Picture-in-picture color television receiver
BACKGROUND OF THE INVENTION 
     The present invention relates to a television receiver for simultaneously displaying a plurality of programs on a single screen of a television receiver. 
     A demand of effective use of a television screen drives one to recently develope a television receiver of the type in which a small size picture (hereinafter referred to as a sub-picture) is displayed in a large size picture (hereinafter referred to as a main picture), those pictures being assigned for different programs, respectively, as disclosed in Japanese Laid-open Patent Publication No. 2419/74, DT-OS No. 2,239,593, and electronics journal &#34;Nikkei Electronics&#34; Dec. 26, 1977, p.p. 127 to 134 published in Japan. This type of the television receiver is generally called a picture-in-picture (PinP) television. The PinP television will be described in brief referring to FIGS. 1 to 3. 
     In FIG. 1 illustrating a concept of the PinP TV, reference numeral 1 designates a TV receiver; 2 a cathode ray tube (hereinafter referred to as a CRT); 3 a main picture portion; 4 a sub-picture portion having a picture different from that in the main picture portion. The main picture and sub-picture portions are so arranged as to individually select different channels for displaying pictures of the selected channels. 
     FIG. 2 diagramatically illustrates an example how the sub-picture is inserted into the main picture. In the figure, reference character I designates the sub-picture before its size is reduced and II the main picture with the sub-picture reduced. It is assumed now that a picture reduction rate of the sub-picture is 1/3 for both the vertical and horizontal sides of the picture. Here, the picture reduction rate is defined as a ratio of a scanning period after it is reduced and a scanning period of the original signal. The scanning lines are extracted from the sub-picture before it is reduced, one for three scanning lines. The horizontal period is time-reduced into 1/3. Then, it is synchronized with the main picture. Finally, thus processed picture is inserted into the main picture. Scanning lines  1  to  3  are exemplary lines of those of the picture before and after it is reduced. 
     FIG. 3 is a circuit construction of part of a conventional PinP TV receiver, which relates to the present invention. In the figure, reference numeral 11 designates an antenna; 12 a sub-picture insertion circuit; 13 a video signal processing circuit; 14 a display device using a CRT; 21 and 31 tuners for the main picture and sub-picture; 22 and 32 IF video detecting circuits for the main picture and sub-picture, respectively. 
     A signal for the sub-picture is stored into a memory 41 under control of a synchronizing (hereinafter abbreviated as a sync) signal obtained from a sync separation circuit 33 and a clock signal from a write clock generating circuit 42. Reference numeral 43 designates a read clock generating circuit. The sub-picture signal from the memory 41 is read out under control of the signal from the sync separation circuit 23 for each scanning line, and then is inserted into a main picture signal. The example is for a case where both the main picture and sub-picture are of the monochrome type. 
     When both the pictures are of the color type, the sub-picture signal is read out from the memory, and the read out one and the main picture signal are composed. The reference level of the color difference signal changes due to temperature drift and aging of the circuit components. As a result, the white balance by the main picture signal is not coincident with that by the sub-picture signal. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a PinP color television receiver in which the demodulated color signal reference level of the second picture signal read out from the memory is made coincident with that of the demodulated color signal of the first picture signal, whereby the white balance by the first picture signal is coincident with that by the second picture signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic elevation view of a conventional PinP television receiver. 
     FIG. 2 is a diagram for illustrating scanning lines when the sub-picture is inserted into the main picture in the conventional PinP television receiver. 
     FIG. 3 is a block diagram of the conventional PinP television receiver. 
     FIG. 4 is a block diagram of an embodiment of a PinP television receiver according to the present invention. 
     FIG. 5 is a block diagram partially including a circuit diagram which is useful in explaining in detail a part of the circuit shown in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of a PinP color television receiver according to the present invention will be described in detail referring to the accompanying drawings. 
     A circuit shown in FIG. 4 corresponds to the conventional circuit shown in FIG. 3 but for further including a means for generating the sub-picture color difference signal and a DC restoration circuit for stabilizing the reference level of the color difference signal in the sub-picture signal. In FIG. 4, like reference numeral designate like or equivalent portions in FIG. 3. The reference level of the color difference signal means an output signal level of an output signal from a demodulator circuit for the color difference signal when the chrominance subcarrier signal is not supplied to the demodulator circuit. 
