JPH05137091A - Fm television receiver - Google Patents

Abstract

PURPOSE:To stabilize the oscillation frequency of a local oscillator to perform the frequency conversion of a reception television FM signal even when a power source is applied or a signal is interrupted transiently. CONSTITUTION:A signal processing circuit 6, when detecting the specific period of a demodulated video signal (a), outputs a pulse voltage signal (b) synchronized with the period. When the pulse voltage signal (b) is obtained based on the acquisition of the pulse voltage signal (b), a mode that a switch 13 makes a hold circuit 14 hold the specific period of the video signal (a) by sampling the video signal (a) by the pulse voltage signal (b), and a switch 16 makes a hold circuit 17 hold a comparison voltage from a comparison voltage generation circuit 15 similarly is set. When no pulse voltage signal (b) is obtained, the switches 13, 16 are always turned on by a fixed voltage, and a mode to hold the mean voltage of the video signal (a) by an integration circuit 9 and the comparison voltage by the hold circuits 14, 17, respectively can be set. A differential amplifier 18 controls the local oscillator 3 by the differential voltage of the hold circuits 14 and 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a second satellite broadcast receiver.
The present invention relates to an FM television receiving apparatus such as a heterodyne receiving apparatus that is suitable for receiving a transmission signal that is mainly FM-modulated (frequency-modulated) and has a fixed frequency for a certain period of time. The present invention relates to an AFC for controlling the oscillation frequency of a local oscillator used for frequency conversion of John FM signals.

[0002]

2. Description of the Related Art In satellite broadcasting that directly receives television broadcasting from a geostationary satellite, a modulation system mainly consisting of FM modulation is used, and this FM modulation system is also AC-coupled and the average value of the modulated signal is a constant frequency. Is often used.

However, since the transmission band is limited, if there is a rapid change in the average value voltage of the modulated signal, for example, when the black screen changes to the white screen, the spectrum of the FM signal goes out of the transmission band. If you want to prevent this, or if you want to prevent the reference DC voltage required for playback with a special video signal from changing depending on the content of the video, set the reference period of the modulated signal to An FM modulation method that fixes the FM frequency to a constant frequency is used.

As an example of such a modulation type receiving circuit,
For example, it is shown in FIGS. 3 and 5 of JP-A-57-135582.

[0005]

The above-mentioned conventional example has a structure for stabilizing the local oscillation frequency of the second heterodyne receiver of the satellite broadcast receiver, and the frequency is constant during steady reception and when the frequency is constant. When the reference period is long, stable control of the local oscillation frequency can be performed, but when the power is turned on or when the signal input is temporarily cut off, the control of the local oscillation frequency is disturbed, and its recovery is delayed. Further, there is a drawback that the control becomes incomplete when the reference period in which the frequency is constant is short.

It is an object of the present invention to provide an FM television receiving apparatus which eliminates the above-mentioned conventional drawbacks and can perform control such that the local oscillation frequency is always stable.

[0007]

To achieve the above object, the present invention provides an AF for controlling the oscillation frequency of a local oscillator.
The C means, the mode in which the signal voltage of the demodulated television signal of the television signal in a specific period is operated by sampling and holding, and the mode in which the average value voltage of the television signal is held and operated are selectively set. And a control means.

[0008]

When the normal TV FM signal is received, A
The FC means samples and holds the level of the demodulated television signal which serves as a reference for a specific period, and controls the oscillation frequency of the local oscillator. Thereby, even if the specific period is short, the oscillation frequency of the local oscillator can be stably controlled. When the power is turned on or the signal is temporarily cut off, the AFC means holds the average voltage value of the demodulated television signal to control the oscillation frequency of the local oscillator, so that the normal reception of the FM signal can be prevented. Until it becomes possible, it enables stable reception of television FM signals.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an FM television receiving apparatus according to the present invention, which is shown as a configuration of a second heterodyne receiving circuit and a signal processing circuit of a satellite broadcast receiver, where 1 is an input terminal, 2 is a frequency mixing circuit, 3 is a local oscillation circuit, 4 is a band pass filter, 5 is an FM demodulation circuit, 6 is a video signal processing circuit, 7 is an output terminal, 8 is an input terminal, 9 is an integrating circuit, and 10 and 11 are A switch, 12 is a fixed voltage generation circuit, 13 is a switch, 14 is a hold circuit,
Reference numeral 15 is a comparison voltage generation circuit, 16 is a switch, 17 is a hold circuit, 18 is a differential amplifier circuit, and 19 is an adder.

