Signal Processing Unit

The present invention relates to a signal processing unit in a television system, and more particularly, to a signal processing unit for outputting one video signal out of two input video signals.

In recent years, such a multiplex television broadcast system has been developed that along with video signals corresponding to ordinary television images, multiplex broadcast signals such as video signals or coded signals corresponding to other images such as characters, figure patterns, etc. are also transmitted during vertical retrace periods, and on a receiver side, either of the ordinary television video signals and the multiplex broadcast signals are selected on demand to display either the ordinary television images or the other images transmitted as the multiplex broadcast. The multiplex broadcast images may be displayed on a part or whole of the image screen. In the United Kingdom and France, such systems have been put in practical use as "TELETEXT" and "ANTIOPE", respectively. In the United States and Japan, it is still in the experimental stage.

The multiplex broadcast signals corresponding to the multiplex broadcast images, which are transmitted during vertical retrace periods, have little information content because the vertical retrace period is short and signals necessitated for interlacing in the next field such as equalizing pulses, horizontal synchronizing pulses, etc. are also transmitted during the same period. Therefore, on the receiver side, the multiplex broadcast signals transmitted and received during the vertical retrace periods are first stored in a memory device. Then, video signals corresponding to the multiplex broadcast images are processed on the basis of the stored signals to obtain three primary color signals (R', G', B'). On the other hand, video signals corresponding to the ordinary television images are derived as three primary color signals (R, G, B) by means of a conventonal video signal processing circuit. Driving a picture tube by means of the three primary color signals, images are reproduced on the image screen of the picture tube. In the following, for the purpose of discriminating the primary color signals corresponding to the ordinary television images from the primary color signals corresponding to the multiplex broadcast images, the former are called simply "original primary color signals," while the latter are called "external primary color signals".

In order to display the multiplex broadcast images on a part or whole of the image screen of the picture tube in place of the ordinary television images, a signal switching unit is necessary for switching the original primary color signals and the external primary color signals to apply one of them to a picture tube. What is required for the signal switching unit is to maintain the pedestal potential and the maximum amplitude level between the original primary color signals and the external primary color signals at the same constant values so that the brightness and contrast of the images may be identical even when switching is effected between the ordinary television images and the multiplex broadcast images. Moreover, it is also important that the signal switching unit should be easily coupled to a video signal processing circuit for processing the video signals corresponding to the ordinary television images to derive the original primary color signals. The signal switching unit should preferably be formed as a monolithic integrated circuit with a few external parts. Furthermore, the signal switching unit is required not to be affected by the voltage variation in a reference voltage for giving a pedestal potential due to the change in the environmental temperature.

In some cases, the maximum amplitude level of the original primary color signals may be higher than that of the external primary color signals. For instance, in the "ANTIOPE" system in France, the maximum amplitude level of the original primary color signals is preset at 3 Vp-p (peak-to-peak), while that of the external primary color signals is preset at 1 Vp-p. In this case, if the maximum amplitude levels of the respective primary color signals are not equalized, a difference is observed in the displaced pictures, especially in their contrasts. In order to equalize the maximum amplitude levels of the respective primary color signals, by way of example, the external primary color signals (R', B', G') may be preliminarily amplified so as to be equalized to the amplitudes of the original primary color signals. However, this method has shortcomings in that the dynamic range of the signal switching unit becomes narrow and it is difficult to provide a contrast control circuit or the like subsequent to the signal switching unit. In another example, it is also possible to equalize amplitude level of the original primary color signals to that of the external primary color signals by reducing the gain of the video signal processing circuit for deriving the primary color signals from the ordinary color television image signals, and to then select the primary color signals by the signal switching unit and amplify them. However, according to this method, the gain of the video signal processing circuit is reduced, and therefore a leakage current of a carrier component contained in a demodulated color difference signal would become large. As a result, the S/N ratio of the displayed picture is degraded.

Thus, the signal switching unit for switching the primary color signals of the ordinary television images and the external primary colors signals of the multiplex broadcast images to output one primary color signal group must fulfil the above-mentioned essential conditions. However, a signal switching unit which can fulfil these essential conditions has not been provided in the prior art.

It is therefore one object of the present invention to provide a signal processing circuit suitable to a monolithic integrated circuit for outputting either original primary color signals or external primary color signals whose pedestal potentials are equalized with each other, and which is not affected by the voltage variation in a reference voltage for defining the pedestal potential caused by the change in the environmental temperature.

