Differential encoding with lookahead feature |
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| 申请号 | US3720875D | 申请日 | 1971-11-03 | 公开(公告)号 | US3720875A | 公开(公告)日 | 1973-03-13 |
| 申请人 | IBM; | 发明人 | WILL P; FRANASZEK P; GROSSMAN D; | ||||
| 摘要 | Analog input information is compacted by a differential encoding process that anticipates abrupt transitions in signal levels and initiates compensatory action in time to prevent the encoded representations of such transitions from being shifted out of phase due to slope overload. Digitized analog signal representations which are to be encoded are first passed through a shift register having L stages, the number L signifying the amount of ''''lookahead'''', i.e., the number of sampled analog elements that are to be analyzed as a group prior to encoding. The contents of the shift register at any instant will furnish the ''''history'''' of variations in the respective levels of the first L signal elements which currently await encoding. By a judicious interpretation of this history, the system is able to select an optimal encoded bit pattern which would most nearly represent in compact digital code form the apparent trend of these variations, and the leading bit or bits of this pattern will be fed out by the encoder. The optimal bit pattern is continually updated as the makeup of the lookahead code group changes. By anticipating rapid changes of level, the phase shift of elements having highly contrasting levels is avoided. | ||||||
| 权利要求 | 1. In a data compaction process of the differential encoding type for converting absolute input values obtained by sampling an analog signal into digitized code symbols that represent only the variations of such input values with respect to each other, a lookahead encoding method which comprises the steps of: a. temporarily storing in a delay means representations of a sequence of input values derived from the sampled analog signal; b. generating for successive availability a series of digital code patterns, each pattern representing a possible sequence of code symbols for tentatively representing in differentially encoded fashion the variations of said sequence of input values; c. as each of said code patterns becomes available, calculating a sequence of output value representations which would be yielded by differentially decoding that code pattern if it were to represent the current sequence of input values; d. as each sequence of output value representations is made available by step c, calculating the value of a performance function that measures the correlation between such output value sequence and the current input value sequence; e. selecting for utilization the code pattern in said series whose calculated performance function indicates optimum correlation between the input and output value sequences; and f. utilizing a chosen portion of said selected code pattern as the differentially encoded representation of a corresponding portion of said input value sequence. 1. In a data compaction process of the differential encoding type for converting absolute input values obtained by sampling an analog signal into digitized code symbols that represent only the variations of such input values with respect to each other, a lookahead encoding method which comprises the steps of: a. temporarily storing in a delay means representations of a sequence of input values derived from the sampled analog signal; b. generating for successive availability a series of digital code patterns, each pattern representing a possible sequence of code symbols for tentatively representing in differentially encoded fashion the variations of said sequence of input values; c. as each of said code patterns becomes available, calculating a sequence of output value representations which would be yielded by differentially decoding that code pattern if it were to represent the current sequence of input values; d. as each sequence of output value representations is made available by step c, calculating the value of a performance function that measures the correlation between such output value sequence and the current input value sequence; e. selecting for utilization the code pattern in said series whose calculated performance function indicates optimum correlation between the input and output value sequences; and f. utilizing a chosen portion of said selected code pattern as the differentially encoded representation of a corresponding portion of said input value sequence. 2. A lookahead encoding method as set forth in claim 1 wherein step f involves utilizing the leading code symbol of the selected code pattern to represent the leading value of said input value sequence, irrespective of any discrepancy which may exist between said leading input value and the output value which will be represented by said leading code symbol when it ultimately is decoded. 3. A lookahead encoding method as set forth in claim 2 wherein the code symbols of said digital code patterns are individual bits, the leading bit of the selected pattern being utilized in step f to form a part of a delta-modulated output signal representing said analog signal in encoded form. 4. A lookahead encoding method as set forth in claim 1 in which the performance function calculated by step d is the summation of values (Vi - Ai)2, where V represents an input value to be encoded, A represents a decoded output value, as calculated by step c, i is an integer whose value ranges from k (the serial number of the leading input value currently stored in said delay means) to k+L-1, and L is the number of inpuT values stored in said delay means, and step e involves selecting the code pattern which yields a performance function of minimum value for a given value of k. 5. Apparatus for encoding absolute input values of a sampled analog signal into digitized code symbols that represent the variations of such input values with respect to each other, said apparatus comprising: a. delay means for temporarily storing a sequence of input values attained by the sampled analog signal; b. digital code pattern generating means for making a series of code patterns successively available, each such pattern representing a possible sequence of code symbols tentatively representing in differentially encoded form the variations of said temporarily stored sequence of input values; c. differential decoding means operable in response to each of said code patterns as it becomes available to generate the sequence of output value representations which would be produced by such a code pattern in the event it were selected to represent the sequence of input values currently stored in said delay means; d. calculating means responsive to each sequence of output value representations generated by said decoding means to calculate the value of a performance function that measures the correlation between such output value sequence and the current input value sequence; e. comparing means for determining which of the performance values calculated by said calculating means most satisfactorily meets a given performance criterion; f. means for storing a chosen portion of the code pattern which constitutes the optimum encoded representation of the current input value sequence, as determined by said comparing means; and g. output means utilizing said stored code pattern portion as a differentially encoded representation of a corresponding portion of said input value sequence. 6. Encoding apparatus of the kind set forth in claim 5 wherein the portion of the code pattern stored in means f is the leading code symbol of the code pattern selected by comparing means e, said output means utilizing said leading code symbol to represent the leading input value currently stored in delay means a. 7. Encoding apparatus of the kind set forth in claim 5 in which the performance function calculated by calculating means d is the summation of values (Vi - Ai)2, where V represents an input value to be encoded, A represents a calculated output value, i is an integer whose value ranges from k (the serial number of the leading input value currently stored in delay means a) to k+L-1, where L is the number of input values stored in said delay means, and comparing means e determines which of said code patterns has the performance function of minimum value. |
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