Signal generator for a color television transmitting system
BACKGROUND OF THE INVENTION 
     This invention relates to a signal generator (color encoder) for a color television system. An object of the invention is an improvement in the picture quality when color encoded signals are reproduced in the picture displayed by a usual color television receiver. 
     FIG. 1 is a block diagram representing a flow of signal processing at a conventional encoder and a usual television receiver, wherein in a transmitter 1, primary color signals R, G and B are converted by a matrix circuit 3 into a luminance signal. The primary color signals are also given to a matrix circuit 4 to produce color-difference signals R-Y and B-Y, the color-difference signals being fed to a modulator 5 and modulated thereat to be output as a chrominance subcarrier. The chrominance subcarrier is passed through a band pass filter 6 and added to the luminance signal at an adder 7 to be fed to a transmission line 8 as a composite color TV signal. A receiver 2 separates the luminance signal and chrominance subcarrier from the composite color TV signal through a band pass filter 9 and feeds the luminance signal and chrominance subcarrier to an amplifier 10 and a modulator 11 respectively, the demodulator 11 demodulating the chrominance subcarrier to produce a color-difference signal. At a matrix circuit 12, the luminance signal output from the amplifier 10 is added to the color-difference signals R-Y, G-Y and B-Y to produce primary color signals R, G and B and send them to a color picture tube (not shown). 
     In the above-noted conventional color television system, the bandwidth of chrominance subcarrier is narrower than that of luminance signal. Especially at the receiver side, the bandwidth of the color difference signal obtained by the demodulation of the chrominance subcarrier is about 500 KHz, which is extremely narrow in comparison with the luminance signal bandwidth of 3 MHz or 4 MHz, resulting in the following defect in picture quality. 
     The defect is that brightness reproduced on the picture has distortion with respect to its original brightness. Such a phenomenon is caused by a cooperative effect of due to the fact that the color picture tube used for the receiver has a non-linear characteristic generally called a γ-characteristic and due to the fact that the bandwidth of color-difference signal is narrow. In other words, if the color picture is high in color saturation, the brightness information reproduced on the picture is influenced by the receiver&#39;s γ-characteristic and is transmitted from the color-difference signal channel as from the well as luminance signal channel. A narrow bandwidth color-difference signal channel cannot accurately reproduce the original brightness information at the picture edge of a saturated color, and the brightness often decreases. 
     Such phenomenon is exemplarily shown in FIG. 2, in which the luminance actually reproduced on the picture tube in response to transient signals of so called color bar is illustrated. The axis of ordinate in FIG. 2 represents relative luminance to the normalized white luminance level, and the axis of abscissa a time period, only a transient response period being shown. As seen from FIG. 2, the transient signal of color bar generally produces a &#34;luminance trough&#34;. At the transient period from the green to magenta, the &#34;luminance trough&#34; is particularly larger. This is the reason for a readily appearing dark part at the boundary of green and magenta when the color bars are displayed by the receiver. Such a luminance distortion which is formed during the transient period of a color picture is a defect in the conventional color television system. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a means to eliminate the aforesaid defect without changing the circuit system at the color television receiver. Such an object is obtained by a signal generator comprising a primary color level detector for producing primary color level signal of the level substantially equal to a sum of squared primary color signals, a color-difference level detector for being supplied with wideband color-difference signals corresponding to the primary color signals, and for producing a color-difference level signal of a level which is substantially equal to a sum of squared low frequency components in the wideband color-difference signal, and a luminance signal producer for generating a luminance signal in response to both the primary color level signal and the color-difference level signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Next, the invention will be detailed with aid of the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a conventional example; 
     FIG. 2 is a graph explanatory of a defect thereof; 
     FIG. 3 is a block diagram of an embodiment of the invention; and 
     FIG. 4 is a block diagram of another exemplary construction of a color-difference level detector applicable to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Luminance L o  of an original picture designated by primary color signals R, G and B is given by the following formula: 
     
         L.sub.o =l.sub.R R.sup.γ +l.sub.G G.sup.γ +l.sub.B B.sup.γ(1) 
    
     where γ: gamma coefficient of a color picture tube, and l R , l G  and l B  : primary color tristimulus values. 
     The luminance given in the above-noted formula is of essential high-fidelity. One object of the invention is to merely change the system at the transmitting side, thereby reproducing correct luminance at the color television receiver. 
     If the luminance signal for this purpose is expressed by Y M  and color-difference signals each by (R-Y) M , (G-Y) M  and (B-Y) M , the luminance reproduced on the picture of a receiver is given in the following formula: 
     
