JP3709866B2 - Digital broadcast receiver - Google Patents

Description

[0001]
[Industrial application fields]
The present invention transmits a first modulated signal in which the carrier signal is suppressed and a second modulated signal in which the carrier wave remains among modulated signals obtained by modulating the carrier signal with at least a video signal. The received modulated signal is input, and a receiving device that receives the first modulated signal having a desired frequency from these modulated signals or the transmitted modulated signal is input. The present invention relates to a receiving apparatus that selectively receives a first modulated signal and a second modulated signal having a desired frequency from among the modulated signals.
[0002]
As the first modulation signal, for example, a digitally modulated high-definition television signal such as QAM (Quadrature Axis Amplitude Modulation) can be considered, and as the second modulation signal, for example, VSB is used. An AM-modulated standard television signal such as (residual sideband amplitude modulation) (specifically, a television signal based on NTSC, PAL, SECAM, etc.) can be considered.
[0003]
[Prior art]
In recent years, with the expansion of television broadcasts, CATV broadcasts, etc., multi-channel broadcasting has been promoted. As a result, reception apparatuses having double superheterodyne frequency converters that can reduce intermodulation interference and the like even during multi-channel reception have come to be used. FIG. 9 shows a conventional double superheterodyne television receiver. In FIG. 9, 1 is a signal input terminal, 2 is a channel selection signal input terminal, 4 is a video and audio signal output terminal, 5 is an input filter (bandpass filter), 6 is a variable gain amplifier, 9 is a first mixer, 10 Is a first local oscillator, 12 is a first IF amplifier, 13 is a second mixer, 14 is a second local oscillator, 15 is a second IF amplifier, 17 is a second IF filter (bandpass filter), 20 is a third IF amplifier, and 22 is A low pass filter, 23 is a PLL (phase locked loop) circuit, 34 is an AM demodulator, and 72 is a first IF filter (band pass filter).
[0004]
An AM-modulated NTSC television signal input as an RF signal from the signal input terminal 1 is divided into bands by an input filter 5, and a band including a desired channel selectively passes through the input filter 5. The passed signal is appropriately amplified or attenuated by the variable gain amplifier 6 so as to obtain a desired reception level for the desired channel, and is input to the first mixer 9. On the other hand, in the first local oscillator 10, feedback control is performed by the PLL circuit 23 and the low-pass filter 22 so as to oscillate at a frequency corresponding to the desired channel based on the channel selection signal input from the channel selection signal input terminal 2. The oscillation output is input to the first mixer 9 as a local oscillation signal. That is, the PLL circuit 23 has a reference signal inside, compares this reference signal with the local oscillation signal from the first local oscillator 10, and passes the error signal to the first local oscillator 10 via the low-pass filter 22. The oscillation frequency of the first local oscillator 10 is controlled so that the error signal becomes zero by outputting, and the frequency division ratio of the frequency divider that generates the frequency for comparison by the channel selection signal Tuning is performed by changing the.
[0005]
The first mixer 9 mixes the local oscillation signal from the first local oscillator 10 with the signal appropriately amplified or attenuated by the variable gain amplifier 6, and outputs the first IF signal. The first IF signal is selectively passed by the first IF filter 72, amplified by the first IF amplifier 12, and then input to the second mixer 13. The second mixer 13 mixes the local oscillation signal from the second local oscillator 14 with the signal amplified by the first IF amplifier 12 and outputs a second IF signal. The second IF signal is amplified by the second and third IF amplifiers 15 and 20, and only a desired band is passed by the second IF filter 17 constituted by a SAW filter or the like, and then demodulated by the AM demodulator 34. The baseband video signal and audio signal are output.
[0006]
The AGC (auto gain control) determines the strength of the input radio wave based on the video signal level detected in the AM demodulator 34, and the gain of the variable amplifier in the AM demodulator 34 is determined by the corresponding AGC signal. In addition to controlling, the gain of the variable gain amplifier 6 is controlled by the same AGC signal output from the AM demodulator 34. The AFC (auto frequency control) detects the frequency difference with the video carrier in the AM demodulator 34, and sets the oscillation frequency of the second local oscillator 14 by the AFC signal output according to the frequency difference. This is done by fine tuning.
