JP3505295B2 - Television broadcast receiver - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a television broadcast receiver mounted on a mobile body such as an automobile to receive terrestrial television broadcast transmitted by an amplitude modulation system.

[0002]

2. Description of the Related Art VHF or UH in a moving body such as an automobile
When receiving the terrestrial television broadcast of F, the received electric field strength largely varies depending on the receiving place. Particularly in urban areas, it is susceptible to multipath interference caused by reflections from structures such as buildings. Multipath interference has a great influence when the radio waves directly arriving from a transmitting antenna of a broadcasting station and the like and the reflected waves from a structure such as an urban building have opposite phases. The degree of obstruction changes as the moving body moves. In addition, a video signal with a wide frequency band using the amplitude modulation method is more susceptible to that than an audio signal with a frequency modulation method, and the frequency of the opposite phase depends on changes in the receiving position within the wide video frequency band. And various changes. The influence of the interference appearing on the received image also changes depending on the frequency having the opposite phase. For example, when the phase is reversed near the video carrier frequency, synchronization disturbance and hue change occur, and when the phase is reversed near the color subcarrier frequency, color disappears, that is, black and white occurs.

Conventionally, a technique has been proposed for preventing the attenuation of a video signal due to transmission and the influence of noise on an image. For example, Japanese Patent Laid-Open No. 4-51293 discloses a prior art for correcting the attenuation of an image signal due to long distance transmission. According to this prior art, a synchronization signal and carrier signal addition means is provided on the transmission side of the image signal, and the synchronization signal and the carrier signal are detected by the detection means provided on the reception side. The transmitted image signal is corrected by correcting the DC gain and the high frequency characteristic in accordance with each detected signal level. Further, in Japanese Patent Application Laid-Open No. 6-070203, a signal-to-noise ratio (hereinafter referred to as "S / N ratio") when an amplification degree of automatic gain control (hereinafter referred to as "AGC") of a video signal is increased. Prior art for the degradation of) is disclosed. According to this prior art, the amplification factor increase / decrease detection signal output from the AGC circuit is used for the amplification stage of the video signal, and the high frequency band of the video signal is corrected by the frequency characteristic correction circuit. With this, even when the amplification degree is increased, noise is prevented from becoming undesired.

A so-called diversity reception system is used as a typical countermeasure against multipath interference. In the diversity receiving method, a plurality of receiving antennas are arranged on a moving body so as to be spaced apart from each other, and the antennas are switched to reproduce a received signal from the antenna in the best receiving state. To prevent noise from appearing on the display screen, the reception status of each antenna is judged based on the signal near the vertical blanking period, and switching is performed within the vertical blanking period of 1/60 seconds. Done.

FIG. 4 shows that the applicant of the present application filed Japanese Patent Application No. 6-1306.
An example of a configuration for preventing the influence of multipath interference proposed by No. 98 will be shown. With this configuration, the multipath interference is detected even during the image display period by detecting the reception state based on the horizontal sync signal of the video signal that should be actually reproduced, and the transmission from the antenna input of the receiver to the detection is performed. Correct the transmission characteristics of the path. The super-heterodyne television broadcast receiver 1 is mounted on a vehicle or the like and used to receive VHF and UHF terrestrial television broadcasts. The high frequency signal received by the antenna 2 is amplified by the high frequency amplification circuit 3 and input to the mixing circuit 4, and is mixed with the oscillation output from the local oscillation circuit 5 in the mixing circuit 4 to become an intermediate frequency signal. The intermediate frequency signal is amplified by the intermediate frequency amplification circuit 6 and then detected by the detection circuit 7 to become a video signal output. The AGC voltage detected from the output of the intermediate frequency amplification circuit 6 by the AGC detection circuit 8 is an IF-AG that adjusts the gain of the intermediate frequency amplification circuit 6.
It is supplied to the intermediate frequency amplifier circuit 6 as C and R
The F-AGC is also supplied to the high-frequency amplifier circuit 3 and has a voltage value that changes according to the gain.

FIG. 5A shows the change in the gain of the intermediate frequency amplifier circuit 6 with respect to the input voltage. FIG. 5B shows a change in output with respect to the input voltage of the intermediate frequency amplifier circuit 6. The gain of the intermediate frequency amplifier circuit 6 is maximum when the input voltage is small, and decreases as the input voltage increases. That is, the output from the intermediate frequency amplifier circuit 6 does not increase for an input voltage exceeding the AGC operation start point. IF-AG
C operates to maintain a constant intermediate frequency signal output for a wide range of input levels.

