| 摘要 |
A codec (coder/decoder), having particular utility in a pulse code modulation (PCM) communications system employing per-line coding and decoding, employs a common digital-to-analog (DAC) converter. A PCM signal to be reconstructed is decoded at two points of time within the basic sampling interval, preferably at twice the basic sampling frequency. In order to realize the desired analog reconstruction level, the PCM signal is first decoded at a first decision level lying on one side of the desired reconstruction level; then modified and again decoded at a different decision level lying on the other side of the desired reconstruction level. The average of the two decoded analog signals then lies on the desired reconstruction level. In a conventional PCM system, the two PCM samples differ by only one least significant bit (LSB) in the PCM word used in setting the common DAC. In a preferred embodiment of the invention, separate storage registers are respectively employed for the encoding and decoding, together with a selector for switching between the two registers during the two desired decoding intervals. In this way, the encoding operation may be carried on during a substantial portion of the basic sampling cycle, being interrupted only for generation of decoded samples. The invention is applicable to PCM systems generally, including compressed pulse code modulation (CPCM) systems operating in accordance with the U-law or the A-law.
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