Patent GB-1523888-A (GB)
Title: Automatic peaking apparatus
IPC: H04N9/77
Filed: 1975-10-16
Abstract: 1523888 Television RCA CORPORATION 16 Oct 1975 [21 Oct 1974] 42522/75 Heading H4F A system for automatically controlling the "peaking characteristics" of television signal processing apparatus in response to the amplitude of colour burst signals, which also suppresses undesired signal components (e.g. chrominance or audio information in a luminance channel) is described. A tapped delay line 156, Fig. 1, is employed in a processing unit 136 in luminance channel 116. Three signals a, b, and c, are extracted from the delay each delayed by a respective amount. These signals are combined at 158 to form Vp 1  by addition and subtraction subsequent to passing individual weighting units 154. Vp 1  is applied to an AGC amplifier 160 having variable gain P determined in inverse relationship to the amplitude of the colour burst component. PVp 1  and V bw1 , the weighted form of b 1 , are added at 164 to produce the desired output Vo 1 . The peaking characteristics of processing unit 136 are determined by algebraically adding delayed video signals a 1  and c, while the bandwidth characteris tics thereof are determined by delayed signal b 1  in combination with the signal Vp 1 . The peak amplitude of the "amplitude versus frequency transfer" (AvFT) characteristics, Fig. 2 (not shown), of the unit, is automatically controlled by controlling the amplitude of Vp 1 . The amplitude transitions of the output signal v ol  of signal processing unit 136 contain a preshoot just before the transition and an overshoot just after the transition. These preshoots and overshoots serve to accentuate amplitude transitions of v ol  so that, for example, an image transition from white to black will be accentuated because the image just before the transition is whiter than it is in the original scene; and, just after the transition, the image is blacker than it is in the original scene. Furthermore, the phase versus frequency transfer characteristics are related to the preshoots and overshoots. For example, a linear phase versus frequency transfer characteristic corresponds to the formation of equal preshoots and overshoots. The preshoots and overshoots are controlled by the signal formed by the summation of amplitude controlled signals associated with taps 152a and 152c. Therefore, although the predetermined gain values of amplitude controlling means 154a and 154c were chosen to be equal and time intervals T 11  and T 21  were chosen to be equal, resulting in a linear phase versus frequency transfer characteristic as manifested by equal preshoots and overshoots, the amplitude controlled signals associated with taps 154a and 154c may be varied to produce unequal preshoots and overshoots to compensate for phase versus frequency non-linearities in other portions of the video signal processing system. Processing unit 136 may be replaced by a similar unit, Fig. 3 (not shown) where it is desirable to provide relatively high frequency peaking consistent with effective trapping. AvFT characteristics of this unit are described w.r.t., Fig. 4 (not shown). Similar processing units may alternatively be provided in the chrominance channel to provide automatic chroma control. In this case unit 570, Fig. 5, relatively accentuates signal in the frequency range the chrominance signals to this end. The amplitude of output Vo 5  is controlled by a feed back loop in response to the amplitude of the colour burst signal. The AvFT characteristics of this unit are similar to those of Vp, Fig. 2 (not shown). Relatively high frequency peaking consistent with a relatively small bandwidth so that for example, chrominance signals may be more readily separated from luminance and sound signals can be achieved by employing the modification of Fig. 6 (not shown) to unit 570, Fig. 5.