Patent GB-781889-A (GB)
Title: Gamma-correction apparatus for a color-television system
IPC: H04N9/69
Filed: 1955-07-04
Abstract: 781,889. Television. HAZELTINE CORPORATION. July 4, 1955 [July 22, 1954], No. 19275/55. Class 40 (3). A gamma-correction arrangement for a colour television system comprises means, at the transmitter, for connecting a conventional (i.e. non-linearized) N.T.S.C. type composite colour signal into a linearized chromaticity type of N.T.S.C. signal (i.e. a signal in which the luminance component is nonlinearly modified and the chrominance component linearly modified) and at the receiver, means for reconverting the received signal to the conventional N.T.S.C. type and then deriving gammaconnected signals for application to the picturereproducing apparatus. By so modifying the signal prior to transmission and then reconverting and effecting gamma correction of the receiver an improvement in the noise susceptibilities of the system results and the arrangement also permits the use of any group of primary colours and results in better constant-luminance reproduction for all colour and signal intensity ranges. Fig. 1 shows the arrangement of the transmitter in which a conventional N.T.S.C. type of composite colour signal is supplied to a filter 20 the characteristic of which (K=<SP>1</SP>/ 5 ) is such that the low-frequency component of the luminance signal (0-2 mc/s) is translated with an amplitude which is five times that of the translated level of the colour modulated sub-carrier wave with its side bands (2-4 mc/s). The output of filter 20 is then supplied to an amplifier 22 having a power characteristic with an exponent = <SP>1</SP>/γ (where y is the voltage/light characteristic of the picture reproducer) and such amplification is shown to result in non-linear translation of the luminance signal and linear translation of the chrominance signal provided the ratio of the chrominance to luminance signal is small, in this case <SP>1</SP>/5. A filter network 23, complementary to filter 20, then re-establishes the relative magnitudes of the low-frequency luminance signal and the chrominance signal and the linearized composite signal (from filter 23) is then transmitted in conventional manner. At a receiver (Fig. 2), the received linearized composite video signal, derived from amplifier 63, is first reconverted to the conventional N.T.S.C. type by means of an amplifier 75 (having a power characteristic with an exponent γ) and filters 74, 76 (equivalent to filters 20, 23 in Fig. 1) and the composite signal is then utilized in conventional manner to produce colour signals which are supplied to the cathode electrodes of the picture reproducing tube 65 via gamma-correcting amplifiers 89a, 89b, 89c, having power characteristics with exponents = γ. In a modification of this latter arrangement which employs only one <SP>1</SP>/ γ  amplifier to provide gamma correction for all of the colour signals the output of filter 76 (Fig. 2), is converted by known means (e.g. by means of the arrangement described in Specification 772,791) to the type of signal which allows of symmetrical sampling (e.g. a "dot-sequential" type of signal) and, after gamma correction by means of an amplifier with a power characteristic exponent = <SP>1</SP>/γ, is supplied to three samplers operating at 120 degrees phase intervals which supply the signals, sequentially to the picture reproducer (Fig. 3, not shown). In a further modification which provides separate channels for the luminance and chrominance signals (i.e. a by-passed, or shunted, luminance channel) the output of the gamma correction amplifier of the Fig. 3 arrangement is supplied to a symmetrical sampler operating at the third harmonic of the colour sub-carrier wave, the sampler output, after translation through an 0-4.1 mc/s filter providing the luminance signal directly and, via a synchronous detection system controlled by a wave at the third harmonic of the subcarrier wave, the three colour difference signals. These latter signals are then combined with the luminance signal to provide the required gamma corrected colour signals (Fig. 3a, not shown). Fig. 5 shows an arrangement in which the gamma correction is effected directly in amplifier 91 without prior conversion to a "dot-sequential" type of signal and the output sampled in a third harmonic symmetrical sampler 96 which supplies (via filter 97), the luminance and chrominance signals over separate channels, (viz. amplifier 87 and amplifier 83 and synchronous detector 84 respectively) for combination in adder circuits 86a, 86b, 86c, in similar manner as described in connection with the arrangements of Figs. 3 and 3a (not shown). By operating the sampler 96 so that it conducts at the phase angle of the red component the blue and green components are sampled with small phase errors and a minimum of colour distortion results. In a modification of the transmitter of Fig. 1 a third harmonic sampler is employed between amplifier 22 and filter 23, the characteristic of the latter and also of filter 20 having the form shown in Fig. 1, or other complementary non-uniform shapes or, if desired, similar uniform pass-bands. With this arrangement any type of N.T.S.C. signal within the limits of the linearized and non-linearized types may be developed depending on the characteristics given to the filters (Fig. 6, not shown).