Patent GB-1300781-A (GB)
Title: Circuit arrangement for colour point adjustment
IPC: H04N9/67
Filed: 1970-03-04
Abstract: 1300781 Colour television PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 4 March 1970 [7 March 1969] 10410/70 Heading H4F Relates to a circuit in a colour television receiver or transmitter by means of which (a) the displayed skin colour may be changed without changing the displayed white point or green intensity, and/or, (b) the displayed green intensity may be changed without changing the displayed skin colour or white point. In the receiver (a) is achieved by effectively adding equal fractions of the received red and blue colour difference signals in opposite senses to the green colour difference signal produced therefrom, and (b) is achieved by effectively adding different fractions of the red and blue colour difference signals to the green colour difference signal produced therefrom, the fractions of the red and blue colour difference signals in (a) and (b) being variable. A circuit for effecting (a) and/or (b) is shown in Fig. 12 in which the received red and blue colour difference signals are applied to terminals 1 and 2 and the modified green colour difference signal is produced at terminal 3. The reproduced skin colour may be adjusted by varying the value of R 1  (the white point and reproduced green intensity remaining constant), and the green intensity may be adjusted without varying the white point or the (possibly corrected) skin colour varying the value of R 5 . Variable resistors R 1  and R 5  preferably follow a logarithmic law which results in a linear variation in the skin colour and the green intensity. The variation in the green intensity occurs because the colour points for red and blue are shifted in the same sense along the sides of the colour triangle towards the green point in such a manner that the relative location of the colour points remains substantially unchanged but the green intensity decreases. The adjustment of the skin colour occurs because the colour points for blue and red are shifted in opposite senses along the sides of the colour triangle in such manner that considerable variations in the skin colour reproduction occur without substantially affecting the white point or the greem intensity. A practical circuit arrangement for a receiver containing integrated circuits is described with refeience to Fig. 13 (not shown). If a fixed reduction of the green intensity is chosen then the circuit of Fig. 12 may be simplified to that shown in Fig. 14. At the transmitter (a) is achieved by adding equal fractions of the red and blue signals in opposite senses to the luminance signal (produced from the red, green and blue signals), to produce a modified luminance signal which is transmitted with modified red and blue colour difference signals formed using this modified luminance signal, and (b) is achieved by subtracting a fraction of the green signal from, and adding different fractions of the red and blue signals to, the luminance signal (produced from the red, green and blue signals) to produce a modified luminance signal which is transmitted with modified red and blue colour difference signals formed using this modified luminance signal, the sum of the different fractions being equal to the fraction of the green signal. A circuit for effecting (a) and/or (b) is shown in Fig. 15 in which amplifier V 1  supplies the normal NTSC type luminance signal, and in which the skin colour may be changed by varying the amplification of V 3  and the green intensity may be changed by varying the amplification of V 2 . A complete mathematical analysis is given in the Specification.