The reception state detection circuit 9 of FIG. 4 detects the disturbance due to multipath interference of the video signal demodulated by the detection circuit 7, that is, the reception state of any part in the video signal band such as the video carrier and the color subcarrier. It is detected that the value is low and is disturbed. The detection result of the reception state detection circuit 9 is input to the frequency characteristic correction circuit 10, and the frequency characteristic correction circuit 1
When it is 0, the transmission characteristic of the part that is lowered in the video signal band is corrected. By improving the transmission characteristics of the intermediate frequency amplification stage, it is possible to prevent image distortion due to the influence of multipath interference.

The AGC voltage from the AGC detection circuit 8 is input to the non-inverting input side of the comparator 11. When the AGC voltage exceeds the operation reference voltage value E applied to the inverting input side of the comparator 11, the RF-AGC is supplied to the high frequency amplifier circuit 3 and the gain of the high frequency amplifier circuit 3 is reduced to be controlled. The operation reference voltage value E of the comparator 11 is determined by the product of the output current I of the constant current source 12 and the resistance value R of the semi-fixed resistor 13, that is, the first equation expressed as E = I × R (1) . If the gain of the high frequency amplifier circuit 3 is reduced when the antenna input is small, the S / N ratio of the high frequency signal is reduced and the S / N ratio of the video signal is reduced, resulting in poor display image quality. RF-AGC is more IF-
The comparator 11 operates so that the AGC starts its operation at an input level higher than the antenna input level at which it starts operating.
The operation reference voltage value E of is defined. FIG. 5C is a graph showing a change in gain characteristic with respect to the input voltage of the high frequency amplifier circuit 3. Such RF-AGC and IF-
Due to the dual operation of the AGC, it is possible to always obtain a stable image detection output having a high S / N ratio even if the received high frequency signal of the antenna 2 has a strong and weak fluctuation. RF-AG
The operation start level of C is set within an adjustable range in which the gain of the intermediate frequency amplifier circuit shown in FIG. It is common to keep.

[0009]

[0010]

The television broadcast receiver shown in FIG. 4 has a high frequency amplification stage and an intermediate frequency amplification stage.
Since it is a super-heterodyne system equipped with a GC, it is possible to stabilize the image output even if the received electric field strength fluctuates greatly. In addition, since the reception status is detected during the horizontal blanking period of the video signal, the reception status is determined according to the video signal to be actually reproduced, and the transmission characteristics of the transmission path corresponding to the detected reception status are corrected. Has been done. For example, when the reception state detection circuit 9 detects a decrease in the video carrier signal, the frequency characteristic of the frequency characteristic correction circuit 10 is corrected so that the transmission characteristic of the lowered video carrier signal is increased by about 15 dB. Such correction of the frequency characteristic can prevent the influence of multipath interference. However, since the input to the intermediate frequency amplifier circuit 6 increases due to the correction of the frequency characteristic, the AGC detection circuit 8 is designed to reduce the gain of the intermediate frequency amplifier circuit 6.
Output decreases, the input to the comparator 11 also decreases, and the gain of the high frequency amplification circuit 3 also decreases. When the gain of the high frequency amplifier circuit 3 which is the input stage of the receiver decreases, the S / N ratio of the high frequency signal decreases, and the detection circuit 7
The S / N ratio of the video output demodulated by is decreased. When the S / N ratio of the video output decreases, the screen becomes difficult to see due to fine flicker due to noise. Even if the image disturbance due to the multipath interference is improved by correcting the frequency characteristic of the transmission path, the change in the state of the intermediate frequency amplification stage due to the correction affects the AGC of the high frequency amplification circuit 3, and as a result, the image quality is improved. It deteriorates.

An object of the present invention is to perform appropriate gain control without deteriorating the S / N ratio even when the frequency characteristic correction means for correcting the frequency characteristic of the intermediate frequency amplification stage operates as a countermeasure against multipath interference. It is to provide a television broadcast receiver that can perform.