         L=l.sub.R {Y.sub.M +(R-Y)}.sup.γ +l.sub.G {Y.sub.M +(G-Y).sub.M }.sup.γ +l.sub.B {Y.sub.M +(B-Y).sub.M }.sup.γ(2) 
    
     assuming L is equal to L o  (assuming a high-fidelity luminance reproduction performance), a desired luminance signal Y M  is obtained by the formulas (1), (2) as follows: 
     
         Y.sup.2.sub.M =l.sub.R R.sup.2 +l.sub.G G.sup.2 +l.sub.B B.sup.2 -{l.sub.R (R-Y).sup.2.sub.M +l.sub.G (G-Y).sup.2.sub.M +l.sub.B (B-Y).sup.2.sub.M }(3) 
    
     where γ≅2 and l R  (R-Y)+l G  (G-Y) +l B  (B-Y)=0 are applied. In formula (3), when (R-Y) M , (G-Y) M  and (B-Y) M  are equal to R-Y, G-Y and B-Y respectively (corresponding to assumed wide band transmission system of the color-difference signal), Y M  =Y is obtained, which is equal to the luminance signal by the conventional system. 
     Color-difference signals (R-Y) M , (G-Y) M  and (B-Y) M  in formula (3), mean the low frequency components in the wideband color-difference signal respectively. By receiving the luminance signal satisfying formula (3), a receiver can display a high-fidelity picture having the same luminance as the original picture. The formula (3) represents an approximated luminance signal which is effective for practical use of the invention, but the invention is not limited to systems using formula (3). In other words, this invention includes all signal generators for color television systems that transmit luminance signals as Y M , to make L o  and L expressed in formulas (1) and (2) substantially equal. 
     FIG. 3 is a block diagram of an embodiment of the invention, in which primary color signals R, G and B are given to multipliers 24, 25 and 26 respectively. The multipliers 24, 25 and 26 generate squared signals having substantially equal levels to the primary color signals respectively, whereby the multipliers 24, 25 and 26 alternatively may be replaced by other non-linear circuits of the same effect. The squared signals are fed into a matrix circuit 27 and added thereat to produce the primary color level signal. These elements 24 to 27 thus constitute a primary color level detector 20. In a case that the primary color signal is a digital signal having a level only of &#34;1&#34; or &#34;0&#34;, the primary color level signal can be expressed by l R  R+l G  G+l B  B and becomes the same signal as the conventional luminance signal, thereby requiring no multipliers 24, 25, 26. Such a primary color level detector is also included in the present invention. 
     On the other hand, wideband color-difference signals R-Y, G-Y and B-Y are given to low-pass filters 30, 31 and 32, and the low frequency components are extracted thereat. The low frequency components of the color-difference signals are fed to a square sum circuit 23, in which the color-difference level signal of the level substantially equal to the sum of squared low frequency components is produced. Alternatively, the square sum circuit 23 may include multipliers 33, 34 and 35 which generate squared signals of low frequency component of each color-difference signal, and a matrix circuit 36 generating color-difference level signal by adding the squared signals in a certain proportion. These elements 30 to 36 thus constitute a color-difference level detector 22. 
     The primary color level signal and color-difference level signal are given to a subtractor 28, in which the difference signal between the two signals is produced and fed to a non-linear circuit 29. The non-linear circuit 29 produces signal of the level substantially equal to a square root of the difference signal level, the signal being output as luminance signal, and the circuit 29 is readily constructed by use of, for example, diodes and resistances. In order to realize the effect of the invention to a minimum, the non-linear circuit 29 may be neglected. A luminance signal output from a luminance signal producer 21, including the subtractor 28 and non-linear circuit 29, is substantially equal to the luminance signal Y M  satisfying the formula (3), whereby the usual television receiver can reproduce pictures of high quality and free from luminance distortion. 
     FIG. 4 shows a modified embodiment of the color-difference level detector 22, in which color-difference signals R-Y, B-Y are given to a modulator 5 and transformed to a phase and amplitude modulated signal, and thereafter limited in bandwidth by a band-pass filter 6. The modulated signal limited in bandwidth is substantially equal to a usual chrominance subcarrier, which is synchronously demodulated by a demodulator 11 to output the low frequency components (R-Y) M , (G-Y) M  and (B-Y) M  of color-difference signals. 
     The aforesaid modulator 5, band-pass filter 6 and demodulator 11 may be equivalent to those used in the conventional example. The low frequency components of color-difference signals are fed to the square sum circuit 23 to thereby produce thereat a color-difference level signal of the level substantially equal to the sum of squared low frequency components. 
     The embodiment of the invention materializes function corresponding to the low-pass filter in the color-difference level detector by use in common of the modulator and band-pass filter used in the conventional encoder, thereby being effective to simplifying the circuit. 
     As seen from the above, this invention is effective to basically decrease the luminance distortion found in the conventional color television transmitting and receiving system without any change in the television receiver, thereby having an extremely large effect.