JPH09139897A - Demodulator for receiving broadcast - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the demodulators in which a front end section are used in common and the selection of the front end section is simplified with miniaturization at a low cost. SOLUTION: Automatic gain control is conducted so as to make a level of a 1st IF signal outputted from a 1st IF variable amplifier circuit 4 and a level of a 2nd IF signal outputted from a 2nd IF variable amplifier circuit 12 constant respectively with a control signal outputted from an IFAGC circuit based on a level of a 2nd IF signal being an output of the 2nd IF variable amplifier circuit 12. Thus, optimum automatic gain control is applied respectively to the 1st TF signal and the 2nd IF signal. A reception broadcast signal selected vi a common front end section to two reception broadcast signals is given to an FM video demodulation circuit 14 and an MSK demodulation circuit 16, in which the signals are separately demodulated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulation device for broadcast reception, and more particularly to a demodulation circuit for frequency-modulated (FM) video and audio signals from a broadcast satellite and minimum shift keying (MSK) from a communication satellite. ) Pulse code modulation (PCM)
The present invention relates to a broadcast receiving demodulation device in which a front end section that is placed in front of a demodulation circuit of a different modulation method such as a demodulation circuit of a music broadcast signal is shared.

[0002]

2. Description of the Related Art Conventionally, in a satellite broadcasting receiver, an intermediate frequency signal (I
The F signal) demodulates the FM-modulated video signal (video and audio), and the MSK-modulated PCM audio signal.

For example, in a conventional satellite broadcast receiving demodulator described in Japanese Patent Laid-Open No. 5-64101, the front end section is shared by an FM demodulation circuit and an MSK demodulation circuit, and
A configuration is disclosed in which an FM demodulation circuit or an MSK demodulation circuit is selected by a switching circuit in association with a channel selection circuit given from the outside.

Further, the channel selection and the AGC are shared so as to avoid the complication of the configuration. For example, in the conventional satellite broadcast receiving demodulator described in Japanese Patent Laid-Open No. 4-53312, the first IF that allows only the desired MSK-modulated signal to pass through using the output signal of the front end section excluding the IF filter as a signal input. A filter, a digital signal processing circuit for obtaining an audio signal after MSK demodulating with the output signal of the first IF filter as an input, and a second I for passing only a desired FM-modulated video signal (video and audio).
An F filter, a baseband signal processing circuit that obtains a video signal and an audio signal after FM demodulation using the output signal of the second IF filter as an input, and a peak detection of the output signal of the second IF filter to perform a front end section. And a peak detection circuit that obtains a control voltage for controlling the gain of the variable gain amplifier of FIG.

[0005]

However, in the above conventional satellite broadcast receiving demodulator, the automatic gain control is performed based on the second IF signal input to the FM demodulation circuit regardless of the FM and MSK demodulation methods. Since (AGC) is performed, optimum AGC is difficult for each of FM demodulation and MSK demodulation waves.

Further, a plurality of expensive IF filters are required for each of FM demodulation and MSK demodulation, which causes an increase in cost, and it is difficult to reduce the size and simplify the device. Further, the selection of the FM-modulated signal and the MSK-modulated PCM audio signal is switched in the second IF signal portion before demodulation, and since the signal here has a high frequency, simplification of the circuit is also difficult. .

The present invention has been made in view of the above points, and is an FM-modulated video signal and audio signal, or MSK.
An object of the present invention is to provide a broadcast receiving demodulation device capable of improving reception performance when receiving (demodulating) one of the modulated PCM audio signals.

Another object of the present invention is to provide a demodulator for broadcast reception, which can share the front end portion, can easily switch the front end portion, and can realize downsizing and cost reduction.

