JPH03781Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention relates to an AFC device for FM television broadcasting that improves the image quality during weak input in an FM television broadcasting receiver.

[Technical background of the invention] Conventionally, most TV broadcast transmissions have been based on AM, and FM has been used only in limited areas such as satellite broadcasting.
method was used.

The most common AFC (automatic frequency control) method in the FM system is to control the average DC potential of the demodulated signal in the discriminator to a constant value, as is often seen in audio FM broadcast receivers. .

FIG. 4 is a block diagram showing the configuration of a conventional AFC method. In this FIG. 4, the input terminal 10
The RF signal introduced from 1 is amplified by an RF amplifier 102 and sent to a mixer 103 by a voltage controlled oscillator 1.
09 and output to the IF filter 104.

This output is used to extract an intermediate frequency signal by an IFIF filter 104, and is inputted to an FM discriminator 106 through a limiter 105, where it is demodulated. This demodulated output video signal is sent to the amplifier 11
The signal is amplified by 0 and taken out from the output terminal 111, and the DC average level is detected by the low-pass filter 107.

A voltage comparator 108 calculates the difference between this DC average level and a reference voltage E ₀ to control a voltage controlled oscillator 109 .

That is, FM discriminator 106 - low pass filter 107 - voltage comparator 108 - voltage controlled oscillator 109 - mixer 103 - IF filter 1
By configuring a loop of 04-limiter 105-FM discriminator 106, the output DC average potential of the discriminator 106 is equal to the reference voltage.
Fixed at E ₀ (or near E ₀ ).

[Background technology and problems]

The conventional AFC method causes almost no inconvenience when receiving audio FM broadcasts, but when the content of the modulated signal is a video signal, the modulation waveform is asymmetrical and contains a synchronization signal, so the synchronization signal cannot be received when the input is weak. There was a drawback that the received image quality deteriorated rapidly due to the omission of .

[Purpose of invention]

This idea was created to eliminate the above-mentioned drawbacks of the conventional technology.It is used for FM TV broadcasting to prevent synchronization signal loss during weak input and improve received image quality.
The purpose is to provide AFC equipment.

[Summary of the idea]

The AFC device for FM television broadcasting of this invention is
In a television broadcast receiver using this method, the input level is detected by a level detection means at the time of a weak input, the output of this detection means is compared with a predetermined reference voltage by a first voltage comparator, and the difference voltage is calculated accordingly. This output controls the switching of the switch to extract one of the two reference voltages, and the extracted reference voltage and the output voltage of the first voltage comparator are connected to the second voltage comparator. The oscillation frequency of the voltage controlled oscillator is varied according to the difference voltage, and the output of the voltage controlled oscillator is applied to a mixer to change the center frequency of the output of the FM discriminator.

[Example of idea]

[Configuration of Embodiment] Next, an embodiment of the AFC device for FM television broadcasting of this invention will be described first from the configuration. FIG. 1 is a block diagram showing the configuration of one embodiment. In this FIG. 1, the
The RF signal is input to an RF amplifier 202.

The output of this RF amplifier 202 is sent to a mixer 203. The output of the voltage controlled oscillator 209 is input to the mixer 203.
Mixed with the output of 09 and sent to IF filter 204
I am trying to output it to .

The output of IF filter 204 is limiter 205
FM Discriminator 206 through (AGC)
It is now being sent to Also, Limita 2
The output of 05 is detected by a level detector 212, and the output thereof is input to the inverting input terminal of a voltage comparator 213. A reference voltage E ₂ is applied to the non-inverting input terminal of the voltage comparator 213.

The output of the voltage comparator 213 controls a switch 214. A reference voltage E ₀ is applied to one terminal of this switch 214, and a reference voltage E ₁ is applied to the other terminal. . The output of the switch 214 is supplied to the non-inverting input terminal of the voltage comparator 20.

On the other hand, the output of the FM discriminator 206 is taken out from an output terminal 211 through an amplifier 210 and applied to an inverting input terminal of a voltage comparator 208 through a low-pass filter 207.

The voltage comparator 208 compares the output voltage of the switch 214 and the output of the low-pass filter 207, and applies the difference voltage to the voltage controlled oscillator 209.

In place of the switch 214, the circuit shown in FIG. 3 can be used. In FIG. 3, resistors R ₁ and R ₂ are connected in series between the power supply Vcc and ground.
The connection point between the resistors _R1 and _R2 is connected to the collector of the transistor Q via the resistor _R3 . The emitter of transistor Q is grounded. The base of transistor Q is connected to the output terminal of voltage comparator 213 via resistor R.

Reference voltages E ₀ and E ₁ at the connection point of resistors R ₁ and R ₂
is generated depending on whether transistor Q is on or off, and the reference voltages E ₀ and E ₁ are E ₀ = Vcc・R ₂ /R ₁ +R ₂ E ₁ Vcc・Rx/R ₂ +Rx (Rx =R ₂ ×R ₃ /R ₂ +R ₃ ).

[Effect of the embodiment]

Next, the operation of the embodiment of FIG. 1 constructed as above will be explained. During normal reception, the
After the RF signal is amplified by RF amplifier 202, it is sent to mixer 203.

The mixer 203 mixes the output of the RF amplifier 202 and the output of the voltage controlled oscillator 209 to generate an IF
Output to filter 204. This IF filter 2
04 extracts the intermediate frequency signal, and the limiter 205
The signal is sent to the FM discriminator 206 through the FM discriminator 206, where it is demodulated. The demodulated signal is amplified by an amplifier 210 and then taken out from an output terminal 211. The operation up to this point is similar to the conventional case shown in FIG.

