GB1385024A - Digital signal handling techniques - Google Patents

Abstract

1385024 Gain control EMI Ltd 4 May 1972 [12 Feb 1971] 4508/71 Heading H4R Digital audio signal has its gain controlled by feeding the digitized audio signal to a multiplier 4 to the other input of which is fed the state of a counter 2 fed with the output of a variable frequency oscillator 1. As described the output of the oscillator increases as the lever 3 is moved either one way or the other from a datum position with movement of the lever one way causing up counting of counter 2 and movement the other way causing down counting. The counter 2 provides an output state which varies logarithmically with the input pulses from generator 1 and a store 5 is provided to register any desired count, and reset the counter to that count at any later time on depression of a recall button. The logarithmic counter Fig. 2, comprises a conventional binary counter 6 with a set of J-K flip-flops 7a to 7l connected by their set inputs to the counter stages and their Q output connected to the other inputs of the following stage, in the direction of least significant counting stage, and also to respective inputs of an AND gate 8. The first J-K flipflop is fed with the pulses to be counted, which are also applied to one input of the AND gate 8, and the AND gate output is taken to the counter input. With a count of zero in counter 6 all Q outputs of the flip-flops are 1's and the pulse stream from the generator feeds directly through AND gate 8 to counter 6. When the counter is half full and the last stage of counter 6 is set, only half the pulses from the generator 1 pass through AND gate 8 to the counter, so the counting rate is halved. When the counter is ¥ full the last but one stage of counter 6 is set, and the pulse rate to the counter is again halved, thus as counting proceeds the counting rate is continually reduced in an approximately logarithmic progression. Also described is a digital low pass filter Fig. 3 (not shown) in which the output of a digital recursive filter (D) fed with a digitized signal is added to the digitized signal to provide base cut or lift.