Another object of the present invention is to provide a signal processing circuit which receives original primary color signals and external primary color signals whose maximum amplitude levels are different from each other, and outputs either one of them after equalizing their maximum amplitude levels.

According to one aspect of the present invention, a signal processing circuit comprises signal switching means adapted to receive a first signal or signals for a first image and a second signal or signals for a second image for outputting either one of them, clamp means for clamping a pedestal potential of the second signal or signals at a reference voltage, comparator means for comparing a pedestal potential of the first signal or signals with the reference voltage, and feedback means responsive to the output of the comparator means for equalizing the pedestal potential of the first signal or signals to the reference voltage.

According to another aspect of the present invention, the signal processing unit further comprises attenuator means having input, output and common terminals, and the first signal is applied to the input terminal of the attenuator means and derived from the output terminal thereof to be applied to the signal switching means. Preferably, the attenuation factor of the attenuator means is preset such that the maximum amplitudes of the first and second signals may be equalized, and the common terminal of the attenuator means is applied with the reference voltage.

In the signal processing circuit according to the present invention, the pedestal potential of the second signal is clamped at the reference voltage, so that the both voltages become equal to each other. The pedestal potential of the first signal is compared with the reference voltage and then equalized to the second signal by the feedback means. Accordingly, either one of the first and second signals, whose pedestal potentials have been equalized to the reference voltage, can be output from the signal switching means. In addition, even if the reference voltage is varied due to the change in the environmental temperature, the pedestal potentials of the first and second signals are still kept equalized to each other by the comparator means and feedback means and by the clamp means, respectively. Accordingly, either one of the first and second signals having the same pedestal potential can be outputted free from the variation of the environmental temperature.

In the case where the maximum signal level of the first signal is higher than that of the second signal, the first signal is applied through the attenuator means to the signal switching means. Accordingly, either one of the first and second signals having the same maximum amplitude level can be outputted without narrowing the input dynamic range of the signal switching means and, moreover, without lowering the gain of the circuit for generating the first signal. At this time, the reference voltage is applied to the common terminal of the attenuator means. Therefore, when the attenuation factor of the attenuator means is preset such that the maximum amplitude levels of the first and second signals may be equalized, the D.C. potential, that is, the pedestal potential of the first and second signals, would not be varied but would be equalized to the reference voltage, and only the A.C. component of the first signal would be attenuated to the same maximum amplitude level as the second signal.

The above-mentioned and other objects, features and advantages of the present invention will become more apparent from the following description of the invention taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram showing one preferred embodiment of the present invention.

FIG. 2 is a block diagram showing another preferred embodiment of the present invention.

FIG. 3 is a circuit diagram showing a part of the blocks shown in FIG. 2 in greater detail.

Now one preferred embodiment of the present invention will be described with reference to FIG. 1. The ordinary television image signal transmitted from a broadcast station is converted into a composite video signal. The composite video signal is applied to a terminal T.sub.10 of a monolithic integrated circuit 10 which acts as a video signal processing circuit for deriving original primary color signals (R, G, B). The composite video signal applied to the terminal T.sub.10 is led to a demodulator 90 where three color difference signals R-Y, G-Y and B-Y are generated. In addition, a luminance signal Y is produced from the video signal of the ordinary television image, and applied to another terminal T.sub.16 of the monolithic integrated circuit 10. The luminance signal Y applied to the terminal T.sub.16 is fed to an RGB matrix circuit 30 through a contrast control amplifier 40 whose gain is controlled according to a voltage applied to a contrast control terminal T.sub.11. The color difference signals R-Y, G-Y and B-Y generated by the demodulator circuit 90 are also applied to the RGB matrix circuit 30. Hence, the RGB matrix circuit 30 generates original primary color signals R, G and B of the ordinary television image in response to the luminance signal Y and the color difference signals R-Y, G-Y and B-Y. The generated original primary color signals R, G and B have their pedestal potentials clamped at the voltage generated by a brightness control circuit 50 in accordance with the voltage applied to a brightness control terminal T.sub.12, by means of pedestal clamp circuits 60, 70 and 80, respectively, and the clamped original primary color signals R, G and B are generated from output terminals T.sub.13, T.sub.14 and T.sub.15, respectively. The above-mentioned video signal processing circuit 10 has been commonly used in conventional television receiver sets and involves nothing novel.