[0012]

SUMMARY OF THE INVENTION The present invention comprises a high frequency amplification stage, an intermediate frequency amplification stage, automatic gain adjusting means for controlling the gains of the high frequency amplification stage and the intermediate frequency amplification stage, and an intermediate frequency amplification stage. In a super-heterodyne television broadcast receiver comprising a frequency characteristic correction means for correcting frequency characteristics and a reception state detection means for detecting the occurrence of multipath interference by monitoring the reception signal, the reception state detection means is: When detecting the occurrence of multipath interference,
Television broadcasting characterized by controlling the frequency characteristic correcting means to suppress the influence of multipath interference and at the same time controlling the automatic gain adjusting means to suppress a decrease in the gain of the high frequency amplification stage. It is a receiver. According to the present invention, the receiving state detecting means monitors the received signal to detect the occurrence of multipath interference, and when detecting the occurrence of multipath interference, controls the frequency characteristic correcting means to detect the multipath interference. At the same time as suppressing the influence, the automatic gain adjusting means is controlled to suppress the decrease in the gain of the high frequency amplification stage. By correcting the frequency characteristics of the intermediate frequency stage to prevent the effects of multipath interference, the output of the intermediate frequency amplification stage increases,
As a result, even if the output for the automatic gain adjustment increases, the decrease in the gain of the high frequency amplification stage is suppressed, so that the S / N ratio can be prevented from deteriorating and appropriate gain adjustment can be performed.

Further, in the present invention, the automatic gain adjustment means increases the threshold value of the antenna input voltage for starting the gain control of the high frequency amplification stage according to the output from the reception state detection means, It is characterized in that a decrease in the gain of the high frequency amplification stage is suppressed. According to the present invention, when the reception state detecting means detects the occurrence of multipath interference, the gain of the high frequency amplification stage is decreased by increasing the threshold value of the antenna input voltage for starting the gain control of the high frequency amplification stage. Is suppressed, the gain reduction for automatic gain adjustment until reaching the raised threshold value is performed on the intermediate frequency amplification stage, and the S / N ratio can be prevented from deteriorating. Further, according to the present invention, the frequency characteristic correcting means of the intermediate frequency amplifying stage is inserted between the band limiting filter for limiting the band of the intermediate frequency signal and the intermediate frequency amplifying stage. According to the present invention, the frequency characteristic correction means is arranged on the output side of the band limiting filter to correct the frequency characteristic of the intermediate frequency signal attenuated by the insertion loss of the band limiting filter. As a result of the frequency characteristic correction, even if a part of the frequency band of the intermediate frequency signal is amplified,
Since there is a margin for the insertion loss of the band limiting filter, it is possible to secure a sufficient dynamic range, perform gain adjustment appropriately, and obtain good image quality.

[0014]

1 shows a simplified electrical configuration of a television broadcast receiver 21 according to an embodiment of the present invention. The television broadcast receiver 21 is mounted on a moving body such as a vehicle, and is used for receiving a terrestrial wave of a television broadcast broadcast by VHF or UHF by a super heterodyne system. The high frequency signal received by the antenna 22 is amplified by the high frequency amplification circuit 23, input to the mixing circuit 24, and mixed with the oscillation output from the local oscillation circuit 25 to become an intermediate frequency signal. The intermediate frequency signal is amplified by the intermediate frequency amplification circuit 26 and demodulated by the detection circuit 27 to become a video signal. AGC detection circuit 28
The AGC voltage detected by the intermediate frequency amplifier circuit 26 is supplied to the intermediate frequency amplifier circuit 26 as an AGC voltage for IF-AGC. The reception state detection circuit 29
A change in the reception state due to multipath interference is detected from the video signal demodulated by the detection circuit 27, and the output is output from the mixing circuit 2
4 and the intermediate frequency amplification circuit 26 are provided to the frequency characteristic correction circuit 30. Frequency characteristic correction circuit 30
Corrects the frequency characteristic of the intermediate frequency amplifier circuit 26 so as to eliminate the influence of multipath interference according to the change in the input reception state. The configurations and operations of the reception state detection circuit 29 and the frequency characteristic correction circuit 30 are basically the same as those of the reception state detection circuit 9 and the frequency characteristic correction circuit 10 shown in FIG.