[0009]

In order to achieve the above object, the present invention according to claim 1 receives a plurality of broadcast signals, each of which has at least an audio signal, which are modulated by different modulation methods, and selects one of them. A front end unit common to a plurality of received broadcast signals, which converts the received received broadcast signal into a first intermediate frequency signal and then into a second intermediate frequency signal;
Automatic gain control means for performing automatic gain control so that the first intermediate frequency signal level and the second intermediate frequency signal level are made constant based on the level of the second intermediate frequency signal output from the front end section. And for the second intermediate frequency signal which is automatically gain controlled by the automatic gain control means,
A plurality of demodulation circuits that separately perform demodulation corresponding to each modulation method of a plurality of broadcast signals, and a demodulation circuit that is operating normally or abnormally among the plurality of demodulation circuits are applied to each demodulation signal of the plurality of demodulation circuits And a switching means for selectively outputting only the output signal of the demodulation circuit operating normally based on the result of the determination.

According to the first aspect of the present invention, the first intermediate frequency signal level and the second intermediate frequency signal level are made constant based on the level of the second intermediate frequency signal output from the front end section. Since automatic gain control is performed as described above, optimum automatic gain control can be performed for each of the first intermediate frequency signal and the second intermediate frequency signal.

According to the first aspect of the present invention, the received broadcast signals selected via the front end section common to the plurality of received broadcast signals are separately demodulated by the plurality of demodulation circuits, respectively. Since the normally demodulated signal is switched and output from the demodulation circuit that demodulates the received received broadcast signal, switching in the front end section and each demodulation circuit can be eliminated.

According to a second aspect of the invention, the switching means in the first aspect of the invention generates a switching signal indicating whether the operation is normal from the audio signal demodulation processing units of the plurality of demodulation circuits, Each of the analog audio signals output from the audio signal demodulation processing unit of the demodulation circuit of is received as an input signal,
It is characterized in that it has a changeover switch for selectively outputting only the output analog voice signal of the voice signal demodulation processing section which is operating normally based on the changeover signal. According to the second aspect of the present invention, the signal switching can be performed at the stage of the analog signal.

According to a third aspect of the present invention, a gain variable for amplifying a first intermediate frequency signal of a received broadcast signal selected from among a plurality of broadcast signals each having at least an audio signal, which are modulated by different modulation systems. A first amplifier circuit of
A frequency conversion circuit for frequency-converting the output first intermediate frequency signal of the first amplifier circuit into a second intermediate frequency signal;
A distributor that distributes the second intermediate frequency signal to the same plurality of receivable broadcast signals, and a variable gain that individually amplifies the plurality of second intermediate frequency signals distributed by the distributor. A plurality of second amplifier circuits of
The level of the second intermediate frequency signal extracted from the one second amplification circuit of the plurality of second amplification circuits is detected, and the gain control signal corresponding to the detected level is generated to generate the second control signal. A first gain control circuit that controls the gains of the first amplification circuit and the second amplification circuit, so that the output second intermediate frequency signal level of the amplification circuit becomes constant, and a plurality of second gain control circuits. A plurality of demodulation circuits that separately perform demodulation corresponding to the respective modulation methods of the plurality of broadcast signals on the plurality of second intermediate frequency signals extracted from the amplifier circuit, and one of the plurality of demodulation circuits. Based on the output signal of the demodulation circuit that demodulates the second intermediate frequency signal extracted from the remaining second amplification circuit excluding the second amplification circuit, the output of the remaining second amplification circuit The remaining second frequency is adjusted so that the intermediate frequency signal level is constant. Switching between the second gain control circuit for controlling the gain of the width circuit and the analog audio signal output from the audio signal demodulation processing unit in the plurality of demodulation circuits, which indicates whether the operation of the audio signal demodulation processing unit is abnormal or normal. And a changeover switch for selecting only an analog audio signal output from one normally operating audio signal demodulation processing unit based on the signal.

According to the third aspect of the present invention, the front end section can be constructed in common by the first amplifier circuit and the frequency conversion circuit, and the first and second intermediate gain control circuits can be used for the first and second intermediate circuits. Gain control can be separately performed for each frequency signal.