[0007]
[Problems to be solved by the invention]
However, the above-described receiving apparatus is for receiving a modulated signal in which a carrier wave remains, such as an AM-modulated standard television signal (specifically, an NTSC television signal). No consideration is given to receiving modulated signals in which the carrier signal is suppressed, such as digitally modulated high-definition television signals such as (orthogonal axis amplitude modulation), and thus the modulation in which the carrier remains It has not been considered to selectively receive both the signal and the modulated signal in which the carrier signal is suppressed.
[0008]
Therefore, purpose of the invention is intended to receive a digital television signal in a good condition.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, a receiver for receiving a digital television signal,
The digital television signal is a signal obtained by data compression of a video signal and an audio signal, and a signal having substantially the same bandwidth as the bandwidth of the analog television signal. A first frequency converting means for converting and outputting a frequency substantially equal to an intermediate frequency signal used for demodulating an analog television signal, and a video signal and an audio signal among the output signals from the first frequency converting means. Extraction means for extracting and outputting a digital television signal of a predetermined channel including a signal; and second frequency conversion means for converting the output signal of the extraction means to a frequency lower than the output signal of the first frequency conversion means And a recording means for recording the output signal from the second frequency converting means in a compressed state without being demodulated. It is sufficient.
[0014]
[Action]
As described above, since the digital television signal is converted into an intermediate frequency signal having a frequency substantially equal to the intermediate frequency for the analog television signal, extraction means for extracting a signal of a predetermined channel from the intermediate frequency signal. However, it is possible to use means having the performance of suppressing the generation of a good interference signal equivalent to the existing filter for the intermediate frequency signal of the analog television signal, and the generation of the interference signal is sufficiently suppressed. Thus, a well-demodulated television signal can be obtained.
[0017]
【Example】
Embodiments of the present invention will be described below with reference to the drawings. In the following description, QAM (Quadrature Amplitude Modulation), QPSK is a modulated signal obtained by modulating a carrier signal with at least a video signal and the carrier signal is suppressed. Digitally modulated high-definition television signals such as (four-phase phase modulation), OFDM (orthogonal frequency division multiplexing) and their complex modulation (hereinafter referred to as QAM (orthogonal axis amplitude modulation) or the like) A fine television signal may be simply referred to as a high-definition television signal), and the second modulated signal in which the carrier wave remains is VSB (residual sideband amplitude modulation) or the like. Among the standard television signals that have been subjected to AM modulation, television signals in accordance with the NTSC system (hereinafter referred to as NTSC television signals that have been subjected to AM modulation in this way). Simply and be described by way of example it is) to say that NTSC signal.
[0018]
FIG. 1 is a block diagram showing a receiving apparatus as a first embodiment of the present invention. The present embodiment is a receiving apparatus capable of receiving a high-definition television signal of a desired channel.
[0019]
In FIG. 1, 1 is a signal input terminal for a television signal, 2 is a channel selection signal input terminal, 3 is a high-definition television signal output terminal, 5 is an input filter (bandpass filter), and 6 is a variable gain amplifier for RF signals. , 9 is a first mixer, 10 is a first local oscillator, 11 is a first IF filter (bandpass filter), 12 is a first IF amplifier, 13 is a second mixer, 14 is a second local oscillator, 15 is a second IF amplifier, 16 is a high-definition television signal IF filter (bandpass filter), 19 is a third IF amplifier, 22 and 24 are low-pass filters, 23 and 25 are PLL circuits, 26 is a channel selection control unit, 28 is a third mixer, 29 Is a third local oscillator, 30 is a low pass filter, 31 is a variable gain amplifier for IF signals, 33 is a demodulator for high definition television signals, 39 is a band pass filter, 0 signal level detector for high-definition television signals, 41 is a low pass filter, 42 is the AGC voltage amplifier.