The AGC voltage from the AGC detection circuit 28 is
Applied to the non-inverting input of comparator 31 for RF-AGC. When the input voltage on the non-inverting input side of the comparator 31 exceeds the inverting input side, the AGC voltage is supplied from the comparator 31 to the high frequency amplifier circuit 23. A constant current I is supplied from the constant current source 32 to the inverting input side of the comparator 31, and a resistor 33 and a resistor 34 and the collector and emitter of the transistor 35 are connected in parallel with the ground. To the base of the transistor 35, the voltage taken out from the connection point of the resistors 37 and 38 for dividing the output voltage of the comparator 36 is input.

The output of the reception state detection circuit 29 is simultaneously input to the non-inverting side of the comparator 36. When the input voltage exceeds the operation start voltage E1 applied to the inverting input side of the comparator 36, the collector-emitter of the transistor 35 is switched on. Transistor 35
In the normal state where the switch is turned off, the operation start voltage as a threshold value given to the inverting input side of the comparator 31 which is the operation point of the RF-AGC is the current I obtained from the constant current source 32 and the semi-fixed resistance. A value E2 determined by the following second equation is set as a product of the resistance value R2 of the resistor 33 realized by the container.

E2 = I × R2 (2) When the transistor 35 is switched on, a part of the current I is shunted to the resistor 34 realized by a semi-fixed resistor, so that the inverting input side of the comparator 31 is connected. The operation starting voltage given to the voltage V.sub.2 drops to E3 as shown in the following third equation.

E3 = I × 1 / (1 / R1
+ 1 / R2) (3) When the operation start voltage of the comparator 36 decreases from E2 to E3, the operation start point of the RF-AGC changes, and even if the AGC voltage is constant, the decrease in the gain of the high frequency amplifier circuit 23 is suppressed, The gain for the antenna input is controlled to be distributed more to the high frequency amplifier circuit 23. At this time, the intermediate frequency amplifier circuit 26 operates using the gain margin shown in FIGS. 5A and 5C.

FIG. 2A shows the change in gain with respect to the input of the intermediate frequency amplifier circuit 26. When the input to the intermediate frequency amplification circuit 26 increases and the output of the AGC detection circuit 28 decreases, the gain of the intermediate frequency amplification circuit 26 also decreases as the voltage of the IF-AGC decreases. For example, when the input of the intermediate frequency amplifier circuit 26 changes from the state (a) to the state (b), the gain decreases along the slope.

FIG. 2B shows a change in gain with respect to the input from the antenna 22 to the high frequency amplifier circuit 23. When the frequency characteristic of the intermediate frequency amplifier circuit 26 is corrected and as a result the output of the AGC detection circuit 28 is lowered and the input level of the non-inverting input side of the comparator 31 is reduced,
The operation start voltage E2, which is the threshold value of the comparator 31, is reduced to E3, and the gain of the high frequency amplifier circuit 23 is changed to AGC.
It can be adjusted so that it does not deteriorate. This makes it possible to correct the frequency characteristic of the intermediate frequency amplification stage without lowering the S / N of the high frequency amplification stage and prevent image distortion due to the influence of multipath interference. Also,
Corresponding to the diversification of the frequency characteristic correction means of the intermediate frequency amplification stage, the equivalent resistance connected to the inverting input side of the comparator 31 is continuously changed, or the current I supplied from the constant current source 32 is continuously changed. It is also possible to continuously change the operation start voltage of the comparator 31 by changing the operation start voltage.

FIG. 3 shows an outline of an electrical configuration of a television broadcast receiver 41 according to another embodiment of the present invention.
For convenience of explanation, the same reference numerals are given to the portions corresponding to the embodiment of FIG. The embodiment of FIG. 3 differs from the embodiment of FIG. 1 in that a band limiting filter 42 that determines the frequency band of the intermediate frequency signal is inserted between the mixing circuit 24 and the frequency correction circuit 30. . Since the frequency characteristic correction circuit 30, which is a frequency characteristic correction means, has a frequency characteristic correction gain in a specific frequency band, its output is limited by the dynamic range. Band limiting filter 42
Is realized by, for example, a surface acoustic wave (SAW) filter and has an insertion loss of about 20 dB. Rather than attenuating the signal whose upper limit is limited by the dynamic range by using the frequency correction circuit 30 first, the dynamic range can be widened by amplifying the previously attenuated signal.