According to a fourth aspect of the present invention, the frequency pass is such that a received broadcast signal having the widest transmission frequency bandwidth is selected from a plurality of broadcast signals, each of which has at least an audio signal and is modulated by different modulation methods. A filter circuit having a bandwidth is provided between the frequency conversion circuit and the distributor. As a result, an expensive filter circuit need only be provided in the front end section.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an embodiment of a broadcast receiving demodulator of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a block diagram of an embodiment of a demodulator for broadcast reception according to the present invention. The embodiment shown in the figure is an example of a satellite broadcast receiving demodulation device that is capable of receiving an FM-modulated video signal (video and audio) sent from a broadcasting satellite and an MSK-modulated PCM music broadcast. .

This satellite broadcast receiving demodulator has a first IF
A first IF amplifier circuit 2 that amplifies a first intermediate frequency (IF) signal output from the parabolic antenna down converter through a signal input terminal 1 and a center frequency of a pass band based on a demodulation (selection) instruction of the input IF signal. It has a variable tuning filter 3 which is variable and suppresses and outputs unnecessary waves.

The first output from the variable tuning filter 3
A first IF variable amplifier circuit 4 that variably amplifies and outputs an IF signal with a first IFAGC signal a, and a tuning signal that instructs selection of an FM-modulated video signal (video and audio) or an MSK-modulated PCM audio signal. A tuning signal input terminal 5 for inputting from a not-shown selection unit or the like is provided. Further, the tuning control voltage for varying the frequency of the local oscillation signal oscillated by the local oscillation circuit 7 by the tuning signal from the tuning signal input terminal 5 and the center frequency of the pass band in the variable tuning filter 3 And a local oscillation circuit 7 that outputs a local oscillation signal for frequency conversion based on the tuning control voltage from the channel selection circuit 6.

Further, the transmission bandwidth is wider than that of the frequency conversion circuit 8 for converting the first IF signal into the second IF signal based on the local oscillation signal from the local oscillation circuit 7 and the MSK-modulated PCM voice signal in the first IF signal. The IF filter 9 is set to have a pass band width required to pass the FM-modulated video signal (video and audio).
The output of the IF filter is divided into two by the distributor 11, one of which is input to the second IF variable amplification circuit 12 and the other of which is input to the second IF variable amplification circuit 15. The second IF variable amplification circuit 15 variably amplifies the second IF signal based on the second IFAGC signal b. An FM video demodulation circuit 14 that outputs a video baseband signal obtained by demodulating the second IF signal is provided on the output side of the second IF variable amplification circuit 12.

The first IF variable amplifier circuit 4 outputs the first IFAGC signal a based on the amplitude level of the second IF signal output from the second IF variable amplifier circuit 12, and the second IF variable amplifier circuit 4 outputs the second IF signal.
An IFAGC circuit 13 is provided for outputting the second IFAGC signal c to the IF variable amplification circuit 12 to perform automatic gain control (AGC). On the output side of the second IF variable amplification circuit 15, an MSK demodulation circuit 16 for performing MSK demodulation and M
Gaussian filters 17 and 18 for band-limiting the SK demodulated I signal and Q signal, respectively, are provided, and an IFAGC circuit 19 is provided on the output side of the Gaussian filter 18.

The IFAGC circuit 19 outputs a second signal based on the amplitude level of one of the I signal and the Q signal (here, the Q signal).
The second IFAGC signal b is output to the IF variable amplification circuit 15. Further, the switching circuit 20 selects the video baseband signal demodulated by the FM video demodulation circuit 14 or the I signal and the Q signal from the Gaussian filters 17 and 18.

