JPH06113229A - Agc circuit - Google Patents

Abstract

PURPOSE:To provide the circuit built in a television receiver or a CATV con verter which is able to follow flutter disturbance and in which a time till a pattern is made stable is decreased at changeover of a TV signal (at channel revision or input signal changeover). CONSTITUTION:A TV signal is detected and a gain of an intermediate frequency amplifier circuit 32 and a high frequency amplifier circuit 30 is automatically controlled based on a detection output. An intermediate frequency automatic gain control section 54 and a high frequency automatic gain control section 46 are separately provided. A time constant is set large in the control section 46 to prevent it that a level of an intermediate frequency (IF) is largely fluctuated due to the effect of external noise or the like On the other hand, the time constant is set small in the control section 54, and then fast level fluctuation (flutter disturbance) is followed. Thus, a constant video (V) detection output is obtained at all times and V sag, H sag are prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AGC (auto gain control) built in a TV (television) receiver, a CATV (community antenna TV system) converter, or the like.
Automatic gain control section) circuit. In particular, the present invention relates to an AGC circuit used when receiving a high definition TV signal or a normal TV signal by MUSE transmission. More specifically, a TV in a CATV system that transmits a TV signal by a cable.
The present invention relates to an AGC circuit suitable for a receiver.

[0002]

2. Description of the Related Art In a CATV system or a TV receiver, an AGC circuit is a circuit for controlling the fluctuation of the image detection output due to the strength fluctuation of the radio wave induced in the receiving antenna and keeping the change of the contrast on the screen constant. is there.

Generally, an AGC circuit creates an AGC control voltage from an image detection output (composite image signal), and an AFC control voltage and an RF (radio frequency) amplifier and a VIF (video intermediate frequemc) are generated by the AGC control voltage.
y: image intermediate frequency) It is configured so that the gain of the amplifier is increased or decreased so that the image level fluctuation due to the strength of the radio wave does not appear in the image detection output. As the AGC circuit, average value type A
A GC circuit, a leading value type AGC circuit, etc. are known.

FIG. 4 shows a conventional AGC circuit. In FIG. 4, the RF input signal is supplied to the image intermediate frequency (VIF) amplifier circuit 12 via the tuner 10, amplified, and detected by the image detection circuit 14. The image-detected signal is amplified by the image amplification circuit 16 and then becomes a detection output.

The detection output from the image amplification circuit 16 is passed through a noise removal circuit 18 and an AGC detection circuit 20 to generate an AGC signal.
The gain of the VIF amplifier circuit 12 is controlled by the IFAGC voltage supplied to the amplifier circuit 22 from the AGC amplifier circuit 22. Further, the signal from the AGC amplifier circuit 22 is supplied to the RF (high frequency) AGC circuit 24, and the RF
The RF AGC voltage from the AGC circuit 24 controls the gain of the tuner 10.

As described above, the AGC circuit uses the detected image signal to judge the strength of the input signal, and controls the gains of the VIF amplifier circuit 12 and the tuner 10 accordingly.

[0007]

When the above-mentioned conventional AGC circuit is applied to a system for receiving a MUSE signal, a keyed AGC which is relatively strong against flutter interference.
The circuit could not be used resulting in peak AGC
Circuit will be used. The flutter disturbance is disturbance in which the level fluctuation is relatively small, but the level fluctuates in a short period of time.

The above peak AGC circuit controls the gain of amplification by the output level after passing the video signal through the integrator. If the time constant of the integrator is not increased, the NTS
In the case of the C signal, a voltage proportional to the leading value of the sync signal cannot be obtained. As described above, in the peak AGC circuit, the time constant of the integrator is increased.
The GC circuit is vulnerable to flutter interference, that is, it cannot follow flutter interference.

An object of the present invention is to provide an AGC circuit which can follow flutter interference. Another object of the present invention is to provide an AGC circuit capable of shortening the time until the screen becomes stable at the time of switching TV signals (when switching channels, switching input signals, etc.).