Furthermore, the output DC average potential obtained at the output end of the FM discriminator 206 is fixed at E ₀ (or near E ₀ ).

On the other hand, the receiver input level decreases and limiter 2
When the output level of 05 decreases, level detector 2
When the output voltage of 12 decreases and becomes lower than the reference voltage E ₂ of the voltage comparator 213, the transistor Q of the switch 214 is turned on by the output of the voltage comparator 213, and the reference voltage E ₀ is switched to the reference voltage E ₁ . .

This reference voltage E ₁ is applied to voltage comparator 208 through switch 214 . At this time, the output of the FM discriminator 206 is applied to the voltage comparator 208 through the low-pass filter 207, and the output of the FM discriminator 206 that has passed through the low-pass filter 207 and the reference voltage _E1 are applied to the voltage comparator 208. The difference voltage is applied to the voltage controlled oscillator 209. Voltage controlled oscillator 209 varies the frequency based on the output of voltage comparator 208 and outputs it to mixer 203 .

In this way, the RF input to the input terminal 201
When the input level of the signal exceeds a certain value, the center DC potential of the FM discriminator 206 is fixed at the reference voltage E ₁ (or near E ₁ ).

Due to the above operation, the reference of the AFC circuit changes at the time of weak input. In other words, by shifting the frequency of the intermediate frequency signal and bringing the synchronizing signal portion closer to the center of the IF filter 204, the loss of the synchronizing signal in the demodulated video signal is prevented and the received image quality is improved.

Furthermore, during strong input, AFC can be controlled so that the center frequency of the entire video signal is fixed at the center of the IF filter 204, so a video signal with less distortion can be obtained.

[Other Examples]

In the above embodiment, the reference voltage switching determination was made based on the input signal level, but the same effect can be expected even if this determination is made based on the magnitude of noise contained in the demodulated signal.

Figure 2 shows an example of this case.
The output of the discriminator 306 is applied to the inverting input terminal of a voltage comparator 308 through a low-pass filter 307, and is applied to the non-inverting input terminal of a voltage comparator 314 through a high-pass filter 312 and a noise detector 313. .

The voltage comparator 314 compares the output of the noise detector 313 and the reference voltage E ₂ and uses the difference voltage between the reference voltages E ₀ and E ₁ applied to the switch 315.
The switched reference voltage
E ₀ or E ₁ is applied to the non-inverting input terminal 308 of the voltage comparator 308 .

The voltage comparator 308 compares this reference voltage with the output of the FM discriminator 306 passed through a low-pass filter, and controls the oscillation frequency of the voltage controlled oscillator 309 based on the deviation voltage. Other configurations and operations are the same as in FIG. 1.

Note that the switch 315 that switches between the reference voltages E ₀ and E ₁ is limited to an analog switch that switches contacts by operating a relay as shown in FIGS. 1 and 4, and a circuit shown in FIG. 3. It's not something you can do.

[Effect of idea]

As described above, according to the AFC device for FM television broadcasting of this invention, in a television broadcasting receiver using the FM system, the input level is detected by the level detection means at the time of weak input, and the output of this detection means and a predetermined value are detected. The first voltage comparator compares the voltage with the reference voltage and outputs it according to the difference voltage, and this output controls the switching of the switch to take out one of the two reference voltages, and outputs the output according to the difference voltage. Voltage and 1st
The oscillation frequency of the voltage controlled oscillator is varied according to the difference voltage, and the output voltage of the voltage controlled oscillator is added to the mixer to generate the FM disc liminator. Since the center frequency of the output is changed, it is possible to prevent periodic signals from being dropped during weak input, and the received image quality is improved.

Along with this, it will be put into practical use by broadcasting satellites.
If applied to FM television broadcast receivers, great effects can be expected.

[Brief explanation of the drawing]

Figure 1 shows this invention for FM television broadcasting.
A block diagram showing the configuration of an embodiment of the AFC device,
Figure 2 shows the AFC of this invention for FM television broadcasting.
Block diagram showing the configuration of another embodiment of the device, No. 3
The figure is a circuit diagram showing an example of the specific circuit configuration of the switch in the AFC device for FM television broadcasting of this invention, and Figure 4 is a circuit diagram showing an example of the specific circuit configuration of the switch in the AFC device for FM television broadcasting of this invention.
FIG. 3 is a block diagram of an AFC device. 203...Mixer, 204...IF filter,
205... Limiter, 206... FM discriminator, 207... Low pass filter, 20
8,213,314...Voltage comparator, 209...
Voltage oscillator, 212... Level detector, 214...
...Switcher, 312...High pass filter, 313
...Noise detector.

Claims (1)

[Claims for Utility Model Registration] (1) Detection means for detecting the level of a received signal received by a television broadcast receiver using the FM method at the time of weak input, and the output of this detection means and a predetermined reference voltage. A first voltage comparator that compares and outputs according to the difference voltage; a switch that switches and outputs one of two reference voltages based on the output of the first voltage comparator; a second voltage comparator that compares one of the two switched and output reference voltages with the output of the detection means and outputs an output according to the difference; The oscillation frequency is controlled according to the output voltage of the above TV broadcast receiver.
An AFC device for FM television broadcasting consisting of a voltage controlled oscillator that outputs to the mixer of this television broadcasting receiver so as to change the center frequency of the FM discriminator. (2) For FM television broadcasting as set forth in claim 1 of the utility model registration claim, wherein the detection means is a detector for noise contained in the output video signal obtained by demodulating with an FM disc discriminator.
AFC device.