[0020]
Now, the operation will be described. The high-definition television signal is subjected to analog / digital conversion and data compression on the transmission side, and then QAM (Quadrature Axis Modulation), QPSK (Quadrature Phase Modulation), OFDM (Quadrature Frequency Multiplexing) as described above. ) And their composite modulation, etc., to obtain a signal having the same bandwidth as that of a television signal according to the normal NTSC system. Such a high-definition television signal is transmitted as an RF signal and input to the signal input terminal 1 shown in FIG. The input RF signal is divided by the input filter 5 into a VHF band and a UHF band (further, the VHF band may be divided into a low band, a middle band, and a high band), and a band including a desired channel is selected. Pass through. The passed signal is appropriately amplified or attenuated by the RF signal variable gain amplifier 6 so as to have a desired signal level for the desired channel, and input to the first mixer 9. The RF signal variable gain amplifier 6 is composed of a dual gate FET or the like. On the other hand, the channel selection control unit 26 outputs data to the PLL circuit 23 according to the channel selection signal input from the channel selection signal input terminal 2. Further, the first local oscillator 10 includes a PLL circuit 23 including a reference oscillator and a frequency divider, a low-pass filter so as to oscillate at a frequency corresponding to the desired channel based on the data output from the channel selection control unit 26. 22, the feedback control is performed, and the oscillation output is input to the first mixer 9 as a local oscillation signal.
[0021]
The first mixer 9 mixes the local oscillation signal from the first local oscillator 10 and the signal appropriately amplified or attenuated by the variable gain amplifier 6 and outputs the first IF signal. Here, the frequency of the first IF signal is set to be equal to or higher than the upper limit frequency of the terrestrial transmission band or CATV transmission band of the NTSC signal in order to reduce intermodulation interference of the received signal. Specifically, in the US and domestic frequency arrangements, the 960 MHz band, 1.2 GHz band, 1.7 GHz are also considered in consideration of mutual interference interference due to the first local oscillation signal, the second local oscillation signal, and its harmonic signal. Band, 2.6 GHz band, 3 GHz band, etc. are set. The first IF signal set in these frequency bands is selectively passed through the first IF filter 11. Since the high-definition television signal requires demodulation with higher accuracy than that of a standard television signal such as an NTSC signal, the first IF filter 11 has a band that does not deteriorate the demodulation characteristics of the high-definition television signal. A bandpass filter having inner flatness and low group delay deviation is used. Specifically, it is composed of a dielectric filter or a SAW filter.
[0022]
The first IF signal is amplified by the first IF amplifier 12 and then input to the second mixer 13. On the other hand, the channel selection control unit 26 outputs data to the PLL circuit 25 in accordance with the AFC signal output from the high-definition television signal demodulator 33. In the second local oscillator 14, feedback control is performed by a PLL circuit 25 incorporating a reference oscillator and a frequency divider and a low-pass filter 24 so as to oscillate at a frequency corresponding to the data output from the channel selection control unit 26. The oscillation output is input to the second mixer 13 as a local oscillation signal.
[0023]
The second mixer 13 mixes the local oscillation signal from the second local oscillator 14 with the signal amplified by the first IF amplifier 12 and outputs a second IF signal. Here, the frequency of the second IF signal is set to be the same as when the current standard television signal is received. More specifically, in the case of the United States, the band is the same as that at the time of NTSC signal reception. Similarly, in the case of domestic use, the 58 MHz band is used.
[0024]
The second IF signals set in these frequency bands are amplified by the second IF amplifier 15 and then input to the high-definition television signal IF filter 16. The high-definition television signal IF filter 16 also requires in-band flatness and a low group delay deviation that do not deteriorate the demodulation characteristics of the high-definition television signal, and needs to eliminate interference from adjacent channels. (That is, because it is necessary that the degree of attenuation be large in the band of the adjacent channel), the SAW filter is configured to pass only the band of the desired channel. The desired IF of the second IF signal is amplified by the third IF amplifier 19 and input to the third mixer 28. In the 3rd mixer 28, the local oscillation signal from the 3rd local oscillator 29 and the signal amplified by the 3rd IF amplifier 19 are mixed, and a 3rd IF signal is output.