In FIG. 1, a plurality of received FM signals are input from an input terminal 1, and a desired FM signal is selected by a frequency mixing circuit 2 and a local oscillation circuit 3 whose oscillation frequency is determined by a tuning voltage from an input terminal 8. And frequency conversion is performed, and the bandpass filter 4 removes interfering signals other than the desired FM signal. The obtained desired FM signal is demodulated into a video signal by the FM demodulation circuit 5, processed by the video signal processing circuit 6 such as waveform shaping, and output from the output terminal 7. The video signal processing circuit 6 also outputs the pulse control signal b corresponding to the reference period of the constant frequency set in the FM signal, and also outputs the control signal c indicating whether or not the pulse control signal b is generated. To do.

The output video signal a of the FM demodulation circuit 5 is also supplied to the switch 10 and the integration circuit 9, and an average DC voltage signal is generated and supplied to the switch 10. The switch 10 is controlled by the control signal c from the video signal processing circuit 6, and when the pulse control signal b is not generated, the DC voltage signal from the integration circuit 9 is generated, and when the pulse control signal b is generated, the FM demodulation circuit 5 is controlled. The demodulated signals a are selected and supplied to the first sample-hold circuit including the switch 13 and the hold circuit 14. The output signal of the first sample hold circuit is supplied to the differential amplifier circuit 18. The output voltage of the comparison voltage generation circuit 15 is the switch 16 and the hold circuit 17.
Is supplied to the second sample hold circuit, and its output signal is also supplied to the differential amplifier circuit 18. These difference signals from the differential amplifier circuit 18 are input to the input terminal 8 by the adder 19.
It is superimposed on the tuning voltage from.

The control of the switches 13 and 16 in the first and second sample and hold circuits is performed by the pulse control signal b selected by the switch 11 controlled by the control signal c or the fixed voltage generating circuit 12. The control signal d is a fixed voltage. This allows
When the pulse control signal b is not generated, the inputs and outputs of the switches 13 and 16 are always conductive, and the pulse control signal b
Is generated, it conducts only during the period of the pulse control signal b.

FIG. 2 shows an example of a video signal which is the receiving processing target of this embodiment. This is an IBA EXPERIMENTAL & DEVELOPMENT REPORT (IBA EXPERIMENTAL & DE).
VELOPEMENT Report 116/81 described in FIG. 5 for the multiplexed analog component (MULTIPLEXED ANALOGU).
E COMPONENT) signal, abbreviated Mac signal (MA
This is a television signal called C).

In this signal, T ₁ period is a reference period indicating the center voltage of the signal, T ₂ is a signal period,
It is composed of a color signal C, a luminance signal Y and a synchronization signal SYNC. In the reference period T ₁ , the reference voltage is necessary for processing the signal, and the FM modulation method is used for transmission so that the voltage in the T ₁ period does not change due to the change in the signal in the T ₂ period. Modulation is used in the T ₁ period so that the frequency is constant regardless of the change in the signal in the T ₂ period.

As a signal obtained by FM-modulating a television signal having such a reference voltage period, that is, an FM signal having a reference frequency period, in addition to the above-mentioned Mac signal, a muse (planned by the Japan Broadcasting Corporation (NHK) ( MUSE) signal. When receiving an FM signal having these reference frequency periods, the receiver also needs to perform frequency control so that the reference frequency period becomes a constant frequency.

