Combination contrast/brightness control for a television receiver

Abstract

A single, combined contrast/brightness control is disclosed herein for a television receiver. A contrast control in the form of a variable potentiometer, is coupled to the output, driver transistor of the video amplifier, which serves to amplify and apply the video signal to the cathode ray tube (CRT) and including AC/DC coupling in the form of a resistor and a capacitor. The DC coupling resistor sets the DC bias level to the output transistor and in particular to its base at a level dependent upon the DC level established across the constrast/brightness control potentiometer. Further, the AC coupling capacitor is selectively charged to further adjust the DC bias of the output transistor to a level with the DC coupling resistor whereby the black level of the output signal applied to the cathode ray tube is maintained relatively constant with regard to a predetermined level. In a preferred embodiment of this invention, the predetermined level is the sync tip and back porch voltages and the charging of the AC coupling capacitor is carried out during the horizontal retrace cycle, at which time a charging circuit is formed through the capacitor whereby it is charged to a potential dependent upon the setting of the contrast control potentiometer, and thus, the bias level to the base of the output transistor is adjusted to set the black level to a predetermined level. In this manner, it is only necessary to use the single contrast/brightness control to set both the contrast and the brightness level of the displayed image upon the CRT.