In the embodiment described above, the reverse method in which the gain decrease amount decreases as the AGC voltage increases as the AGC is used, but the forward method in which the gain decrease amount also increases in response to the increase in the AGC voltage is used. It can also be used. In this case, the threshold value of the comparator 31 may be increased according to the frequency correction.

[0023]

As described above, according to the present invention, in a super-heterodyne television broadcast receiver having automatic gain adjusting means for a high frequency amplifying stage and an intermediate frequency amplifying stage, changes in the receiving state due to multipath interference. Is detected to correct the frequency characteristic of the intermediate frequency amplification stage, and at the same time, the decrease in gain due to automatic gain adjustment of the high frequency amplification stage is suppressed. By correcting the frequency characteristics of the intermediate frequency amplification stage to prevent the effects of multipath interference, even if the input and output of the intermediate frequency amplification stage increase and the output for automatic gain adjustment increases, By suppressing the decrease of the gain due to the automatic gain adjustment, the S / N ratio of the received signal does not decrease and the image quality does not deteriorate.
It is possible to prevent the influence of multipath interference, which is unavoidable on the television broadcast receiver mounted on the moving body, while performing the appropriate gain control and maintaining the image quality, without lowering the S / N ratio.

Further, according to the present invention, a decrease in gain due to automatic gain adjustment of the high frequency amplification stage can be suppressed by increasing the threshold value for starting gain control with respect to the antenna input voltage. Further, according to the present invention, the effect of multipath interference is prevented by inserting the frequency characteristic correction means of the intermediate frequency amplification stage between the band limiting filter for limiting the frequency band of the intermediate frequency signal and the intermediate frequency amplification stage. Even if the frequency characteristic correction of the intermediate frequency amplification stage is performed, the dynamic range can be secured and the deterioration of the image quality can be prevented.

[Brief description of drawings]

FIG. 1 is a block diagram showing an outline of an electrical configuration of a television broadcast receiver according to an embodiment of the present invention.

FIG. 2 is a graph showing changes in gain and AGC voltage with respect to inputs to the intermediate frequency amplification circuit and the high frequency amplification circuit of FIG.

FIG. 3 is a block diagram showing a schematic electrical configuration of a television broadcast receiver according to another embodiment of the present invention.

FIG. 4 is a block diagram showing an outline of an electrical configuration of a conventional television broadcast receiver.

5 is a graph showing a change in gain with respect to an AGC voltage of the intermediate frequency amplifier circuit, a change in output with respect to an input of the intermediate frequency amplifier circuit, and a change in gain with respect to an AGC voltage of the high frequency amplifier circuit in FIG.

[Explanation of symbols]

21,41 Television broadcasting receiver 22 antenna 23 High frequency amplifier circuit 24 mixing circuit 25 local oscillator 26 Intermediate frequency amplification circuit 27 Detection circuit 28 AGC detection circuit 29 Reception state detection circuit 30 Frequency characteristic correction circuit 31, 36 Comparator 33,34 resistance 35 transistor 42 Band limiting filter

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H04N 5/52 H04B 1/10

Claims (3)

(57) [Claims] 1. A high frequency amplification stage, an intermediate frequency amplification stage, automatic gain adjusting means for controlling the gains of the high frequency amplification stage and the intermediate frequency amplification stage, and a frequency characteristic correction for correcting the frequency characteristic of the intermediate frequency amplification stage. In the super-heterodyne television broadcast receiver comprising a means and a reception state detection means for monitoring the received signal to detect the occurrence of multipath interference, the reception state detection means detects the occurrence of multipath interference. Sometimes, the frequency characteristic correcting means is controlled to suppress the influence of multipath interference, and at the same time, the automatic gain adjusting means is controlled to suppress the decrease in the gain of the high frequency amplification stage. John Broadcast Receiver. 2. The high frequency amplification means increases the threshold value of an antenna input voltage for starting gain control of the high frequency amplification stage according to an output from the reception state detection means. The television broadcast receiver according to claim 1, wherein a decrease in gain of the stage is suppressed. 3. The frequency characteristic correction means of the intermediate frequency amplification stage is inserted between a band limiting filter for limiting the band of the intermediate frequency signal and the intermediate frequency amplification stage. The described television broadcast receiver.