Patent GB-1189456-A (GB)
Title: Fascsimile Equalizer
IPC: H04N1/409
Filed: 1967-08-23
Abstract: 1,189,456. Digital facsimile receivers; equalizing. XEROX CORP. 23 Aug., 1967 [25 Aug., 1966], No. 38859/67. Headings H4F and H4R. The invention relates to a digital facsimile receiver which incorporates a multitap delayline equalizer, Fig. 3 (not shown), of the type in which the delay between taps equals the system clock interval and the tap outputs are connected to a summing circuit through individual gain controls which permit the selection of adjustable positive or negative portions of the delayed signals. The controls require to be adjusted until the equalizer output comprises an undistorted signal. The invention is concerned with the use of the receiver during the transmission of regular test pulses to display or record the output of the equalizer in a manner which facilitates the adjustment of the gain controls. The signal from the equalizer is applied to a series of gates 64 which are actuated by time displaced sample pulses from a source 62. The pulses are synchronized by a clock 60 so as to recur at the rate of the received test pulses and are time displaced by the system clock intervals. One sample pulse T 0  coincides with the anticipated time of the test pulse; the remaining sample pulses precede and follow pulse T 0 . The gate outputs are applied through low-pass filters 68, which act as signal stores, to cross-point detectors 76. In the detectors the signal samples are compared with ramp voltages from a source 78 and produce pulse outputs at instants when the ramp amplitudes equal the sample amplitudes. In this manner pulses are produced whose time of occurrence represent the sample amplitudes. The ramp source 78 is synchronized with the operation of the facsimile receiver 24 and the voltages applied to the detectors are displaced successively in time. The facsimile receiver is provided with a lamp 72 mounted on a rotating element 70 which carries the recording element (not illustrated) past a record sheet. The lamp is exposed to a phosphorescent screen 82 and receives the pulses from detectors 76 through OR gate 80. The result is the production of a display, Fig. 5, of a pattern of light dots 84 at positions relative to reference lines 86 which indicate the amplitudes of the signal samples. The controls 88 for the delay line gain controls are located adjacent screen 82 in alignment with the displays of the signal samples which they control. The controls are adjusted to bring the samples to zero. In this respect though, the signal at time T 0  corresponds to the desired equalized pulse and should have a selected amplitude. To this end, the gate 64, Fig. 4, for the T 0  sample is preceded by a differential amplifier 66 which receives a reference signal amplitude and furnishes an output equal to the difference of the signal from the reference amplitude. The T 0  control may be a gain control at the input of the delay instead of a control in the tap output. Another form of display system is described with reference to Fig. 6 (not shown) based on a drum recorder and using a line array of lamps. Reference is also made to the use of the facsimile recorder to record the signal samples. Adjustment of the controls 88 is facilitated by an arrangement, Fig. 8 (not shown), based on an assembly of gears by which each individual adjustment, whether positive, negative or zero, is preset and upon completion a single operation of a push button causes all the adjustments to be made simultaneously.