FIG. 2 is a block diagram showing a detailed structure of the switching circuit 20 in FIG. In FIG. 2, the switching circuit 20 includes a video processing circuit 201 that processes the video baseband signal output from the FM video demodulation circuit 14 and sends the output video signal, and four-phase phase modulation (QPSK) in the video baseband signal. QPSK demodulation circuit 202 for converting the voice signal of the above into a PCM voice signal, and this QPSK demodulation circuit 20.
2, a PCM audio processing circuit 203 for converting the PCM audio signal output from the analog signal into an analog signal and transmitting the audio signal, and a Viterbi decoding circuit for decoding the I signal and the Q signal output by the Gaussian filters 17 and 18 into a high-order multiplexed signal. 204, a high-order multiplexed signal separation circuit 205 that generates and outputs this high-order multiplexed signal to a PCM audio signal, and a PCM audio signal output by the high-order multiplexed signal separation circuit 205 is converted into an analog signal to convert the audio signal The output audio signals from the PCM audio processing circuit 206 and the PCM audio processing circuits 203 and 206 to be sent are supplied to the fixed contact, and the movable contact outputs the output audio signal based on the switching signal at the time of normal (or abnormal) demodulation. And a changeover switch 207 for automatically selecting one of the two.

Next, the operation of this embodiment will be described. In FIGS. 1 and 2, the first IF signal input to the first IF signal input terminal 1 is amplified by the first IF amplifier circuit 2 and then the center frequency of the pass band is adjusted by the tuning control voltage output from the tuning circuit 6. It is input to the variable tuning filter 3 which is variable, and unnecessary waves are suppressed here. Variable tuning filter 3
The output first IF signal of is input to the first IF variable amplifier circuit 4, and the first IFAGC signal a from the IFAGC circuit 13 is output.
The signal is variably amplified by and then supplied to the frequency conversion circuit 8.

On the other hand, the tuning circuit 6 outputs a tuning control voltage to the local oscillation circuit 7 for giving a variable instruction of the frequency of the local oscillation signal in response to the tuning signal from the tuning signal input terminal 5. As a result, in the frequency conversion circuit 8, the first IF
The first IF signal output from the variable amplifier circuit 4 and the local oscillation signal having a frequency corresponding to the tuning signal from the local oscillation circuit 7 are frequency-converted, and the first IF signal from the selected broadcasting station is the second IF having a predetermined frequency. Converted to a signal.

This second IF signal is input to the IF filter 9 to suppress unnecessary waves. The IF filter 9 is a bandpass filter (BPF), and the passband characteristic thereof is the FM-modulated video signal (video and audio) as described above.
The bandwidth is set to pass the transmission bandwidth of. The second IF signal having information from the selected broadcasting station extracted from the IF filter 9 is divided into two by the distributor 11, one of which is passed through the second IF variable amplification circuit 12 and the IF AGC circuit 13 and the FM video demodulation circuit. 14, and the other is supplied to the second IF variable amplification circuit 15.

The iFAGC circuit 13 is an FM video demodulation circuit 1
The signal level input to 4 is detected, and the gains of the second IF variable amplification circuits 4 and 12 are variably controlled so that the input signal level of the FM video demodulation circuit 14 becomes constant. The second IF signal from the second IF variable amplification circuit 12 is input to the FM video demodulation circuit 14 and demodulated therein to be an output video baseband signal.

First IFAGC from IFAGC circuit 13
The signal a and the second IFAGC signal c are the levels of the first IF signal supplied to the first IF signal input terminal 1, that is,
The range of gain suppression (amplification reduction) in each of the first IF variable amplification circuit 4 and the second IF variable amplification circuit 12 differs depending on the received electric field strength.

FIG. 3 is a diagram for explaining the range of gain suppression (amplification reduction) in the first IF variable amplifier circuit 4 and the second IF variable amplifier circuit 12. FIG. 3 shows an example of the voltage characteristics of the first and second IFAGC signals a, b, and c with respect to the first IF signal level (received electric field strength). However, in FIG. 3, the second IFAGC signal voltages b and c are overlaid.
That is, the second IFAGC signal voltage b described later is
The characteristic is equivalent to the GC signal voltage c and is a voltage characteristic suitable for each demodulation circuit. Further, the lower the voltages of the first and second IF AGC signals a and c (b), the more the respective IF gains of the first IF variable amplification circuit 4 and the second IF variable amplification circuits 12 and 15 are suppressed (amplification lowered). Has been done.

