Patent GB-1264387-A (GB)
IPC: G06V30/194
Filed: 1969-05-01
Abstract: 1,264,387. Active filters; vocoders. TEXAS INSTRUMENTS Inc. 1 May, 1969 [27 May, 1968], No. 22238/69. Headings H3U and H4R. [Also in Divisions G3 and G4] A non-linear signal processor, or a method of training it, involves using an input signal and a previous output signal to obtain a further signal which is combined with a desired output signal to produce a signal which replaces the further signal and is used to derive a contemporary output signal by normalization or division. In Fig. 1, each quantizer 11, 14, 15 samples its input and quantizes the sampled value to mark one of four leads. Each block T is a delay equal to the time between successive sampling instants, and each block POL is a polarity inverter so that the adder 17 or 20 which it feeds effectively subtracts the inverter's input. In a learning mode (switches 21a, 23a, 27a, closed) an input waveform u i  from a source 10 is sampled and quantized at 11 and the difference of it and a one-time delayed version of itself sampled and quantized at 15. The outputs of quantizers 11, 15, 14 in combination select one of a matrix 12 of capacitor stores and a corresponding one of a matrix 13 of capacitor stores and read them out to adders 20, 26, respectively, the adder outputs being stored back in the respective same capacitors via paths 36, 38 and the first divided by the second at 24 to normalize the first, the quotient going to an adder 30 providing the system output x i . This output is fed back via one-time delay 32 to the adder 30, the adder 20 (via inverter 23) and the quantizer 14. The other input z; shown to adder 20 is the desired current value of the output x i . Adder 26 also receives a signal representing the number one from a source 27. Thus each sample of the input u i  (the subscript i indicates the sample number) results in updating of the stores 12, 13 in dependence on the values already stored, the desired current output z;, &c. An operating mode has switches 21a, 23a, 27a open. The quantizer 15 could be eliminated as could the upper input to adder 20 and/or adder 30. Selection of capacitors in stores 12, 13 could also depend on further quantizers fed by differentlydelayed versions of the output x i  (e.g. via delay 32<SP>1</SP>), and on further quantizers fed by further inputs u i . Fig. 6 (not shown) shows such a modification in which each storage array 12, 13 is doubled to provide two outputs x i  in parallel, there being four inputs u i  in parallel. An average of the sum of the four inputs u i  controls variable gains given to all the inputs to the four adders corresponding to 20 and 26 of Fig. 1 except the inputs from the storage arrays, so that high inputs give high gain, the error signals thus being emphasized (during training) when the signal/noise ratio is high, in effect. As a modification, array 13 need not be doubled. Fig. 2 (not shown) shows the Fig. 1 embodiment, except that quantizer 15 is not used, showing the storage arrays 12, 13 in detail, a 2-position stepping switch being used at each capacitor to connect it for reading and writing in turn when selected by a respective AND gate, resistor adders also being shown. Fig. 7 (not shown) shows an entirely digital version of the Fig. 1 embodiment (minus quantizer 15), respective magnetic tapes providing the input u i  and the desired output z i  and receiving the output x i , the input u i  addressing a word in a magnetic store, a portion of the word being selected (after read-out) by the feedback quantity x i-1 . The system may be implemented on a generalpurpose digital computer. The system can be used to filter signal from a mixture of signal and noise after training with the mixture applied to the input u; and the signal alone applied to the "desired output" input z i . If a delay is included in the z i  input during training with this set-up, a smoother is produced instead of a filter, whereas if the delay is placed in the u i  input instead, a predictor is produced. The predictor can be used in data and speech compression, since if a predictor can estimate the next transmitted bit, that bit need not be transmitted, an identical predictor being used at the receiver to reconstruct the message.