Numerical filter and digital data transmission system including said filter

Claim 1

A digital filter to the input of which the samples of an analog signal are applied, the spectrum of said analog signal being restricted to a frequency of fm which is half the sampling frequency, the filter comprising: 2n 1 elementary halfband pass filter cells of the same type which are grounded in n cascade-arranged stages, the first stage including 2n 1 cells, the pth stage including 2n p cells, p indicating the number of any stage and varying between 1 and n from the first to the laSt stage, the last stage including one cell the output of which is the output of the filter; means for splitting up the incoming series of samples of frequency 2fm into 2n 1 interlaced series of the frequency 2fm/2n 1 which are separately applied to the cells of the first stage; means inserted between succeeding stages, the pth stage and the (p + 1)th stage, for combining pairwise the 2n p outgoing series of the pth stage in order to constitute 2n (p 1) interlaced series of regularly distributed samples of frequency 2fm/2n (p 1) which are applied to the 2n (p 1 ) cells of the (p + 1 )th stage; means for supplying the cells with clock signals the frequency and the phase of which are in correspondence with the frequency and the phase of the samples applied thereto; means at the input and at the output of each cell for reversing the sign of one out of every two incoming and outgoing samples; means in each cell for inhibiting its filter function; a terminal at each stage for controlling the sign reversal of all cells of the stage and a terminal at each stage for controlling the inhibition of all cells of the stage, the filter passband being variable in width and position in steps having a band width of fm / 2n dependent on the value of the binary number with n bits applied to the n terminals for controlling the reversal and of the binary number with n bits applied to the n ternimals for controlling the inhibition state.