[0010]

According to a first aspect of the present invention, a gain of an intermediate frequency circuit is detected in an AGC circuit which detects a TV signal and automatically controls the gains of the intermediate frequency amplifier circuit and the high frequency amplifier circuit based on the detected output. An intermediate frequency automatic gain control unit for automatic control and a high frequency automatic gain control unit for automatically controlling the gain of a high frequency amplifier circuit are separately provided, and the intermediate frequency automatic gain control unit has its time constant set small, and
The high frequency automatic gain control is characterized by an AGC circuit whose time constant is set to a large value.

A second aspect of the present invention is an AGC circuit for detecting the TV quality and automatically controlling the gains of the intermediate frequency amplifying circuit and the high frequency amplifying circuit based on the detected output. Switching detection means for detecting this switching, switching means for storing the high-frequency amplification circuit automatic gain control voltage based on the TV signal currently being received immediately before the switching of the TV signal, and the switching detection means. When switching of TV signal is detected by
Means for supplying an automatic gain control voltage in the storage means to a high frequency amplifier circuit when the other TV signal is received.

[0012]

In the first invention, the intermediate frequency automatic gain control section and the high frequency automatic gain control section are separately provided. Then, in the high frequency automatic gain control section, the time constant is set to a large value, which makes it possible to prevent the level of the intermediate frequency (IF) from greatly changing due to the influence of external noise or the like. On the other hand, in the intermediate frequency automatic gain control unit, the time constant is set to be small so that it is possible to follow a rapid level fluctuation (flutter interference). As mentioned above, the first
According to the AGC circuit of the invention, a constant video (V) detection output can be obtained, and V sag and H sag can be prevented.

Further, in the second invention, when the TV signal is switched, the automatic gain control voltage stored in the storage means is supplied to the high frequency amplifier circuit. As a result, when the TV signal is switched, the level of the automatic gain control voltage to the high frequency amplifier circuit does not change, and the time until the screen becomes stable can be shortened.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. In each of the following examples, the present invention provides C
It is applied to the AGC circuit in a TV receiver that receives ATV high-definition (MUSE transmission).

Further, in FIGS. 1A, 1B and 1C,
An embodiment of an AGC circuit according to the first invention is shown in FIG.
3 (A), (B) and FIGS. 3 (A), (B) show an embodiment of the AGC circuit according to the second invention. [I] AGC Circuit According to First Invention First, FIG. 1A shows an AG circuit according to a first embodiment of the present invention.
The C circuit is shown.

In FIG. 1A, an RF input signal is passed through a tuner 30 to a video intermediate frequency (VIF) amplifier circuit 3
The image is detected by the image detection circuit 34. The video-detected signal is sent to the video amplifier circuit 36.
After being amplified by, it becomes the detection output.

The detection output from the video amplification circuit 36 is
An RFAGC voltage is passed through the noise removal circuit 38, the AGC detection circuit 40, the AGC amplification circuit 42, and the RFAGC circuit 44, and the tuner 30 is generated by the RFAGC voltage.
Gain is controlled. In addition, these circuits 38, 40,
The RFAGC unit 46 is composed of 42 and 44. In the tuner 30, the level fluctuation is slow, so RFA
In the GC section 46, the time constant is set large, that is, the time constant of the AGC detection circuit 40 is set large and R
The response of the FAGC voltage is set to be slow, so that the tuner 30 can be prevented from greatly changing the level of the intermediate frequency (IF) due to the influence of external noise or the like.

The detection output from the video amplification circuit 36 is the noise removal circuit 48, the AGC detection circuit 50, and the AG.
The IF AGC voltage is passed through the C amplification circuit 52 and the IF
The gain of the video intermediate frequency (VIF) amplifier circuit 32 is controlled by the AGC voltage. These circuits 48,
An IFAGC unit 54 is configured by 50 and 52. Since the intermediate frequency level is stable in the VIF amplifier circuit 32, it is possible to achieve an IFAGC loop with a small time constant. Therefore, the IFAGC unit 54
Then, the time constant is set to be small, and therefore the time constant of the noise removing circuit 48 is also set to be small, and the noise removing circuit 48 may be omitted in some cases. in this way,
Since the time constant is set small in the IFAGC unit 54, the VIF amplifier circuit 32 can follow a rapid level fluctuation (flutter interference).

