Patent GB-856595-A (GB)
Title: Video signal compensation
IPC: H04N5/208
Filed: 1957-05-13
Abstract: 856,595. Television. RADIO CORPORATION OF AMERICA. May 13, 1957 [May 25, 1956], No. 15179/57. Class 40(3). In a television system in which image fields are scanned line by line, compensation for loss of detail in the direction of field changes due to the finite dimensions of scanning electron beams is effected by combining with the signals due to one line signal due to one or more other lines of the same field, the signals due to the various lines being brought into time coincidence by passage through delay devices. Fig. 4 illustrates an arrangement in which a signal due to one line has combined with it in reversed polarity signals due to both the preceding and succeeding lines. The signals appear at 20 and are applied to a combining circuit 26<SP>1</SP> over three paths. The first is the upper path which includes no delay, the second include a delay device 22<SP>1</SP> introducing a delay corresponding to one line scan and the third branches from the second after delay device 22<SP>1</SP> and includes a further delay device 22<SP>11</SP> also introducing a delay corresponding to one line scan. By this means, signals due to three successive lines are brought into time coincidence, that due to centre line of the three at any time comprising the main signal and having subtracted from it in combining stage 26<SP>1</SP> the signals due to the other two lines. The delays may be introduced by magnetic tape or wire recorders, lumped capacitance-inductance networks, mercury columns or storage tubes, although as illustrated quartz crystal devices are used, the signal being modulated on a carrier wave before application to the devices and recovered subsequently, by demodulators. For this purpose modulator 32 and oscillator 34 is provided in the path before delay device 22<SP>1</SP> and demodulators 36 and 36<SP>11</SP> provided following the respective delay devices. In practice the delays are made slightly less than one line, adjustable trimming delays 38 and 38<SP>11</SP> being included to build out the delays to exactly one line. Provision is made to adjust the magnitude of signals to be subtracted from the main signal. Furthermore, low-pass filters 28<SP>1</SP> and 28<SP>11</SP> are included in the channels of the signals to be subtracted whereby the high-frequency components of the final compensated signal are effectively peaked. This, it is stated, has the effect of also compensating for loss of detail in the line scan direction. Reference is made to an arrangement in which the peaking starts at low frequencies, reaches a maximum at one-eighth of the highest frequency and thereafter continues substantially level for the remainder of signal band. Fig. 5 (not shown) illustrates a modification of the arrangement shown in Fig. 4 in which the signal derived through the first and third paths are combined and passed through a common low-pass filter before subtraction from the main signal derived through the second path. In a simplified arrangement, Figs. 1 and 2 (not shown), only a single delay line is employed, the main signal having subtracted from it the signal due to only either the preceding of succeeding line. The Specification includes a detailed circuit of this arrangement, Fig. 3 (not shown). Reference is made (without giving details) to more complex arrangements in which compensation is effected with aid of signals from more than two lines, certain of the signals being added to the main signal instead of being subtracted.