Digital identification of tone signal on telephone line - multiplying samples by sine and cosine of tone and accumulating products

摘要

Tone signals on incoming lines in a telephone exchange are identified by digital processing of successive samples taken of the line voltage. Each sample is multiplied by sine and cosine of one possible tone frequency and the result accumulated in a pair of stores over four samples. This is done in four channels in parallel, each channel being staggered in time by one sample period. The accumulated result in each channel is squared to make it positive, and compared with a threshold to determine whether that frequency is present. The process is repeated for each of the possible tone frequencies, after having first reset the store contents to zero. The output is in the form of a statement of which tone, if any, is present on each of the lines being monitored.