As described above, according to the first embodiment of the present invention, since the time constant is set large in the RFAGC section 46, the intermediate frequency (IF) is affected by external noise and the like.
Of the IFA can be prevented from changing greatly,
Since the time constant is set small in the GC unit 54, it is possible to follow a rapid level fluctuation (flutter interference).
Therefore, a constant video (V) detection output can always be obtained, and V sag and H sag can be prevented.

Next, FIG. 1B shows an AGC circuit according to a second embodiment of the present invention. The same parts as those in the first embodiment of FIG. 1A are designated by the same reference numerals. And the description is omitted.

In FIG. 1B, an intermediate frequency (IF) amplifier circuit 56 is arranged between the tuner 30 and the video intermediate frequency (VIF) amplifier circuit 32, and the block 58 in the RFAGC section 46 is 3 shows a video detection circuit. The VIF amplification circuit 32, the video detection circuit 34, the video amplification circuit 36, and the IFAGC unit 54 are configured by one IC, and the RFAGC unit 46 is configured by one IC.

At this time, the IFAGC unit 54 and RFAG
The C section 46 is composed of both peak AGC,
In the case of peak AGC, the time constant of IFAGC is small, so flutter interference can be followed.

Next, FIG. 1C shows an AGC circuit according to a third embodiment of the present invention. The same parts as those of the first embodiment of FIG. 1A are designated by the same reference numerals. And the description is omitted. The embodiment of FIG. 1C is configured by a keyed AGC using the sample hold circuit disclosed in another application of the present applicant (Applicant case No. 46-20180).

There are the following modifications in the first invention. In each embodiment, the noise elimination circuit is provided in common for the IFAGC section and the RFAGC section.

Both the IFAGC section and the RFAGC section are the other applications of the applicant (Applicant Case No. 46-2018
It is configured by the keyed AGC disclosed in 0). -If it is a normal NTSC signal, IFAGC section and RF
It is configured by a keyed AGC together with the AGC section.

The present invention is applicable not only to cable transmission but also to wireless transmission. [Ii] AGC circuit according to the second invention When the TV signal is switched (when the channel is changed or the input signal is switched, etc.), the input level of the AGC circuit fluctuates greatly. At this time, in the AGC circuit, RF
If the time constant of the AGC section is large, the RFAGC section is slow to respond, and it takes a considerable time for the screen to stabilize. Therefore, the second invention has been proposed.

First, FIG. 2A shows an AGC circuit according to a fourth embodiment of the present invention. The same parts as those of the AGC circuit according to the first embodiment of FIG. Is attached and the description is omitted.

In FIG. 2A, in the RFAGC section 46, the code from the RFAGC circuit 44 is converted into digital data by the A / D converter 60 and then supplied to the CPU 62. The output from the CPU 62 is the D / A converter 6
It is converted into analog data at 4 and supplied to the tuner 30 as an RF AGC voltage. The CPU 62 includes an input signal switching detection circuit 66 and a channel switching detection circuit 6
8 and the memory circuit 70 are connected.

The CPU 62, in a normal receiving state,
The digital data from the A / D converter 60 is directly D /
The signal is supplied to the A converter 64 to perform RFDAGC. On the other hand, when the input signal switching detection circuit 66 detects the switching of the input signal or the channel switching detection circuit 68 detects the switching of the channel (at the time of switching the TV signal), the CPU 62 at this time (immediately before). ) Store the digital RF AGC voltage in the memory circuit 70. After the input signal switching and channel switching are performed, C
The PU 62 reads out the digital RF AGC voltage stored in the memory circuit 70, converts it into a rounded data by the D / A converter 64, and supplies it to the tuner 30 as an RF AGC voltage. As a result, the RFAGC voltage does not change before and after input signal switching and channel switching,
The time until the screen stabilizes can be shortened.