Therefore, as can be seen from FIG. 3, the first IF
In the first IF variable amplifier circuit 4 to which the AGC signal a is applied, the first IF AGC signal a is changed from strong electric field strength to medium electric field strength.
Changes from a low voltage to a high voltage and its gain is stabilized. In addition, the second IF to which the second IF AGC signal c is applied
In the variable amplifier circuit 12, the second IFAGC signal c changes from a low voltage to a high voltage with a medium electric field strength to a weak electric field strength, and automatic gain control is performed to make the gain constant.

Returning to FIG. 1 again, the second IF signal supplied from the distributor 11 to the second IF variable amplifier circuit 15 is the second IFAGC from the IFAGC circuit 19 here.
The signal b is variably amplified and supplied to the MSK demodulation circuit 16 to be MSK demodulated into an I signal and a Q signal. The I signal and the Q signal are baseband signals, and the passband width of the IF filter 9 in the front end portion is the same as that of the MS as described above.
Since the band width is wider than that required for K demodulation, a band limitation is required separately. Therefore, the necessary band limitation is performed by the Gauss filters 17 and 18 which are low-pass filters having a Gaussian distribution type pass characteristic.

The IFAGC circuit 19 includes a Gaussian filter 1
The amplitude level of the Q signal from 8 is detected, and the second IFAGC
The signal b is generated and output to the second IF variable amplification circuit 15,
The signal level input to the MSK demodulation circuit 16 is made constant. The amplitude levels of both the I signal and the Q signal may be arithmetically averaged, and this value may be used as the amplitude level detection value.

The video baseband signal demodulated from the FM video demodulation circuit 14 and the I and Q signals from the Gauss filters 17 and 18 are supplied to the switching circuit 20 having the configuration shown in FIG. The output signal from is selected.

In FIG. 2, the video baseband signal output from the FM video demodulation circuit 14 is the video processing circuit 20.
The signal is processed in 1 and converted into a video signal, which is directly output from the switching circuit 20. On the other hand, since the QPSK-modulated PCM audio signal is superimposed on the above-mentioned video baseband signal, it is input to the QPSK demodulation circuit 202, where QPSK
After being K demodulated and converted into a PCM audio signal, it is PCM-decoded by the PCM audio processing circuit 203 and converted into an analog audio signal.

Further, the I signal or Q signal whose band is limited by the Gaussian filters 17 and 18 of FIG. 1 is supplied to the Viterbi decoding circuit 204 of FIG.
The PCM audio signal is converted into a PCM audio signal by the high-order multiple signal separation circuit 205
Further, it is PCM-decoded by the PCM audio processing circuit 206 in the subsequent stage and converted into an analog audio signal.

The changeover switch 207 has a movable contact piece of PC.
The switching signal output from the M audio processing circuit 203 is switched to select one of the output audio signals of the PCM audio processing circuits 203 and 206. In other words, the changeover by the changeover switch 203 is performed by the PCM audio processing circuit 203 or the PCM audio processing circuit 206, whichever has normally performed the PCM audio processing (that is, the demodulation circuit side of the modulation system of the selected frequency). This is done with the above switching signal so as to select the analog audio signal from the PCM audio processing circuit (for The one of the PCM audio processing circuit 203 and the PCM audio processing circuit 206 that has performed the abnormal PCM audio processing sends a switching signal to the change-over switch 123 to perform the normal PCM audio processing or the PCM audio processing circuit 203. The output analog audio signal of the PCM audio processing circuit 206 may be selected.

As described above, according to this embodiment, the switching between the FM video signal accompanied by the audio signal and the MSK-modulated PCM audio signal containing only the audio signal depends on the modulation method of the selected frequency. Since only the analog audio signal after demodulation is switched, it is not necessary to switch the front end portion of the switching circuit 20 at the front stage, and only the simplest analog stage is required. As a result, the front end section is a FM-modulated video signal (video and audio), MS.
It is commonly used for K-modulated PCM audio signals.

