DELTA MODULATION SYSTEM USING A CONSTANT CODE LENGTH LESS THAN THE AVAILABLE CODE LENGTH WITH AUTOMATIC RANGE SHIFT WITHIN THE AVAILABLE CODE LENGTH

摘要

A train of binary-weighted pulses, each of which may have positive or negative significance, is transmitted to represent the difference between a current analogue signal sample and the immediately preceding sample. Twelve binary weights are available for coding the difference between samples, but only five are used at a time, which group of five is used depending on the previous values of the coded difference. An input sample is applied to the base of T1, which together with T2, forms a longtailed pair comparator. C1 stores a charge corresponding to the amplitude of the preceding sample. A switching network PCC is set to select a group of five pairs of positive and negative pulses PP, PN, a positive or negative pulser being selected in accordance with the output of the comparator to steer appropriate charges into Cl until it is charged to the same level as the new sample. If the coded difference is less than 01111 for a plurality of cycles, this fact being stored in a ferrite core store CM, PCC is controlled to select the next lower group of pulsers to increase the accuracy of coding. If the coded difference reaches 11101 (equivalent to 29 in range 2 -24) the next higher group of pulsers is selected by PCC. The decoder is similar in operation. Incoming pulses select positive or negative pulsers PP, PN to alter the charge on Cl, from which the output is taken through a filter. A store CM and network PCC control the steering of incoming pulses to the appropriate group of pulsers in a manner similar to that employed in the coder.