of stored sequences (Z(1), . . . , Z(q)) each of which represents a data character thereby determining measures of affinity representing errors for each stored sequence;
• g) identifying the existence of a transmitted data character when, during the step of comparing, at least one error does not exceed a preselected bound;
• and
• h) selecting, from the identifying step, as the transmitted character the character represented by the stored (Z(1), . . . , Z(q)) which yields the smallest error during the comparing step.

A system for asynchronous transmission of digital data over a non-ideal communications channel comprising:

• a) means for generating halfwave sinewave segments of varying frequencies;
• b) means for encoding streams of data characters as a plurality of said halfwave sinewave segments, joined phase-coherently, wherein each segment constitutes frame, data, or fill elements, thereby producing a continuous, asynchronous frequency-shift keyed data signal;
• c) means for transmitting said frequency-shift keyed data signal over said non-ideal communications channel;
• d) means for receiving a perturbed frequency-shift keyed data signal of said kind;
• e) means for measuring time intervals between preselected indicia points of said perturbed frequency-shift keyed data signal;
• f) means for comparing finite numbers of consecutive time intervals of said kind with sequences of stored times to thereby determine the information content of the said perturbed frequency-shift keyed data signal;
• and
• g) means for calculating and storing said sequences.

Apparatus for receiving and decoding sequences of digital data transmitted over a communications channel as sequences of phase-coherent halfwave sinewave signals in which a preselected number of sequential halfwaves of the sequences are representative of a data character, comprising:

• a) signal input means for receiving the transmitted sequences of halfwave sinewave signals;
• b) means connected to the input means for measuring the time periods between consecutive zero crossings or peaks of the received signals;
• and
• c) computer means connected to the measuring means and having a memory storing at least one representation of each possible transmitted data sequence as a sequence of time periods for comparing each received sequences of time periods to each of the stored sequences of time periods, and for outputting as a transmitted data stream the one represented by the stored sequence closest to the received sequence.

A method for decoding a frequency-shift keyed ac signal transmitted over a non-ideal communications channel comprising the steps of:

• a) receiving a frequency-shift keyed ac signal from the non-ideal communications channel;
• b) generating a sequence {E(n)} of positive numbers by measuring the time intervals between consecutive peak values or consecutive zero crossings of the received frequency-shift keyed ac signal;
• c) comparing said sequence {E(n)} to each of a set of stored sequences each representing a stream of none, one or more data characters thereby determining error margins for the stored sequences;
• d) identifying each of said sequences having an error margin less than a preselected bound;
• and
• e) selecting as the transmitted stream of none, one or more data characters the one from the identified sequences which corresponds to the stored sequence which yields the smallest error margin during the comparing step.