FIG. 3 is a timing chart showing the signals of the respective parts in FIG. 1, and the operation of this embodiment will be described below.

When the received FM signal is a signal whose frequency is constant during the T ₁ period and whose frequency changes according to the video signal during the T ₂ period, the demodulated signal a of the FM demodulation circuit 5 has a DC voltage DC during the T ₁ period. And the T ₂ period is the video signal PS. In the video signal processing circuit 6, the pulse control signal b corresponding to the T ₁ period is generated from the synchronization information of the video signal PS. Also, from the video signal processing circuit 6, when the power is turned on,
Alternatively, when the reception signal is temporarily interrupted due to channel selection or the like, the control signal c corresponding to the time T ₀ until the pulse control signal b is generated is generated.

As an example, the voltage of the pulse control signal b generated in the T ₁ period is high voltage and low voltage in other periods, and the control signal c is high voltage in the T ₀ period and low voltage in other periods. Assuming that the output fixed voltage of the fixed voltage generation circuit 12 is almost the same as the high voltage of the pulse control signal b in the T ₁ period,
The switch 11 selects a fixed voltage in the period T ₀ , and selects and outputs the pulse control signal b in the period in which the control signal c is not generated.

In the period T ₀ , the switch 10 is operated by the integrating circuit 9
Output signal e of the switch 10 becomes the average value voltage of the demodulation signal a of the FM demodulation circuit 5, and in this case, the switches 13 and 16 output the output voltage d of the switch 11 at a high voltage. Since it is a fixed voltage, it is always in a conductive state, and the output voltage f of the switch 13 and the output voltage h of the hold circuit 14 are average value voltages from the integrating circuit 9. Further, the output voltage g of the switch 16 and the hold circuit 1
Since the output voltage i of 7 is the comparison voltage from the comparison voltage generation circuit 15, the differential voltage between them is obtained from the differential amplification circuit 18, and this compares with the average value voltage of the demodulation signal a of the FM demodulation circuit 5. The oscillation frequency of the local oscillation circuit 3 is controlled so that the output voltages of the voltage generation circuit 15 become equal.

Therefore, even if the oscillation frequency of the local oscillator circuit 3 fluctuates due to temperature or the like and is detuned, the video signal a can be quickly obtained from the FM demodulation circuit 5 by the action of the average value AFC, and the video signal processing circuit. The pulse control signal b in 6 is smoothly generated. However, in this period T ₀ , the reference frequency changes according to the average value of the video signal, so that the reference voltage DC of the demodulation signal a changes, and the luminance signal of the video signal changes to the white or black side. ..

Next, when the video signal a is obtained from the FM demodulation circuit 5 and the pulse control signal b is generated in the video signal processing circuit 6, the output signal e of the switch 10 becomes this video signal a, and the output voltage of the switch 11 is outputted. d becomes the pulse control signal b. Therefore, the switches 13 and 16 are controlled by the pulse control signal b.
During the period T _{1 and} cut off during the other period T ₂ , the output signal f of the switch 13 becomes the reference voltage DC for the period T ₁ of the video signal a, and the output voltage g of the switch 16 changes the comparison voltage generation circuit. The output voltage of 15 is output for the period T ₁ and is charged by the hold circuits 14 and 17, respectively, and is held as the voltages h and i for the next period T ₂ .

A differential voltage between the hold voltages h and i is obtained from the differential amplifier circuit 18.
4, 17 are charged during the T ₁ period as shown, and T ₂
The first and second sample-and-hold circuits are set to have equal charge and discharge time constants, and the differential amplifier circuit 1 in the T ₂ period has a characteristic of discharging little by little during the period.
The change in the output differential voltage of No. 8 is made small. That is, in this embodiment, the oscillation frequency of the local oscillation circuit 3 is controlled so that the reference voltage DC of the demodulated signal a in the T ₁ period and the output voltage of the comparison voltage generation circuit 15 become equal, but the hold circuit in the T ₂ period. By making the discharge time constants of 14 and 17 large and making the discharge time constants of the reference voltage and the comparison voltage the same, the fluctuation of the output differential voltage of the differential amplifier circuit 18 can be significantly reduced, and the T ₂ period can be reduced. The fluctuation of the oscillation frequency of the local oscillation circuit 3 is reduced.

