GB1523729A - Digital filtering - Google Patents

Abstract

1523729 Digital filter W & T AVERY Ltd 20 Nov 1975 [10 Dec 1974] 53257/74 Heading H3U Apparatus to average a series of digital inputs comprises a filtering stage having a first store of capacity M which receives and stores inputs and also produces the sum of the stored inputs, a second store which stores the sum and supplies it to an output, and means which acts on the application of N new inputs to the filtering stage to remove the oldest N inputs from the first store (but only if it already contains M inputs) and updates it with the N new inputs and which also adjusts the sum being stored in the second store by subtracting the sum of removed inputs and adding the sum of the updating inputs. The process can be repeated in a second filtering stage with a store capacity R and changes made in P, and in a third stage with a store capacity S and changes of T inputs. Preferably R equals M and S equals 2M; alternatively R equals M + m 1 , S equals 2 (M Œm 2 ) where m 1 and m 2 are small compared with M and are selected so that the product M(MŒm 1 ) 2(M # m 2 ) is at least approximately equal to 2M<SP>3</SP>. The second store in each filtering arrangement may have a divider associated with it to divide the output by the capacity M, R or S of the first store to provide an output for processing by subsequent stages. The apparatus may be used with a weightresponsive transducer having an analogue output, an A/D converter, and a sampler to provide inputs to the apparatus to form weighing equipment. The apparatus provides a linear output response to a step change in the input, whereas prior art filters have responded exponentially and thus more rapidly. The frequency response is said to resemble a decaying sine wave with the RMS value of the peaks being equivalent to a lower gain beyond cut off than in prior air filters, and thus less noisy.