On the other hand, multiplex broadcast image signals representing characters, figure patterns, etc. which have been transmitted during a vertical retrace period, are fed to a multiplex broadcast image signal processing circuit 500. As described previously, the multiplex broadcast image signal processing circuit 500 first stores the multiplex broadcast image signals and then processes the stored signals to generate external primary color signals R', G' and B' at its output.

The original primary color signals R, G and B fed from the video signal processing circuit 10 and the external primary color signals R', G' and B' fed from the multiplex broadcast image signal processing circuit 500 are respectively applied to separate groups of primary color signal input terminals T.sub.103, T.sub.104 and T.sub.105, and T.sub.106, T.sub.107 and T.sub.108. The two red primary color signals R and R' are applied to an R-switching circuit 110 through the terminals T.sub.103 and T.sub.106, respectively. Likewise, the two green primary color signals G and G' and two blue primary color signals B and B' are respectively applied to a G-switching circuit 120 and a B-switching circuit 130. The R-, G- and B-switching circuits 110, 120 and 130 apply either one of the original primary color signals R, G and B and the external primary color signals R', G' and B' to output circuits 190, 200 and 210, respectively, in response to the control signal level applied to a switching control terminal T.sub.115. The output circuits 190, 200 and 210 finally control the pedestal potentials and the amplitude levels of the primary color signals R, G and B or R', G' and B' to be fed to output terminals T.sub.111, T.sub.112 and T.sub.113 in accordance with the voltages applied to an output contrast control terminal T.sub.109 and an output brightness control terminal T.sub.110, respectively. The primary color signals R, G and B or R', G' and B' derived from the output terminals T.sub.111, T.sub.112 and T.sub.113 are amplified by output amplifiers (not shown) and then applied to a picture tube (not shown) to display a picture image on an image screen. The output brightness control terminal T.sub.110 and the output contrast control terminal T.sub.109 are respectively connected to potentiometers associated with a brightness adjusting knob and a contrast adjustment knob provided on a television receiver set.

The external primary color signals R', G' and B' fed from the multiplex broadcast image signal processing circuit 500 have their pedestal potentials clamped at a reference voltage V.sub.ref supplied from a reference voltage source 180 by means of pedestal clamp circuits 140, 150 and 160, respectively, and are then applied to the R-, G- and B-switching circuits 110, 120 and 130, respectively.

Among the pedestal potentials of the original primary color signals R, G and B fed from the video signal processing circuit 10, the pedestal potential of the red primary color signal R, selected as a representative signal, is compared with the reference voltage V.sub.ref by means of a comparator 170. The comparator 170 can operate only when a gate pulse is applied to a gate pulse input terminal T.sub.114 to compare the above-mentioned two potentials. The gate pulse is applied only during a pedestal potential period of the primary color signals R, G and B. This is for the purpose of preventing the A.C. components of the original primary color signals R, G and B from being varied by the output of the comparator 170, as will become apparent from the description below. Accordingly, the gate pulse is generated at the same timing as the clamp pulses applied to the pedestal clamp circuits 60, 70 and 80 and 140, 150 and 160. If desired, the gate pulse is subjected to level conversion and then applied to the comparator 170.

The output of the comparator 170 is applied via a comparator output terminal T.sub.100 to an external voltage holding circuit 300. This voltage holding circuit 300 generates and holds a voltage at its output end in response to the output of the comparator 170 via the terminal T.sub.100. The output voltage of the voltage holding circuit 300 is applied to a brightness control terminal T.sub.12 of the video signal processing circuit 10. Accordingly, the pedestal potentials of the original primary color signals R, G and B fed from the video signal processing circuit 10 are determined depending upon the output voltage from the voltage holding circuit 300. Thus, the output voltage of the voltage holding circuit 300 is controlled by the output of the comparator 170, and the output of the comparator 170 is varied in accordance with the voltage difference between the reference voltage V.sub.ref of the reference voltage source 180 and the pedestal potential of the red primary color signal R applied from the input terminal 103. In other words, a feedback loop is established through the comparator 170 and the voltage holding circuit 300 so that the reference voltage V.sub.ref and the pedestal potential may be equalized to each other. Owing to the existence of this feedback loop, the pedestal potentials of the original primary color signals R, G and B fed from the video signal processing circuit 10 become equal to the pedestal potentials of the external primary color signals R', G' and B' fed from the multiplex broadcast image signal processing circuit 500.

