Decimation filter arrangement

Claim 1

A decimation filter for converting a time-discrete input signal having a sampling frequency f i to a time-discrete output signal having a sampling frequency f u , such input signal consisting of sequential components x(q), q=. . . -2, -1, 0, 1, 2, . . . , and such output signal consisting of sequential components y(n), n=. . . -2, -1, 0, 1, 2, . . . , the ratio f u /f i being irrational; such filter comprising: (a) means for producing a first sequence of clock pulses ki(q) at said rate f i ; (b) means for producing a second sequence of clock pulses au(n) at said rate f u ; (c) filter coefficient genrating means for producing a group of N filter coefficients, N being a predetermined integer, such soefficient generating means comprising: (c1) deviation component generating means connected to the first and second clock pulse generating means and which, in response to each n th second sequence clock pulse, produces a deviation component d n of a magnitude proportional to the ratio between the time interval Tad(n) between such n th second sequence clock pulse and the first clock pulse closest thereto; such deviation component generating means comprising: means for gernating at a sampling rate f i samples of a periodic signal having a period 1/f u and which linearly varies between two limit values, selection means connected to said periodic signal sample generating means for selecting those two consecutive periodic signal samples of which one is above and the other is below a predetermined threshold value, subtracting means connected to said selection means for forming two difference components, the first such difference component being equal to the difference between said first limit value and the magnitude of the first of said selected consecutive periodic signal samples and the second such difference component being equal to the difference between said second limit value and the magnitude of the second of said selected consecutive periodic signal samples, adding means connected to said subtracting means for deriving the sum of said two difference components, and dividing means connected to said adding means for producing the deviation component d n by dividing one of the difference components by the sum of both of said difference components; (c2) and calculating means connected to said deviation component generating means and responsive to the deviation component d n produced thereby to produce said group of N filter coefficients in accordance with the relation a.sub.n (m)=h](d.sub.n +m)V.sub.O ] wherein m=0, 1, 2, . . . N-1, a n (m) is the n th filter coefficient of the group of N filter coefficient to be produced following occurrence of the n th first sequence clock pulse, h[(d n +m)V O ] is the impulse response h(v) of a FIR-filter, v being a continuous variable, and V O is a predetermined constant; (d) a multiplying arrangement coupled to the output of said filter and to said coefficient generator and to which is applied, in response to the n th second sequence clock pulse, N consecutive components x(r-m) of the input signal x(n), r being an integer, and the N filter coefficients a n (m) produced by the coefficient generator; such multiplying arrangement being adapted to multiply each input signal component x(r-m) by an associated filter coefficient a n (m); (e) and an adding arrangement coupled to said multiplying arrangement for adding the N products a n (m).x(r-m) produced thereby, the sum of such N products being the filter output signal component y(n).