Patent GB-1268927-A (GB)
Title: Vertical and horizontal aperture equalization
IPC: H04N5/208
Filed: 1969-03-31
Abstract: 1,268,927. Television. RCA CORPORATION. 31 March, 1969 [1 April, 1968], No. 16714/69. Heading H4F. An aperture distortion correction signal for correcting a television video signal in both the horizontal and vertical directions is derived by applying the video signal directly, A, and via a two-line period delay C to the inputs of an amplifier 20, and via a one-line period delay B to the input of an inversion amplifier 18, the output of amplifier 18 being delayed, 22, for onehalf the period of a frequency, e.g. the colour subcarrier frequency, near the upper end of the video signal frequency range, which frequency is an odd multiple of one-half the horizontal line scanning frequency, and combined with the output of amplifier 20. The inputs A and C to amplifier 20 are attenuated by a factor of four and added together so that with ganged contacts 25a and 25b respectively closed and open (not shown) a vertical aperture correction signal of the form “(A + C) - B is produced at terminal 26. Amplifier 20 may be an inversion amplifier instead of amplifier 18. Horizontal aperture correction is also produced when the ganged contacts 25a and 25b are in the positions shown, the receiving end of delay device 22 being terminated by a resistor 24 of a value to prevent reflections of the wave energy impressed on the sending end of the device. The sending end of the device 22 is terminated by an impedance higher than the characteristic impedance of the device and thus causes reflections of wave energy impressed on the receiving end of the device. The signals from amplifier 20 are impressed on the delay device 22, and travel to the sending end from which they are reflected back to the receiving end where they are added to the unreflected signals from amplifier 20 and the signals from amplifier 18. At the subcarrier frequency, i.e. the frequency which is "combed", and at multiples of the line scanning frequency intervals around the subcarrier frequency the function “(A+C)-B is thus equal to zero, whereas maximum peaking of the luminance signal is produced around the subcarrier frequency except at the above null points. A compromise between the "combing" effect and the peaking may be achieved by selecting delay device 22 such that its delay is other than onehalf the period of the subcarrier frequency. The correction signal will not then be zero at the subcarrier frequency but maximum peaking will be achieved at a frequency having a period twice the delay time of device 22. In Fig. 2 the video signal is modulated, 28, on a carrier to produce a signal compatible with delay devices 14 and 16, and demodulated, 32, 34. Also an additional peaking circuit 36, 40, 38, 42 and 44 is included so that it is possible to keep the response of the aperture correction signal minimum at the subcarrier frequency and peak the response at some other frequency (delay device 42 selected so that its delay time is equal to one-half the period of the frequency at which peaking is desired). Delay device 22 may be replaced by two terminated delay devices in series each having a delay time equal to one-half the period of the frequency to be combed, amplifier 20 attenuating the A and C signals by a factor of two and being connected to the first delay device input as well as terminal 26, and amplifier 18 being connected to the junction of the two seriesconnected delay devices. The video signal at input 12 should be obtained before gamma correction and the aperture correction signal added to the video signal after gamma correction.