When the input signal is switched or the channel is switched, the CPU 62 stores the (immediately before) digital RF AGC voltage in the memory circuit 70, but the CPU 62 does not receive the latest digital RFA currently being received.
The GC voltage may be constantly stored in the memory circuit 70, and the latest digital RF AGC voltage in the memory circuit 70 may be output to the D / A converter 64 when the input signal is switched or the channel is switched.

Next, FIG. 2B shows an AGC circuit according to a fifth embodiment of the present invention. The same parts as those in the second embodiment of FIG. 1B are designated by the same reference numerals. And explain.
Further, FIG. 3A shows an AG according to a sixth embodiment of the present invention.
A C circuit is shown, and the same parts as those in the third embodiment shown in FIG. Also,
FIG. 3B shows an AGC circuit according to a seventh embodiment of the present invention. The same parts as those of the third embodiment of FIG. 1C are designated by the same reference numerals and the description thereof will be omitted. . Note that FIG.
In the AGC circuit of the seventh embodiment of (B), digital processing is performed in a keyed AGC using a sample hold circuit as disclosed in another application of the applicant (Applicant case No. 46-20180). I am doing, RFAGC
In the unit 46, the noise removing circuit 38, the A / D converter 60,
Sampling circuit 72, CPU 62, D / A converter 6
4, the AGC detection circuit 40, and the RF AGC circuit 42 are connected in this order.

According to the AGC circuits of the fifth, sixth and seventh embodiments shown in FIGS. 2B, 3A and 3B, the AGC circuits shown in FIG. The same effect as that of the AGC circuit of the first embodiment can be achieved, that is, the RFAGC voltage does not change before and after the input signal switching and the channel switching, and the time until the screen becomes stable can be shortened.

[0033]

As described above, according to the first embodiment, the time constant is set large in the high frequency automatic gain control section, so that the intermediate frequency level is prevented from largely changing due to external noise or the like. In addition, since the time constant is set small in the intermediate frequency automatic gain control section, it is possible to follow the flutter disturbance.
Therefore, a constant video detection output can always be obtained, and V Sagho,
H sag can be prevented.

Further, according to the second aspect of the present invention, when the TV progress is switched, the automatic gain control voltage stored in the storage means is supplied to the high frequency amplifier circuit. The level of the automatic gain control voltage to the amplifier circuit does not change, and the time until the screen stabilizes can be shortened.

[Brief description of drawings]

1A, 1B, and 1C show AGC circuits according to a first embodiment, a second embodiment, and a third embodiment of the present invention, respectively.

2A and 2B respectively show an AGC circuit according to a fourth embodiment and a fifth embodiment of the present invention.

3A and 3B respectively show an AGC circuit according to a sixth embodiment and a seventh embodiment of the present invention.

FIG. 4 shows a conventional AGC circuit.

[Explanation of symbols]

 30 ... Tuner 32 ... Video intermediate frequency amplification circuit 46 ... RFAGC section 54 ... IFAGC section 62 ... CPU 66 ... Input signal switching detection circuit 70 ... Memory circuit

Claims (2)

[Claims] 1. An AGC circuit for detecting a TV signal and automatically controlling the gains of an intermediate frequency amplifier circuit and a high frequency amplifier circuit based on the detected output, an intermediate frequency automatic gain control section for automatically controlling the gain of the intermediate frequency circuit. And a high frequency automatic gain control section for automatically controlling the gain of the high frequency amplification circuit, the intermediate frequency automatic gain control section is set to have a small time constant, and the high frequency automatic gain control has the time constant. Is set to a large value, an AGC circuit. 2. An AGC circuit for detecting a TV signal and automatically controlling the gains of the intermediate frequency amplifier circuit and the high frequency amplifier circuit based on the detected output, the TV signal being currently received is switched to another TV signal. At this time, switching detection means for detecting this switching, and the TV currently being received immediately before the TV signal is switched.
Storage means for storing a voltage for high-frequency amplification circuit automatic gain control based on a signal; and, when the switching of the TV signal is detected by the switching detection means, when another TV signal is received, the automatic storage in the storage means. A means for supplying a gain control voltage to a high-frequency amplifier circuit, the AGC circuit.