[0025]
Here, the frequency of the third IF signal is set to be lower than that of the second IF signal so that it matches the transmission rate of the high-definition television signal and the carrier frequency when writing to a storage medium such as an optical disk to be described later. Therefore, the demodulation and writing circuits are simplified. Further, by setting the frequency of the third IF signal low, the amplifier after the third mixer 28 (only the variable gain amplifier 31 in FIG. 1 is used, but the required input signal level to the demodulator 33 for the high-definition television signal) For example, an operational amplifier having a high linearity and a high gain operation region can be used.
[0026]
The third IF signal passes through the low-pass filter 30 and is appropriately amplified or attenuated by the IF signal variable gain amplifier 31 and then input to the high-definition television signal demodulator 33, where demodulation according to the modulation method is performed. As a result, a data-compressed high-definition television signal is output from the output terminal 3. The output signal is input to a digital signal processing circuit (not shown) that performs data expansion, digital / analog conversion, and the like, and is output to a high-definition television as a video signal and an audio signal or data.
[0027]
Further, the AGC for RF signal inputs the third IF signal that has passed through the low-pass filter 30 to the band-pass filter 39, passes only the desired band in the third IF signal, and detects the signal level of the band portion as the detector 40. The AGC voltage is generated by the low-pass filter 41 and the AGC voltage amplifier 42 from the signal level and applied to the RF signal variable gain amplifier 6.
[0028]
FIG. 2 is an explanatory diagram for explaining the operation of the bandpass filter 39 in FIG. In FIG. 2, the vertical axis represents the level, the horizontal axis represents the frequency, 50 represents the passband of the bandpass filter 39, 51 represents the frequency spectrum distribution of the high-definition television signal, and 52 represents the frequency spectrum distribution of the NTSC signal. , 53 is a video carrier of the NTSC signal, 54 is a color subcarrier of the NTSC signal, and 55 is an audio carrier of the NTSC signal.
[0029]
Now, when an NTSC signal is transmitted through the same channel as the high-definition television signal of the desired channel and is input to the signal input terminal 1, each signal has a frequency arrangement as shown in FIG. In FIG. 2, the frequency is reversed, and the frequency arrangement is reversed left and right with the center frequency in FIG. 2 as the axis). Among these, when receiving a high-definition television signal of a desired channel, the AGC compares the signal level of the high-definition television signal shown in 51 with a reference value and generates an AGC signal from the difference, This is performed by appropriately adjusting the amount of amplification or attenuation according to the AGC signal. At this time, in the AGC for RF signal, the signal level of the high-definition television signal is detected by the detector 40. At this time, the NTSC signal transmitted through the same channel is input to the detector 40 simultaneously with the high-definition television signal. Then, the detector 40 superimposes the high energy video carrier 53, the audio carrier 55 and the color subcarrier 54 on the high definition television signal in the NTSC signal, and the signal level of the high definition television signal is Since the signal level is detected as a level higher than the actual high-definition television signal level, the RF signal AGC operates so as to be higher than the actual signal level of the high-definition television signal. As a result, the input level to the high-definition television signal demodulator 33 becomes insufficient.
[0030]
Therefore, in this embodiment, even when the NTSC signal is transmitted through the same channel as the high-definition television signal of the desired channel and is input to the signal input terminal 1, the detector for operating the RF signal AGC properly is provided. A band-pass filter 39 is provided in front of 40, and its pass band is set to a portion excluding each carrier 53, 54, 55 of the NTSC signal as indicated by 50 in FIG. As a result, the levels of the respective carriers 53, 54, 55 of the NTSC signal input to the detector 40 are significantly reduced, and the amount of superposition at the detector 40 is reduced, so that the RF signal AGC is operated properly. be able to.