Owing to the above-mentioned feedback loop, the voltages of the original primary color signals R, G and B fed from the video signal processing circuit 10 are controlled to be equal to the reference voltage V.sub.ref. Accordingly, if the comparator 170 is continuously operating, the A.C. components of the original primary color signals R, G and B would be adversely affected. Therefore, the comparator 170 is actuated only during the period when a gate pulse generated at the same timing as the clamp pulse applied to the pedestal clamp circuits 60, 70 and 80 and 140, 150 and 160 is applied to the terminal T.sub.114. If necessary, the same pulse as the clamp pulse could be used as the gate pulse.

As is apparent from the above description, the pedestal potentials of the original primary color signals R, G and B and the pedestal potentials of the external primary color signals R', G' and B' become equal to each other. Therefore, even if the multiplex broadcast image is displayed on a part or whole of the image screen by switching the signals to be derived from the output terminals T.sub.111, T.sub.112 and T.sub.113 by means of the R-, G- and B-switching circuits 110, 120, and 130, the brightness and contrast of the displayed picture image on the image screen would be the same. It is to be noted that the above-described embodiment is designed for the case where the maximum amplitude levels of the original primary color signals R, G and B and the external primary color signals R', G' and B' are equal to each other.

Furthermore, even if the reference voltage V.sub.ref fed from the reference voltage source 180 is varied due to the change in the environmental temperature, the output of the comparator 170 will vary by the corresponding amount. As a result, the aforementioned feedback loop operates so that the pedestal potentials of the primary color signals R, G and B may vary in accordance with the variation of the reference voltage V.sub.ref. In other words, even if the environmental temperature changes, the pedestal potentials of the original primary color signals R, G and B and the pedestal potentials of the external primary color signals R', G' and B' are kept equal to each other. Accordingly, no special circuit arrangement for compensating a temperature change is necessitated.

In addition, owing to the existence of the feedback loop including the pedestal clamp circuits 60, 70 and 80, the comparator 170, and the voltage holding circuit 300, the original primary color signals R, G and B and the external primary color signals R', G' and B' can be inputted to the signal processing circuit 100 by D.C. coupling, and so, the circuit arrangement is suitable to be formed in a monolithic integrated circuit. Furthermore, it is to be noted that for the purpose of equalizing the pedestal potentials of the original primary color signals R, G and B and the external primary color signals R', G' and B', no circuit modification has been made in the video signal processing circuit 10 nor in the multiplex broadcast image signal processing circuit 500. In other words, a television receiver set that can also receive a multiplex broadcast without undesired variation in brightness and contrast of a displayed picture image can be realized by adding the signal processing circuit according to the present invention to a conventional television receiver set. As described above, the signal processing circuit according to the present invention can be formed in an integrated circuit for the interface use that is preferable for multiplex broadcast.

FIG. 2 shows another preferred embodiment of the present invention in which component parts having the same functions as those shown in FIG. 1 are given like reference numerals or symbols, and further description thereof will be omitted here. The embodiment shown in FIG. 2 includes countermeasures for the case where the maximum amplitude levels of the original primary color signals R, G and B is higher than those of the external primary color signals R', G' and B'. To that end, attenuators 220, 230 and 240 each having input, output and common ends are provided in the signal processing circuit 100. The respective input ends of the attenuators 220, 230 and 240 are connected to the terminals T.sub.103, T.sub.104 and T.sub.105, and the output ends thereof are connected to the R-, G- and B-switching circuits 110, 120 and 130, respectively. The common ends of the attenuators 220, 230 and 240 are connected to the reference voltage (V.sub.ref) terminal of the reference voltage source 180. The output of the attenuator 220 chosen as a representative one of the attenuators 220, 230 and 240 is applied to the comparator 170. The attenuation factors of the respective attenuators 220, 230 and 240 are preset such that the maximum amplitude levels of the original primary color signals R, G and B may become equal to those of the external primary color signals R', G' and B'.