     In FIG. 4, 41B designates a B-Y color difference signal memory, 41R an R-Y color difference signal memory, 44 a G-Y color difference signal matrix, 45B, 45G and 45R DC restoration circuit for the respective color difference signals, 46B, 46G and 46R color difference signal inserting circuits for composing the main picture and the sub-picture, 15 a video signal processing circuit, 24 a main picture color signal processing circuit, 34 a sub-picture color signal processing circuit. 
     The sub-picture color difference signal is properly processed by the sub-picture color signal processing circuit 34. Color difference signals B-Y and R-Y produced from the circuit 34 are stored in the memories 41B and 41R from which these signals are read out in a manner similar to that described relating to FIG. 3. The B-Y color difference signals read out from the B-Y color difference signal memory 41B and the R-Y color difference signal memory 41R are inputted into the G-Y color difference signal matrix circuit 44 which in turn produces the G-Y signal. 
     The three color difference signals thus obtained are supplied to the color difference signal inserting circuits 46B, 46G and 46R, through DC restoration circuits 45B, 45G and 45R, respectively. Those signals are added to the respective color difference signals of the main picture obtained from the main picture signal processing circuit 24, respectively, and the added ones are supplied to the CRT 14 for display. 
     The operation of the DC restoration circuit 45B will be described in detail with reference to FIG. 5. The DC restoration circuit 45B is divided into a control signal generating section 110 which receives the reference level of the main picture and produces a control signal corresponding to the reference level, and a DC restoration section 120. The operations of the DC restoration circuits 45G and 45R are the same as that of the DC restoration circuit 45B, and therefore no explanation of those circuits will be given. In FIG. 5, like reference numerals designate like or equivalent portions in FIG. 4. In the Figure, reference numeral 101 designates a DC restoration control signal generating circuit; 102 a power source; 103 a transistor operating as an emitter follower; 104 and 105 capacitors; S 1  and S 2  switch circuits. 
     A switch circuit S 1  included in the DC restoration circuit 45B is closed for a given period of a period having no B-Y color difference signal read out from the B-Y memory 41 by a control signal a obtained from the DC restoration control signal generating circuit 101. The given period corresponds to a main picture scanning period (a in FIG. 2) of the horizontal scanning period and the horizontal flyback period. As a result of the closing of the switch circuit S 1 , the capacitor 105 is charged up to the emitter potential of the transistor 103. During the period other than the above-mentioned one, the switch circuit S 1  is kept open. The DC restoration is performed in this way. 
     The base of the transistor 103 is connected through the switch S 2  to the B-Y color signal difference terminal of the main picture color signal processing circuit 24. The switch circuit S 2  is closed only during a period that the reference level of the B-Y color difference signal of the main picture is produced, for example, the horizontal flyback period. As a result, the capacitor 104 is charged up to the reference level of the main picture B-Y color difference signal. During the other period than the above-mentioned one, when the switch circuit S 2  is opened, the base potential of the transistor 103 is maintained at the reference level of the main picture B-Y color difference signal. 
     Accordingly, in the DC restoration circuits 45B, 45G and 45R, the reference level of the sub-picture color difference signal depends on the reference level of the main picture color difference signal. As a result, the white balance of the main picture may be coincident with that of the sub-picture on the CRT screen. 
     As seen from the foregoing, no shift of the white balance of the main picture from that of the sub-picture is attained even when the reference level of the main picture, for example, changes due to temperature drift, by DC-restoring the reference level of the color difference signal of the sub-picture signal with the reference level of the sub-picture color difference signal. 
     The above-mentioned embodiment has been described by using the PinP television receiver in which the sub-picture (or a size-reduced picture) is inserted into the main picture (or a non-reduction picture). It is evident from the foregoing, however, that the present invention is applicable for a television receiver of the type in which a part of the non-reduction picture or a part of an expanded picture is used as the sub-picture or two or more sub-picture are used.