[0031]
FIG. 3 is a circuit diagram showing an example of the circuit configuration of the bandpass filter 39 in FIG. In FIG. 3, 56 is an input terminal, 57 is an output terminal, 58 and 59 are common terminals (ground terminals), C1, C2, C3, C4, C5, C6, C7 and C8 are capacitors, and L1, L2, L3 and L4. , L5, L6, L7, and L8 are inductors. The circuit shown in FIG. 3 is obtained by converting a sixth-order elliptic function type low-pass filter into a band-pass filter by frequency conversion, which is one of filter design methods, and can obtain a steep filter characteristic compared to a Butterworth filter of the same order. In the NTSC signals transmitted on the same channel, the carrier part having high energy has a large attenuation and is suitable for the configuration of the bandpass filter 39 described above.
[0032]
On the other hand, IF signal AGC is performed by applying the AGC signal generated in the high-definition television signal demodulator 33 to the IF signal variable gain amplifier 31.
[0033]
Now, in generating each AGC signal, the RF signal AGC is detected with a relatively long time constant, and the IF signal AGC is set to a relatively short time constant, with a short period such as flutter by an airplane or the like. When the signal level changes, the IF signal AGC follows the change. Further, in order to minimize the degradation of the received signal-to-noise ratio when the gain is attenuated, the gain attenuation starts from the IF signal variable gain amplifier 31 as the input signal level increases, and corresponds to the level change of the flutter and the like. Therefore, the RF signal variable gain amplifier 6 starts to attenuate, leaving an amount of attenuation for this purpose.
[0034]
Here, the reason why the AGC for IF signal and the AGC for RF signal are provided separately will be described. In many cases, an AD converter that converts an analog signal into a digital signal is provided at the input section in the high-definition television signal demodulator 33. The allowable range of the input signal level of the AD converter is smaller than the input level range of the high-definition television signal when the high-definition television signal is broadcast on the ground wave. In order for the high-definition television signal demodulator 33 including the AD converter to operate normally, an AGC independent of the high-definition television signal demodulator 33 is required. Therefore, unlike the conventional AGC signal generation method at the time of receiving the standard television signal described in FIG. 9, the AGC signal is separately generated from the third IF signal and used for RF signal AGC.
[0035]
Further, as described above, the AFC inputs the AFC signal from the high-definition television signal demodulator 33 to the channel selection control unit 26, and the channel selection control unit 26 passes through the PLL circuit 25 in accordance with the AFC signal. The oscillation frequency of the two local oscillator 14 is finely adjusted. In generating the AFC signal, a band pass filter having the same pass band as the band pass filter 39 is provided in the input section in the high-definition television signal demodulator 33 to separate the signal, and an analog circuit such as FM detection. By using AFC, AFC can be easily performed at high speed.
[0036]
In addition, when an NTSC signal is transmitted on the same channel as the high-definition television signal of the desired channel and is input to the signal input terminal 1, in order to avoid interference from the NTSC signal, the high-definition television signal is used. A demodulator 33 is provided with a notch filter for removing high energy video carrier, audio carrier and color subcarrier in the NTSC signal, or the spectrum of the high definition television signal is preliminarily transmitted on the transmission side. In other words, it is necessary not to arrange them in the vicinity of the high energy video carrier, audio carrier and color subcarrier in the NTSC signal.
[0037]
Further, at the time of channel selection, a band switching voltage for switching the band including the desired channel to pass through the input filter 5 is sent from the channel selection control unit 26 to reduce the number of signals input to the RF amplifier 6 and the first mixer 9 for input. Suppresses the occurrence of intermodulation interference between signals. Further, at the time of channel selection, a signal indicating that the desired channel has been changed is sent from the channel selection control unit 26 to the high-definition television signal demodulator 33, and the high-definition television signal demodulator 33 is returned to the initial state, It is set so that demodulation can be started smoothly.
