A television receiver monitor can accept video input signals of standard definition, having horizontal scanning frequency of 15.734 KHz (1H) or signals of higher definition with a higher scanning frequency of nominally 2.14H or about 33.6 KHz. Standard definition, SD or 1H input signals are processed to enable display at a double scanning frequency of 2H. Higher definition input signals with horizontal scanning frequencies of slightly greater than 2H are processed by analog circuits and then displayed. In an exemplary receiver monitor because the display operates with a scanning frequency in the order of double the standard definition rate, standard definition signals require up conversion to form a 2H or double frequency scan rate prior to display. Typically standard definition signals are encoded with color information according to the NTSC standard thus prior to up conversion it is necessary decode the NTSC signal into its luminance and color components which are then digitized to form a digital signal bit stream. Typically this 1H digital bit stream is processed by a de-interlacer, which de-interlaces or up converts the bit stream from 1H to a 2H scanning frequency. The resulting double frequency signal is digital to analog converted to form an analog 2H signal for subsequent analog processing and display.
Prior to the analog to digital converter or (ADC) processing the input signals are subject to automatic gain control, AGC, which ensures that a substantially constant amplitude signal is provided for digital conversion. This AGC system monitors the sync pulse amplitude of the luminance signal coupled for digitization to determine the degree of correction required. For example, if the incoming sync is too large or small, the AGC system assumes that the video signal originated with a standard video to sync ratio, and decreases or increases the overall signal gain to restore a standard sync amplitude. This system works fairly well except in cases where the video to sync ratio is significantly different from the nominal ratio of 2.5:1 (100/40 IRE).