Here, by setting the output impedances of the FM demodulation circuit 5, the integration circuit 9 and the comparison voltage generation circuit 15 which are connected to the first and second sample hold circuits when the switch is conductive, the first, The charging time constant of the second sample-hold circuit can be made small. Therefore, due to the above-mentioned holding effect, a stable sampling AFC can be configured even with the pulse control signal b of a short period.

FIG. 4 is a circuit diagram showing a specific example of the AFC circuit configuration of a main part in the embodiment shown in FIG. 1, Q ₁
~ Q ₁₁ is a transistor, C1, C2 are capacitors,
The same symbols are attached to the portions corresponding to.

In the figure, switches 10, 11, 1
Using the gate switch of the CMOS to 3,16, the output transistor Q ₃ are the emitter follower constant voltage circuit of the transistor Q _1, the output transistor Q ₂ and the comparison voltage generation circuit 15 of the integrating circuit 9, to which the demodulation signal a is input As a result, the output impedance is reduced, the charging time constant determined by the output impedance and the capacitances C1 and C2 of the hold circuits 14 and 17 is reduced, and charging is performed by conducting the switch for a short period.

At the time of holding, the transistors Q ₄ , Q ₅ ,
Q ₆ and Q _7, Q _8, a multi-stage impedance of the output side as viewed from the capacitor C1, C2 to connect Q ₉ significantly by increasing the discharge time constant, as well as reduce the voltage variation of the hold, the two The hold circuits 14 and 17 have the same discharge time constant. Therefore, the difference between the base voltages of the transistors Q ₁₀ and Q ₁₁ of the differential amplifier circuit 18 does not change, the output voltage of the differential amplifier circuit 18 does not change, and a good hold effect can be obtained for a long period.

[0027]

As described above, according to the present invention,
The AFC means of the local oscillator is a normal transmission television F.
While the M signal is being received, the frequency of the local oscillator is controlled by sampling and holding the signal voltage of the video signal during the specific period. Therefore, even if the specific period is short, the frequency control of the local oscillator is stabilized. Further, even when the power is turned on or the signal is temporarily cut off, the sample hold circuit holds the average value voltage of the video signal to control the frequency of the local oscillator. The transition to

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an FM television receiver according to the present invention.

FIG. 2 is a diagram showing an example of signals to be used in the present invention.

FIG. 3 is a timing diagram showing signals of various parts in FIG.

FIG. 4 is a circuit diagram showing a specific example of a circuit configuration of a main part in the embodiment shown in FIG.

[Explanation of symbols]

 2 frequency mixing circuit 3 local oscillation circuit 4 band pass filter 5 FM demodulation circuit 6 video signal processing circuit 9 integration circuit 10 and 11 switch circuit 12 fixed voltage generation circuit 14 hold circuit 15 comparison voltage generation circuit 16 switch 17 hold circuit 18 differential Amplifier circuit 19 Adder

Claims (1)

[Claims] 1. The first television FM signal is received and the second frequency is converted by the frequency converter including the local oscillator and the mixer.
In the FM television receiving apparatus for frequency-converting to the television FM signal and demodulating the second television FM signal by the FM demodulator to obtain a television signal, AFC means for controlling the oscillation frequency of the local oscillator, Control means for selectively setting a mode in which the AFC means operates by sampling and holding a signal voltage of the demodulated television signal in a specific period and a mode in which an average value voltage of the television signal is held and operated. An FM television receiving apparatus comprising: