GB2229877A

GB2229877A - Video intermediate frequency processing circuit

Info

Publication number: GB2229877A
Application number: GB9003614A
Authority: GB
Inventors: Takesi Adachi
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1989-03-08
Filing date: 1990-02-16
Publication date: 1990-10-03
Anticipated expiration: 2010-02-16
Also published as: DE4006059A1; KR900015525A; GB9003614D0; JPH02118373U; JP2515086Y2; KR940010614B1; GB2229877B

Description

1

SPECIFICATION

Title of the Invention:

VIDEO INTERMEDIATE FREQUENCY PROCESSING CIRCUIT Background of the Invention:

Field of the Invention:

This invention relates to video intermediate frequency processing circuits to be used for television receivers or video tape recorders and more particularly to a video intermediate frequency processing circuit intended to make a video signal component a high band, to improve the same phase distortion of a color signal component and to reduce the buzz of a sound signal component.

Description of the Related Art:

Conventionally, in an inter-carrier system which is one of signal receiving systems in television receivers and the like, a video carrier component, color carrier component and sound carrier component having had amplified processing Therefore, intermediate simplified, the frequencies converted by a tuner have been and detected by the same intermediate frequency circuit having one kind of common pass band.

in the inter-carrier system, whereas the frequency processing circuit can be for example, the video signal and sound 2 signal are likely to interfere with each other. For the counter-measures, the sound signal level is made lower than the video signal level or a sound trap corresponding to the beat frequency between the video and sound carriers is provided to prevent the sound signal from mixing into the video.

Fig. 4 is a block diagram showing a conventional video intermediate frequency processing circuit.

In Fig. 4, a television signal from an antenna 21 is converted to an intermediate frequency signal (which shall be mentioned as an IF signal hereinafter) by a tuner 22 and is then fed to a video intermediate frequency amplifying circuit (which shall be mentioned as a VIF circuit hereinafter) 24 through a picture surface acoustic wave filter (which shall be mentioned as a PSF filter hereinafter) 23 having a predetermined band characteristic. The signal having had the IF amplified by the VIF circuit 24 is fed to a video detecting circuit (which shall be mentioned as a DET circuit hereinafter) 26 having a tank circuit 25 for synchronous detection to obtain a video detecting output (composite video output) at an output terminal 28. An AGC circuit 27 generating an AGC voltage proportional to the amplitude of the IF signal on the basis of the video detecting output and controlling the gain of the VIF circuit to make the video detecting output constant is provided between the above mentioned DET circuit 26 and VIF circuit 24.

In the above mentioned formation, the band characteristic of the PSF circuit is as shown in- ' Fig. 5(a).

That is to say, the responses of the color carrier 3 component C and video carrier component P are lower by 6 dB than the response within the pass band and the response of the sound carrier component S is of a characterisic lower by about 18 dB. The band characteristic of the composite video output obtained at the output terminal 28 is as shown in Fig. 5(b) in which the response of the color carrier component C is lower than the response of the video carrier component P. The composite video output has the color component C separated through a color band-pass filter (not illustrated) having such band characteristic as is shown in Fig 5(c). In Fig. 5(c), the hatched part represents a cross color component (such as a high band component of a luminance signal) coming into the color band. By the way, in Fig. 5, the abscissa represents correlative frequency relations of a video carrier component P, color carrier component C and sound carrier component S.

In the conventional circuit formed as in the above, there are such problems as in the following.

First of all, as the color carrier component is reduced by 6 dB, the video band will not be exanded enough and the video carrier component will be reduced in the band. Second, as the sound carrier component is reduced by about 18 dB, the buzz sound will be increased by mixing with the video carrier component and particularly the sound quality at the time of detuning will deteriorate. Third, the beat by the color and sound carrier components will enter the video band and will appear as a color and sound beat obstruction. Fourth, the same phase distortion of the color carrier component will be large and the cross 4 color comoponent into the color band will be also large. Summary of the Invention:

An object of the present invention is to provide a video intermediate frequency processing circuit intended to make a video component a high band, to reduce a sound buzz, to reduce a color and sound beat obstruction, to improve the same phase distortion of a color component and to improve a cross color obstruction.

That is to say. the video intermediate frequency processing circuit of the present inventiion is characterized by comprising:

a first band-pass filter having a predetermined video band characteristic including a color band and passing a video carrier component having had the frequency converted by a tuner; a second band-pass filter having a predetermined color and sound band characteristic and passing a color and sound carrier component having had the frequency converted by the above mentioned tuner; a first video intermediate frequency amplifying circuit formed of a balance input type amplifier and amplifying the video carrier component from the above mentioned first band-pass filter; a second video intermediate frequency amplifying circuit formed of a balance input type amplifier and amplifying the color and sound carrier component from the above mentioned second band-pass filter; a first detecting circuit detecting the video carrier component from the above mentioned first video intermediate frequency amplifying circuit; 1 a second detecting circuit detecting the color and sound carrier component from the above mentioned second video intermediate frequency amplifier; and an AGC circuit generating an AGC voltage on the basis of the output of the above mentioned first detecting circuit and controlling the gains of the above mentioned first video intermediate frequency amplifying circuit and second video intermediate frequency amplifying circuit.

Brief Description of the Drawings:

Fig. 1 is a block diagram of a video intermediate frequency processing circuit, showing an embodiment of the p resent invention.

Fig. 2 shows circuit diagrams for explaining a balance input type amplifier used in the video intermediate frequency amplifying circuit in Fig. 1 and an unbalance input type amplifier.

Fig. 3 shows explanatory diagrams for explaining the band characteristics in the essential parts in Fig. 1.

Fig. 5 shows explanatory diagrams for explaining the band characteristics in the essential parts in Fig. 4.

Detailed Description of the Preferred Embodiments:

Fig. 1 is a block diagram of a video intermediate frequency processing circuit, showing an embodiment of the present invention.

In this diagram, a television signal from an antenna 1 has the frequency converted by a tuner 2 and is input as an 6 IF signal into a first picture surface acoustic wave filter (which shall be mentioned as a PSF-1 circuit hereinafter) 3 for processing video signals and at the same time into a second picture surface acousic wave filter (which shall be mentined as a PSF-2 circuit hereinafter) 4 for processing color and sound signals. The band characteristics of the PSF-1 circuit 3 and PSF-2 circuit 4are as shown respectively in Figs. 3(a) and (b). The IF signal having passed through the PSF-1 circuit 3 enters a video detecting circuit (which shall be mentioned as a DET-lcircuit hereinafter) 7 having a tank circuit 6 for synchronous detection through a first video intermediate frequency amplifying circuit (which shall be mentined as a VIF-1 circuit hereinafter) 5 to obtain a video detecting output (composite video output) at its output termianl 8. An AGC circuit 9 generating an AGC voltage proportional to the amplrtude of the IF signal and controlling the gain of the VIF-1 circuit 5 is provided between the above mentioned DET-1 circuit 7 and VIF-1 circuit 5. On the other hand, the IF signal having passed through the PSF-2 circuit 4 enters a detecting circuit (which shall be mentioned as a DET-2 circuit hereinafter) 11 through a second video intermediate frequency amplifying circuit (which shall be mentioned as a VIF-2 circuit hereinafter) 10to obtain a color and sound detecting output at its output terminal 12. By the way, the AGC voltage of the above mentioned AGC Circuit 9 is used also to control the gain of the VIF-2 circuit 10.

Such balance input type amplifier as is shown in Fig. 2(a) is used for both of the above mentioned 7 VIF-1 circuit 5 and VIF-2 circuit 10. In Fig. 2(a), the VIF amplifier comprises two transistors Q1 and Q2 forming a differential couple, a transistor Q3 forming a current source connected to the common emitter of the transistors Q1 and Q2, resistances Rl and R2 connected between the respective collectors of the transistors Q1 and Q2 and a voltage source terminal 16 and a resistance R3 connected between the emitter of the transistor Q3 and a reference potential point. By the way, a direct current voltage source +Vcc is connected to the voltage source terminal 16. A signal from the PSF-1 circuit 3 or PSF-2 circuit 4 is input into the input terminals 13 and 14 connected to the bases of the transistors Q1 and Q2. An AGC voltage is input into the terminal 15 connected to the base of the transistor Q3. An amplifying signal is taken out of the output terminals 17 and 18 connected to the collectors of the transistors Q1 and Q2 and is fed to the DET-1 circuit 7 or DET-2 circuit 11 in the next step. Or many of such amplifying circuits as are shown in Fig. 2(a) may be connected in many steps so that an amplifying signal may be taken out and may be fed to the DET-1 circuit 7 or DET-2 circuit 11 in the next step.

When such balance input type amplifier is used, if an input signal is applied between the bases of the two transistors Q1 and Q2, the output signal will be takn out as a difference voltage between the collectors of the transistors Q1 and Q2 on the basis of the difference between the collector currents of the transistors Q1 and Q2 and therefore, even if noises come in from outside, they will be canceled with each other between the 8 collector outputs and no noise will appear in the output signal. When the collector currents of the transitors Q1 and Q2 are varied by applying the AGC voltage to the base of the transistor 03, the gain will be able to be easilly controlled. In the circuit in Fig. 2(a), the balance input type by two transistors 0, and Q, has been explained but a double balance type amplifying circuit utilizing four transistors can be also used.

By the way, against the above described balance input type amplifier, an unbalance input type amplifier is such amplifier comprising a transistor 04 and resistances R4 and R5 as is shown in Fig. 2(b). Here is shown an emitter earthed type amplifier wherein an input signal is fed to the base of the transistor Q4 and an output signal is taken out of the collector and the gain is controlled by controlling the collector current by applying an AGC voltage to the base. However, there is a defect in the formation that the performance of removing foreign noises is low.