Accordingly, the A.C. components of the original primary color signals R, G and B fed from the video signal processing circuit 10 are attenuated by the attenuators 220, 230 and 240, respectively, so that original primary color signals R, G and B having the same maximum amplitude levels as the external primary color signals R', G' and B' are applied to the R-, G- and B-switching circuits 110, 120 and 130. In other words, the input signal levels applied to the switching circuits 110, 120 and 130 are equalized to the lower levels, and therefore, the effective input dynamic range of the switching circuits 110, 120 and 130 is not decreased. Moreover, since the above-mentioned method is different from the method of lowering the amplitude gain of the original primary color signals R, G and B by manipulating the video signal processing circuit 10, a leakage current of a carrier component contained in the color difference signal is not increased. In other words, the original primary color signals R, G and B derived from the video signal processing circuit 10 are in themselves attenuated, and the carrier component is therefore attenuated together with the signal component. As a result, there is substantially no degradation in the S/N ratio of the output signal applied to the picture tube for displaying television picture image. The D.C. levels of the outputs of the attenuators 220, 230 and 240 are equalized to the reference voltage V.sub.ref by the effects of the feedback loop including the comparator 170. The common ends of the attenuators 220, 230 and 240 are connected to the reference voltage V.sub.ref. Consequently, the D.C. voltage of the original primary color signals R, G and B, that is, their pedestal potentials, are the same at the input ends and at the output ends of the attenuators 220, 230 and 240, and thus there is no variation in the D.C. potentials. Accordingly, the input signal to the attenuator 220, 230 or 240 may be applied to the comparator 170 to obtain the same the pedestal potential as the reference voltage V.sub.ref.

As described above, by providing the attenuators 220, 230 and 240 within the signal processing circuit 100, either one of the original primary color signal group (R, G, B) and the external primary color signal group (R', G', B') whose pedestal potentials and maximum amplitude levels have been equalized, can be selectively output. Accordingly, in the same manner as the first preferred embodiment shown in FIG. 1, by adding the signal processing circuit according to the present invention to the conventional television receiver set, it becomes possible to display multiplex broadcast images in addition to or in place of the ordinary television images.

FIG. 3 shows detailed circuit constructions of the attenuator 220, R-switching circuit 119, comparator 170 and voltage holding circuit 300 in FIG. 2. Other circuit blocks having the same functions as those shown in FIG. 2 are given like reference numerals, and further description thereof will be omitted here. In addition, since each of the attenuators 230 and 240 and each of the G-switching circuit 120 and the B-switching circuit 130 in FIG. 2 have the same circuit constructions as those of the attenuator 220 and the R-switching circuit 110 illustrated in FIG. 3, for simplicity of the illustration, only the signal paths relating to the red original primary color signal R and the red external primary color signal R' within the signal processing circuit 100 according to the present invention are shown in FIG. 3.

In FIG. 3, the attenuator 220 comprises a resistor R.sub.100 connected between the input and output ends of the attenuator and a resistor R.sub.101 connected between the output and common ends.

A red original primary color signal R through the attenuator 220 is applied to the base of a transistor Q.sub.100 within the R-switching circuit 110. The emitter of the transistor Q.sub.100 is grounded via a resistor R.sub.102. The collector of the transistor Q.sub.100 is connected to the emitter common junction point of transistors Q.sub.101 and Q.sub.102. A red external primary color signal R' applied via a terminal T.sub.106 is fed to the base of a transistor Q.sub.103 within the R-switching circuit 110. The emitter of the transistor Q.sub.103 is grounded via a resistor R.sub.103, and its collector is connected to the emitter common junction point of transistors Q.sub.104 and Q.sub.105. The resistors R.sub.102 and R.sub.103 are selected to have the same resistance value. The bases of the transistors Q.sub.101 and Q.sub.105 are connected in common and are also connected to the switching control terminal T.sub.115. The bases of the transistors Q.sub.102 and Q.sub.104 are connected in common and are also connected via an inverter I.sub.100 to the switching control terminal T.sub.115. The collectors of the transistors Q.sub.101 and Q.sub.104 are connected in common and also connected to a power supply terminal T.sub.116 that is not shown in FIG. 2. The collectors of the transistors Q.sub.102 and Q.sub.105 are commonly connected via a load resistor R.sub.104 to the power supply terminal T.sub.116 and are further connected to the output circuit 190 shown in FIG. 2.

