Patent GB-1334250-A (GB)
Title: Self adaptive filter and control circuit therefor
IPC: H04B3/14
Filed: 1970-12-21
Abstract: 1334250 Adaptive filters AUSTRALIA COMMONWEALTH OF 21 Dec 1970 [22 Dec 1969] 60513/70 Heading H4R In a self adaptive transversal filter, e.g. for use as a line equalizer or for synthesizing an echo signal to cancel the leak across the go and return paths of a hybrid junction, an error signal is developed which is a measure of the degree of conformity between the output signal of the filter and a desired signal, and the error signal is correlated with the input signal to derive adjustment signals for the multipliers of the transversal filter. The correct adjustment of the filter multipliers is determined iteratively and for each signal sample sequence fed into the transversal filter a complete iteration is performed on the filter contents with repeated adjustment of the error signal to take account of the filter correction just derived and subsequent recalculation of further filter adjustment to match an output sample from the filter to a desired signal sample and the final adjustment of the filter determined by the iteration is set into the filter multipliers before a subsequent signal sample is fed to the filter. In one embodiment, Fig. 3, each sample of the signal on the input of the four wire side of a hybrid 49 is converted to digital form in converter 44 and fed to the shift register 45 forming part of the digital transversal filter 87. The filter 87 synthesizes the echo signal passing from the input to output on the four wire side of the hybrid. The synthesized echo is subtracted from the actual echo in subtractor 50 leaving an error signal if the synthesized signal does not match the actual echo. A delay 61 provides time synchronization between the actual echo signal and the synthesized echo. The filter control system 86 includes a shift register 51 into which input signal samples X -m  to X m  are stored and a shift register in which corresponding error values Y m  to Y o  are stored. The values of input signal and error signal are correlated in multiplier 62 and integrator 66, the sign of the correlation result taken by limiter 70 and the resulting value fed to impulse response increment store 67. The values in the store 67 are then multiplied by the contents of the input signal store 51 and modify the values of error signal in store 59 to the values which would have occurred had the impulse response values stored in register 47 in the transversal filter 87 been modified by the increment values in the register 67. At the same time the increment values in register 67 are used to modify the response values stored in register 47. The new values of error signal in register 59 are again correlated with the input signal from store 51 to determine further increments which are stored in register 67, and used to further modify the error signal in register 59. The iteration continues until the error values stored in register 59 are sufficiently small. The error values are monitored by a device 81 and if the error is above a predetermined value e.g. due to outgoing speech on line 82, the impulse response values in store 47 are not adjusted. In a modified system, Fig. 4, the input signal on line 90 is autocorrelated with time spaced samples of itself and the coefficients stored in register 105. In addition time spaced input signal samples are cross correlated with the difference between the hybrid output signal, line 103, and the transversal filter output, from summing network 101, at the output of subtractor 102, and the cross correlation values stored in register 104. The impulse response increments in 99 are adjusted in accordance with the signs of the cross correlations C o  to C m , and iteration proceeds by modifying the values of cross correlation C o  to C m  by subtraction therefrom of the values of the autocorrelation fns. A -m  to A  +m  multiplied by the impulse response increments #r o  to #r m .