Further, as described above, since AGC is not performed only based on the second IF signal, FM demodulation and MS
Optimal AGC is performed for each of the K demodulated waves,
Reception performance is improved. Further, an expensive IF filter for FM demodulation and MSK demodulation is not required, which enables cost reduction, size reduction and simplification.

In this embodiment, two PCM audio processing circuits 203 and 206 are provided as shown in FIG. 2, but the PCM audio processing circuits 203 and 206 perform the same processing, and the configuration is the same. Since they are the same, they can be shared. In this case, an output signal from the QPSK demodulation circuit 202 and the high-order multiple signal separation circuit 205 is selected by the change-over switch as an input of one PCM voice processing circuit. The selection by this change-over switch may be performed by providing a video presence / absence determination circuit inside the video processing circuit 201, determining the presence / absence of a video signal by this circuit, and switching according to the determination result.

Further, in the above embodiments, the demodulation circuits for receiving and demodulating the satellite broadcast signals of two different modulation systems are respectively provided, but the reception for receiving broadcast signals of three or more different modulation systems is provided. The present invention can be similarly applied to a device.

[0041]

As described above, according to the present invention,
Based on the level of the second intermediate frequency signal output from the front end unit, automatic gain control is performed so as to keep the first intermediate frequency signal level and the second intermediate frequency signal level constant, respectively. Since optimum automatic gain control is performed for each of the first intermediate frequency signal and the second intermediate frequency signal, demodulation characteristics with high performance corresponding to the input range can be obtained, and the reception performance can be improved.

Further, according to the present invention, the received broadcast signals selected via the front end section common to the plurality of received broadcast signals are separately demodulated by the plurality of demodulation circuits, respectively, and are selected. By switching and outputting the normally demodulated signal from the demodulation circuit that demodulates the received broadcast signal, switching in the front end section and each demodulation circuit is unnecessary, so it is easy to select the signal after demodulation of each modulation method. You can

Further, according to the present invention, the analog audio signals output from the audio signal demodulation processing units in the plurality of demodulation circuits are converted based on the switching signal indicating whether the operation of the audio signal demodulation processing units is abnormal or normal. Since the configuration has a changeover switch that selects only the analog voice signal output from the normally operating voice signal demodulation processing unit, the changeover switch at the simplest analog stage can be used, and the front end Combined with the fact that the parts are shared and the IF filter is provided only in the front end part, the device can be downsized, simplified, and reduced in cost.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a broadcast receiving demodulation device according to the present invention.

FIG. 2 is a block diagram showing a detailed configuration of a switching circuit shown in FIG.

FIG. 3 is a diagram for explaining a range of gain suppression in the IF variable amplification circuit shown in FIG.

[Explanation of symbols]

 2 1st IF amplification circuit 3 Variable tuning filter 4 1st IF variable amplification circuit 6 Channel selection circuit 7 Local oscillation circuit 8 Frequency conversion circuit 9 IF filter 11 Distributor 12, 15 2nd IF variable amplification circuit 13, 19 IFAGC circuit 14 FM video demodulation Circuit 16 MSK demodulation circuit 17, 18 Gaussian filter 20 Switching circuit 201 Video processing circuit 202 QPSK demodulation circuit 203, 206 PCM audio processing circuit 204 Viterbi decoding circuit 205 Higher-order multiplex signal separation circuit 207 Changeover switch

Claims (5)