The pedestal potential of the original primary color signal R is compared with a reference voltage V.sub.ref of a reference voltage source 180 by means of the comparator 170. Hence, the output of the attenuator 220 is applied to the base of a transistor Q.sub.107, and the reference voltage V.sub.ref is applied to the base of a transistor Q.sub.108. The emitters of the transistors Q.sub.107 and Q.sub.108 are connected in common and are also connected to the collector of a transistor Q.sub.106. The emitter of the transistor Q.sub.106 is grounded via a resistor R.sub.105 and its base is connected to the gate pulse input terminal T.sub.114. Accordingly, the transistor Q.sub.106 forms a constant current source for the transistors Q.sub.107 and Q.sub.108, and at the same time controls the operation period of the comparator 170 because the transistor Q.sub.106 becomes conductive only when the gate pulse is applied to the terminal T.sub.114. The collectors of the transistors Q.sub.107 and Q.sub.108 are respectively connected to the power supply terminal T.sub.116 through collector-emitter paths of transistors Q.sub.109 and Q.sub.110. The bases of the transistors Q.sub.109 and Q.sub.110 are connected in common and are connected also to the collector of the transistor Q.sub.110. Accordingly, the transistors Q.sub.109 and Q.sub.110 form an active load for the transistors Q.sub.107 and Q.sub.108. The output of the comparator 170 is derived from the collector of the transistor Q.sub.107. This output of the comparator 170 is applied via the terminal T.sub.100 to the voltage holding circuit 300.

The voltage holding circuit 300 comprises resistors R.sub.301 and R.sub.302 for dividing the operating voltage V.sub.cc applied to the terminal T.sub.116. One end of a resistor R.sub.303 is connected to the junction point between the resistors R.sub.301 and R.sub.302, and the other end is grounded through a capacitor C.sub.300. The junction point between the resistor R.sub.303 and the capacitor C.sub.300 is connected to the terminal T.sub.100 and is also connected to a terminal T.sub.12 of a video signal processing circuit 10. The signal processing circuit 10 also has a ground terminal T.sub.117 that is not shown in FIG. 2, and the resistors R.sub.102, R.sub.103 and R.sub.105 each have one end connected to the ground terminal T.sub.117.

Now, description will be made on the operation of the circuit shown in FIG. 3. The red original primary color signal R fed from the video signal processing circuit 10 has a maximum amplitude level of 3 Vp-p. On the other hand, the red external primary color signal R' has a maximum amplitude level of 1 Vp-p. The resistances of the resistors R.sub.100 and R.sub.101 are preset respectively at 2 K.OMEGA. and 1 K.OMEGA., so that the attenuation factor of the attenuator 220 is equal to 5/8. Accordingly, the red primary color signal R passed through the attenuator 220 has a maximum amplitude level of 1 Vp-p which coincides with that of the red external primary color signal R'.

The red external primary color signal R' has its pedestal potential clamped at the reference voltage V.sub.ref of 2 V by means of a pedestal clamp circuit 140, and then it is applied to the base of the transistor Q.sub.103.

The red original primary color signal R passed through the attenuator 220 is applied not only to the base of the transistor Q.sub.100 in the R-switching circuit 110 but also to the base of the transistor Q.sub.107 in the comparator 170. As noted previously, the transistor Q.sub.106 becomes conductive only during the period when the gate pulse is applied to the terminal T.sub.114, so that the comparator 170 is brought into an operable condition. The application of the gate pulse to the terminal T.sub.114 is effected when the red original primary color signal R presents its pedestal potential, and therefore, the transistors Q.sub.107 and Q.sub.108 therefore compare the pedestal potential of the red original primary color signal R with the reference voltage V.sub.ref. Assuming now that the reference voltage V.sub.ref is higher than the pedestal potential of the red original primary color signal R, the current flowing through the transistor Q.sub.108 is large, while the current flowing through the transistor Q.sub.107 is small. The transistors Q.sub.109 and Q.sub.110 not only form an active load, but also constitute a current-mirror circuit. Hence a current equivalent to the current flowing through the transistor Q.sub.110 flows through the transistor Q.sub.109. However, the current flowing through the transistor Q.sub.107 is small. Consequently, a part of the current flowing through the transistor Q.sub.109 outflows via the terminal T.sub.100 into the voltage holding circuit 300 as a charging current for the capacitor C.sub.300.

