Abstract: A convergence correcting apparatus is used in a projection television receiver including a CRT having an aspherical fluorescent screen and a projection lens apparatus which is attached to the display screen of the CRT and has aspherical plastic lenses. In the digital convergence correcting apparatus, a digital memory stores digital correction data corresponding to correction values of convergence adjustments points indicative of a plurality of points on the display screens divided in the horizontal and vertical directions. A multiplexer reads out predetermined digital correction data from the digital memory and time sharingly multiplexes. At least one digital/analog converter receives the time sharingly multiplexed data from the multiplexer and generates an analog signal. Sample and hold circuits extract a predetermined correction signal from the analog output of the digital/analog converter and sample and hold them.

Abstract: A noise reduction circuit comprising: a high-pass filter for extracting the high-frequency component of an image signal; a coring circuit for extracting a faint signal component included in the extracted high-frequency component as noise; a low-pass filter for extracting the low-frequency component of the image signal; an adder for adding the cored high-frequency component and the low-frequency component extracted by the low-pass filter; and a coring control circuit which detects the level of a luminance signal and controls the coring range of the coring circuit according to the level of the luminance signal.

Abstract: In a focusing voltage output circuit for a cathode ray tube, a variable impedance circuit is connected in series with a variable resistor for adjustment of focusing voltage. The impedance of the variable impedance circuit is changed in response to a video luminance signal or a beam current to change the focusing voltage so as to optimize the focus tracking characteristic.

Abstract: There is disclosed an electronic selection apparatus comprising a surface acoustic wave comb filter, in which the local oscillator frequency of an electronic tuner is swept up to a frequency higher than a peak frequency of the surface acoustic comb filter corresponding to a desired channel number (an upward direction), and then swept toward the opposite direction (a downward direction) until a predetermined number of peaks of the comb filter is counted while checking the presence of an AFC signal, whereby an erroneous tuning operation may be prevented, provided that the signal of the desired channel is present within a predetermined range in the vicinity of the receiving frequency of the desired channel.