[Claims] 1. A second intermediate frequency after receiving a plurality of broadcast signals, each of which has at least an audio signal, modulated by different modulation methods, and converting the received broadcast signal selected from the plurality of broadcast signals into a first intermediate frequency signal. A front end portion common to the plurality of received broadcast signals, which is converted into a signal; and a first intermediate frequency signal level and the first intermediate frequency signal level based on a level of a second intermediate frequency signal output from the front end portion. 2 to the automatic gain control means for performing automatic gain control so as to keep the intermediate frequency signal levels of 2 constant, and the plurality of broadcasts with respect to the second intermediate frequency signal which is automatically gain controlled by the automatic gain control means. A plurality of demodulation circuits that separately perform demodulation corresponding to respective modulation methods of signals, and a demodulation circuit that is operating normally or abnormally among the plurality of demodulation circuits A demodulating device for broadcast reception, comprising: a switching unit that makes a determination based on each demodulated signal of the demodulating circuit and selectively outputs only an output signal of a demodulating circuit that is operating normally based on the determination result. 2. The switching means generates a switching signal indicating whether the operation is normal from the audio signal demodulation processing units of the plurality of demodulation circuits, and outputs the switching signal from the audio signal demodulation processing units of the plurality of demodulation circuits. 2. An analog audio signal as an input signal, and a changeover switch for selectively outputting only the output analog audio signal of the audio signal demodulation processing unit which is normally operating based on the switching signal. The broadcast receiving demodulator described. 3. A variable gain first amplifier circuit for amplifying a first intermediate frequency signal of a received broadcast signal selected from among a plurality of broadcast signals each having at least an audio signal, which are modulated by different modulation methods. A frequency conversion circuit that frequency-converts the first intermediate frequency signal output from the first amplifier circuit into a second intermediate frequency signal; and the plurality of broadcasts capable of receiving the second intermediate frequency signal. A plurality of dividers that respectively divide into a plurality of signals, and a plurality of variable gain second amplifying circuits that individually amplify the plurality of second intermediate frequency signals respectively distributed by the dividers; Detecting the level of the second intermediate frequency signal extracted from one of the second amplification circuits of
A gain control signal corresponding to the detection level is generated so that the output second intermediate frequency signal level of the one second amplifier circuit becomes constant, and the first amplifier circuit and the one second A first gain control circuit for controlling respective gains of the amplifier circuits; and a plurality of second gain circuits extracted from the plurality of second amplifier circuits.
A plurality of demodulation circuits for separately demodulating the intermediate frequency signals corresponding to the respective modulation methods of the plurality of broadcast signals, and the rest of the plurality of demodulation circuits excluding the one second amplification circuit The second intermediate frequency signal level of the remaining second amplifier circuit is constant based on the output signal of the demodulation circuit that demodulates the second intermediate frequency signal extracted from the second amplifier circuit of And a second gain control circuit for controlling the gain of the remaining second amplifier circuit, and an analog audio signal output from an audio signal demodulation processing unit in the plurality of demodulation circuits, And a changeover switch for selecting only an analog audio signal output from one normally operating audio signal demodulation processing unit based on a switching signal indicating whether the operation of the demodulation processing unit is abnormal or normal. For broadcasting reception Adjusting unit. 4. A filter circuit having a frequency pass bandwidth for selecting a received broadcast signal having a widest transmission frequency bandwidth among a plurality of broadcast signals each having at least an audio signal, which are modulated by different modulation methods. 4. The demodulator for broadcast reception according to claim 3, wherein is provided between the frequency conversion circuit and the distributor. 5. The first plurality of broadcast signals, each of which has at least an audio signal and are modulated by different modulation methods, are multiplexed signals of a video signal and a QPSK audio signal are modulated by a first modulation method. The broadcast signal and the second broadcast signal in which the PCM audio signal is modulated by the second modulation method are included. The audio signal processing unit in the plurality of demodulation circuits decodes the PCM audio signal into an analog audio signal. Whether the video signal demodulation circuit unit in the demodulation circuit for the first broadcast signal outputs a video signal which has a common PCM audio processing unit for conversion and which is input to the common PCM audio processing unit 4. The broadcast receiving demodulator according to claim 3, further comprising a selecting unit that selects the signal based on a signal indicating whether or not it is present.