×

Generalized interpolative method for digital/analog conversion of PCM signals

  • US 4,467,316 A
  • Filed: 06/03/1981
  • Issued: 08/21/1984
  • Est. Priority Date: 06/03/1980
  • Status: Expired due to Fees
First Claim
Patent Images

1. A method for effecting digital to analog conversion of PCM signals by a generalized interpolative procedure, comprising the steps of:

  • sampling the PCM signals at a first sampling rate and effecting intermediate storage of each sampled PCM signal word in succession;

    supplying each stored word in succession to a first input of a first digital adder;

    dividing each word at the output of the adder into a first part representing the higher value portion of the adder output word and having a length shorter than that of the adder input word, and a second part representing the lower value part of the adder output word;

    conducting the second part of each adder output word through a digital, nonrecursive counter feedback filter having a degree greater than one at a clock pulse rate which is an integral multiple of the sampling rate, and supplying each resulting word at the filter output to a second input of the first digital adder where it is added to the stored word at the first adder input;

    transferring the first part of each adder output word at the clock pulse rate to a system output;

    setting the degree and pulse response of the filter to values minimizing the quantizing error, due to the difference in length and value between each adder output word first part and the corresponding sampled signal word, within the useful frequency band of the resulting signal in such a manner as to cause the power content of the quantizing error in that band to be negligible compared to the quantizing error power contained in the PCM signals;

    converting the successive adder output word first parts at the system output into an analog signal to a digital/analog converter having a small number of available output values; and

    passing the analog signal at the output of the converter through an analog lowpass filter for suppressing periodic continuations of the useful signal frequency band created by the sampling and for sufficiently attenuating the additional concentrated quantizing distortions due to the length of the adder output word first part being shorter than that of the word at the adder first input, wherein said step of conducting comprises;

    storing in respective locations of a fixed value memory representations of the filter coefficients;

    feeding the second part of each adder output word through a shift register chain having taps which are connected to the address input of the fixed value memory, the taps being so arranged that at each of them there appears one identical-value bit of the output word second parts;

    reading the representations out of one of the memory locations together with the bit pattern present at the shift register chain taps;

    summing the read out representations in an accumulative circuit connected between the output of the memory and the second digital adder input in such a manner that after a number of clock pulses corresponding to the length of each adder output word second part, a new word appears at the accumulator circuit output.

View all claims
  • 1 Assignment
Timeline View
Assignment View
    ×
    ×