Fig. 3 shows band characterisics in the respective parts of the circuit in Fig. 1. In Fig. 3, the ordinate represents responses (in dB) and the abscissa represents correlative relations of the respective frequencies of a video carrier component P, color carrier component C and sound carrier component S. Fig. 3(a) shows a video band characteristic of the above mentioned PSF-1 circuit 3. This is a characteristic of a high band type C as shown extending compared in Fig.

to the band of the color carrier component with the characteristic of the PSF circuit 5. -In addition, the response of a sound 9 carrier component is zero. Fig. 3(b) shows a band characteristic of the above mentioned PSF-2 circuit 4 which is a high band type characteristic of an asymmetric double peak type including a band of the color carrier component C and sound carrier component S. Fig. 3(c) shows output responses of the above mentioned DET-1 circuit 7 in which the video component is made a high band in response to the characteristic in Fig. 3(a). Fig. 3(d) shows output responses of the above mentioned DET-2 circuit 11 in which such output responses as are shown by the broken line are obtained in response to the characteristic in Fig. 3(b). In Fig. 3(d), the solid line represents color band components, the one-point chain line represents sound band components and the hatched part represents cross color components (such as high band components of a luminance signal) entering the color band.

In the above formation, as the video carrier component and color and sound carrier component are separated and processed in the PSF-1 circuit 3 and PSF-2 circuit 4 and respectively independent filters are formed, both components will be able to be reduced in interfering with each other so that the video component may be made a high band, the color and sound component may be separated and processed so as to be of a high color picture quality (the same phase distortion may be reduced and the cross color obstruction may be improved) and the buzz component mixing into the sound component may be reduced.

Further, as the input system in the VIF step is made a balance type input form in both of the video processing step and the color and sound processing step as shown in Fig. 2(a), the performance of removing the obstruction caused by an obstructing signal coming from outside will be able to be improved. Also, in the IF AGC loop, as the video processing step and color and sound processing step are controlled with the same AGC voltage, the gain will be able to be made the same by the AGC loop of the same type and no disadvantage will be produced by the separating process.

Therefore, in the present apparatus as televisiion receivers the picture quality and sound quality By the way, in the above picture surface acoustic wave invention, such video can be made high in mentioned embodiment, a filter is used to separate a video carrier component and a color and sound carrier component from each other but the present invention is not limited to it and any band-pass filter having the same band characteristic will do.

By the way, the present invention is not limited to the above mentioned embodiment and can be variously modified without deviating from the subject matter of the invention.

Claims

What is claimed is:

1. A video intermediate frequency processing circuit characterized by comprising:

a first band-pass filter having a predetermined video band characteristic including a color band and passing a video carrier component having had the frequency converted by a tuner; a second band-pass filter having a predetermined color and sound band characteristic and passing a color and sound carrier component having had the frequency converted by said tuner; a first video intermediate frequency amplifying circuit formed of a balance input type amplifier and amplifying the video carrier component from said first band-pass filter; a second video intermediate amplifying circuit formed of a balance input amplifier and amplifying the color and sound carrier component from said second band-pass filter; a first detecting circuit detecting the video carrier component from said first video intermediate frequency amplifying circuit; a second detecting circuit detecting the color and sound carrier component from said second video intermediate frequency amplifier; and an AGC circuit generating an AGC,voltage on the basis of the output of said first detecting circuit and frequency type 12 controlling the gains of said first video intermediate frequency amplifying circuit and second video intermediate frequency amplifying circuit.

2. A video intermediate frequency processing circuit according to claim 1 characterized in that said first band-pass filter is formed of a picture surface acoustic wave filter.

3. A video intermediate frequency processing circuit according to claim 1 characterized in that said second band pass-filter is formed of a picture surface acoustic wave filter.

4. A video intermediate frequency processing circuit according to claim 1 characterized in that, in said first video intermediate frequency amplifying circuit, the respective emitters of a first and second transistors are connected in common, the respective collectors of said first and second transistors are connected to a direct current voltage source respectively through a first and second loads, a current source circuit having a third transistor is connected between the common emitter of said first and second transistors and a reference potential point, a signal from said first band pass filter is input between the bases of said first and second transistors, an AGC voltage from said AGC circuit is input into the base of said third transistor and an amplifying signal is taken out between the collectors of said first and second transistors.

5. A video intermediate frequency processing circuit according to claim 1 characterized in that, in said second video intermediate frequency amplifying 1 13 circuit, the respective emitters of a first and second transistors are connected in common, the respective collectors of said first and second transistors are connected to a direct current voltage source respectively through a first and second loads, a current source circuit having a third transistor is connected between the common emitter of said first and second transistors and a reference potential point, a signal from said second band path filter is input between the bases of said first and second transistors, an AGC voltage from said AGC circuit is input into the base of said thirdtransistor and an amplifying signal is taken out between the collectors of said firt and second transistors.

Published 1990a The Patent Office. State Mouse-6671 High Holborn. LondonWC1R4TP FILirther copies maybe obtained from The Patent OfficeWes Branch.. St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multaplex techniques itL St Mary Cray, Kent, Con. 1187