| Popis: |
Analog multiplexing and coding in the D2 Channel Bank is discussed in this article. Multiplexing of the message signals is accomplished in two stages. In the first stage, groups of 12 channels are multiplexed together using resonant transfer gates. The resulting eight buses,' each carrying pulse-amplitude-modulated signals of 12 channels, are then multiplexed in the second stage. The samples of all 96 channels are presented to a single coder. The demultiplexing plan follows the inverse of the multiplexing plan. The output of the decoder is first divided into eight buses, and the final demultiplexing is accomplished in groups of 12 channels. Because the decoding is accomplished by an asynchronous time-shared decoder, storing and stretching of the analog samples is necessary to permit removal of the time jitter due to the queuing process. The coder used in D2 is a nonlinear coder using a compression characteristic called the 15-segment approximation to the µ = 255 law. To ensure the success of the coder development, a stage-by-stage binary coding plan was chosen. The first stage determines the polarity of the signal, and the succeeding binary stages determine the amplitude of the compressed signal one digit at a time. To achieve accuracy in the coder with available devices, automatic zero-setting circuits are used in a feedback loop to control offset deviations. This is in addition to the use of precision resistors and precision power supplies for the remaining critical parts of the coder. In order to achieve comparable accuracy in the decoder, the same stage-by-stage arrangement is also used. Again, automatic zero-set feedback loops are used to control drifts. The performance of the coder/ decoder combination has met the objectives. |
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