GB1387428A - Filtering system - Google Patents

Abstract

1387428 Active filters; frequency selective amplifiers COMMUNICATIONS SATELLITE CORP 11 Jan 1972 [11 Jan 1971] 38930/74 Divided out of 1387427 Headings H3U and H3W A filter system for separating unwanted energy at odd multiples of frequency H/2 in a frequency band A 1 to A 2 (A 1 <A 2 ) from wanted energy at harmonics of frequency H up to frequency A 2 comprises means for comb filtering the entire band up to frequency A 2 to extract the unwanted energy, a band-pass filter for passing all the unwanted energy between frequencies A 1 and A 2 and means for subtracting the band passed unwanted energy from the original signal. The band-pass filter output is preferably constant within 3 db. through the pass-band and the filter incorporates a phase equalizer so that it has a linear phase shift characteristic (Fig. 7). As shown (Fig. 5) the input signal is delayed by H1 and fed to the plus input of differential amplifier 30. To the other amplifier input is supplied the sum of the input signal and the input signal delayed by 2H. The output of amplifier 30 has the desired comb characteristic and is band-pass filtered and subtracted at 34 from the input signal delayed by H plus a delay 35 equal to that of the band-pass 32. The filter may be used for separating luminance and chrominance signals in a NTSC TV signal or for removing sampling energy interleaved in a PCM system operating at less than the Nyquist frequency. A low-pass filter with phase equalization (Fig. 8) for use in such a system is also described.