The resistors R.sub.301 and R.sub.302 in the voltage holding circuit 300 have resistances of 1 K.OMEGA. and 5.1 K.OMEGA., respectively, and the resistance and capacitance values of the resistor R.sub.303 and the capacitor C.sub.300 are chosen to be 56 K.OMEGA. and 2.2 .mu.F, respectively, to establish a large time constant for holding a voltage. Under a stationary state, the capacitor C.sub.300 has been charged through the resistor R.sub.303 and is in a predetermined voltage holding condition. Since the power supply voltage V.sub.cc is 12 V, the voltage across the capacitor C.sub.300 is about 9 V.

However, when a charging current is fed from the transistor Q.sub.109, that is, when the reference voltage V.sub.ref is higher than the pedestal potential of the red original primary color signal R, then the voltage across the capacitor C.sub.300 will rise. As a result, the voltage applied to the brightness control terminal T.sub.12 of the video signal processing circuit 10 also rises, so that the pedestal potential of the red original primary color signal R is raised by means of the brightness control circuit 50 and the pedestal clamp circuit 60. On the other hand, in the event that the pedestal potential of the red original primary color signal R is higher than the reference voltage V.sub.ref, a current flowing through the transistor Q.sub.107 is larger than that through the transistor Q.sub.108. However, the transistor Q.sub.109 cannot feed an ample current to the transistor Q.sub.107. Consequently, a current is fed from the capacitor C.sub.300 to the transistor Q.sub.107. In other words, the capacitor C.sub.300 is discharged and hence the voltage accross the capacitor C.sub.300 is lowered. As a result, the pedestal potential of the red original primary color signal R is lowered. Owing to the above-mentioned feedback loop, the pedestal potential of the red original primary color signal R can be equalized to the reference voltage V.sub.ref. Since the common terminal of the attenuator 220 is connected to the reference voltage V.sub.ref point, variation of a D. C. component of the primary color signal R does occur.

As described above, a red original primary color signal R and a red external primary color signal R' whose pedestal potentials as well as maximum amplitude levels have been equalized to each other, are applied to the R-switching circuit 110.

Assuming now that the switching signal at a high level has been applied to the terminal T.sub.115, then the transistors Q.sub.101 and Q.sub.105 become conducting. On the other hand, a signal produced by inverting the switching signal through the inverter I.sub.100 is applied to the bases of the transistors Q.sub.102 and Q.sub.104, and these transistors Q.sub.102 and Q.sub.104 are therefore turned OFF. Accordingly, the red original primary color signal R applied via the transistor Q.sub.100 does not appear across the resistor R.sub.104 because the transistor Q.sub.102 is held OFF. On the other hand, the transistor Q.sub.105 is held ON, and hence the red external primary color signal R' appears across the resistor R.sub.104 via the transistor Q.sub.103 and is applied to the output circuit 190. In other words, when a high level signal is applied to the terminal T.sub.115, the red external primary color signal R' is output.

On the other hand, when a switching signal at a low level is applied to the terminal T.sub.115, the transistors Q.sub.101 and Q.sub.105 are turned OFF, while the transistors Q.sub.102 and Q.sub.104 are turned ON. As a result, the red original primary color signal R would be applied to the output circuit 190. Since the resistors R.sub.102 and R.sub.103 have the same resistance value (2.2 K.OMEGA.) as described above, the transistors Q.sub.100 and Q.sub.103 have the same amplification gain. In addition, the transistors Q.sub.101, Q.sub.102, Q.sub.104 and Q.sub.105 form a dual differential type switching circuit. As a result, the relationships of the pedestal potentials and the maximum amplitude levels between the two input signals R and R' applied to the R-switching circuit 110 can be maintained, and one of the signals can be selectively applied to the output circuit 190.

As described above, according to the present invention, there is provided a signal processing circuit applicable to a signal switching circuit which can not be effected by a change in the environmental temperature and which maintains pedestal potentials and maximum amplitude levels of two signal groups equal to each other, and which is also suitable for formation in an integrated circuit.

It is to be noted that, in the case where the original primary color signals and the external primary color signals have the same maximum amplitude level, the attenuator 220 shown in FIG. 3 becomes unnecessary. In addition, the constructions of the comparator 170 and the switching circuit 110 are not necessarily limited to the illustrated circuit arrangement. Moreover, the signal processing circuit according to the present invention can be matched with any other video signal processing circuit having at least a brightness control terminal and primary color signal output terminals.