GB859996A - Improvements in or relating to signal transmission group-delay equalising apparatus - Google Patents

Abstract

859,996. Phase equalizing networks. POSTMASTER GENERAL. June 27, 1957 [April 5, 1956], No. 10507/56. Class 40(8). [Also in Group XL(c)] A variable group-delay equalizer comprises two parallel connected branches each having different group-delay/frequency characteristics (where # = d #/d # is the group-delay, # the phase shift between input and output and # the angular frequency) provided by all-pass equaliser networks A, B, and variable amplifier or attenuator means 5 so that the combined group-delay/ frequency characteristic taken as a vector sum of the two outputs, may be made variable between the limits set by the fixed delay equalisers A, B. Moreover, a phase shift means 8, e.g. a length of cable, is inserted in one path to equalise the phase shift in each branch at or near the mid-band frequency. The equalisers A, B in each branch also are such that approximately the same group-delay d #/d # occurs at the midband frequency in each branch. A delta pad 1 may supply the two inputs. The gain of both paths may be inversely varied by a differential gain control 9 applied to variable gain valves. The group delay equalisers 6 may each be a series pair of bridged-T networks or lattice networks. One of the parallel paths may consist only of a length of cable and the other comprise a group-delay equaliser with inverted parabola characteristic having a maximum delay at the mid-band frequency. The equalisers may be inserted at intervals in a multi-channel frequency modulation link to reduce distortion, and each branch may be variably gain controlled to compensate automatically for variations from a desired standard and linearise the over-all group-delay/frequency characteristic.