[0038]
As described above, according to the present embodiment, the first IF filter 11 and the second IF filter 16 are bandpass filters having in-band flatness and low group delay deviation that do not deteriorate the demodulation characteristics of high-definition television signals. By using, high-definition television signals can be demodulated with higher accuracy than standard television signals such as NTSC signals, and as a result, high-definition television signals can be received.
[0039]
Further, by converting the third mixer 28 into a third IF signal having a frequency lower than the frequency of the second IF signal, an operational amplifier having a high linearity and a high gain can be used as an amplifier after the third mixer. Therefore, even when an NTSC signal is transmitted on the same channel as the high-definition television signal of the desired channel and is input to the signal input terminal 1, the cross-modulation due to both signals can be reduced, so a high-definition television with a low error rate. John signal can be demodulated. Also, by converting to the third IF signal, the signal frequency handled by the high-definition television signal demodulator 33 is lowered, so that the circuit configuration can be simplified.
[0040]
Further, even when the NTSC signal is transmitted through the same channel as the high-definition television signal of the desired channel and is input to the signal input terminal 1, the RF signal AGC detects the signal level of the high-definition television signal. By providing a band-pass filter 39 that removes each carrier with high energy in the NTSC signal in front of the device 40, the AGC for RF signal can be properly operated, and the desired high-definition television signal can be demodulated. Is possible.
[0041]
FIG. 4 is a block diagram showing a connection method when a recording / reproducing apparatus is connected to the receiving apparatus shown in FIG. 4, parts that perform the same operations as in FIG. 1 are assigned the same numbers as in FIG. 1, and descriptions thereof are omitted. In addition, 35 is a recording / reproducing apparatus, and 36 is a connection terminal.
[0042]
For standard television signals such as the current NTSC signal, recording / playback devices such as video tape recorders have become widespread, recording programs that are being viewed while watching programs, and programs that are not being viewed, or automatically by setting the time Recording is possible. In these recordings, a baseband signal of a standard television signal such as an NTSC signal is recorded on a magnetic tape. In a high-definition television signal, the amount of information is large and the baseband signal is a digital signal. Since the frequency band required for recording is wide, recording on a magnetic tape is not easy.
[0043]
Therefore, in the recording / reproducing apparatus 35 shown in FIG. 4, a writable optical disk, a magneto-optical disk or the like having a large recording capacity and suitable for recording digital signals is used as a recording medium in order to record a high-definition television signal. Use.
[0044]
By the way, as a recording method to these optical disks, magneto-optical disks, etc., there are generally a method of recording as a digital signal and a method of recording by FM modulation. When a digital signal is input, the former can be recorded more efficiently. However, as in this embodiment, when a signal whose transmission band is compressed by multi-value digital modulation is input, it is demodulated. The recording efficiency is better when the modulated signal is FM-modulated and recorded than when it is recorded as a digital signal later.
[0045]
Therefore, as a method of connecting the recording / reproducing device 35 to the receiving device shown in FIG. 1, as shown in FIG. 4, a connection terminal 36 for outputting the third IF signal output from the IF signal variable gain amplifier 31 to the outside is provided. And a recording / reproducing apparatus 35 is connected thereto.
[0046]
At this time, when the recording medium is an optical disk, the upper limit frequency of the FM modulation is determined from parameters such as the track pitch, the number of rotations of the disk, the required storage capacity, and the required CN ratio. Considering the characteristics, the frequency is from about 10 MHz to several tens of MHz. Further, the upper limit frequency of the modulated signal needs to be about two thirds of the upper limit frequency of the FM modulation, and is about 10 MHz or less to about a dozen MHz. Therefore, if the frequency of the third IF signal is set as the upper limit frequency of these modulated signals, a high-definition television signal can be recorded on the optical disk with the same simple circuit as the conventional recording / reproducing apparatus. In the case of these optical disks and the like, the recording / reproducing apparatus 35 incorporates an FM modulator in the input unit and an FM demodulator in the output unit.
[0047]
FIG. 5 is a block diagram showing a receiving apparatus as a second embodiment of the present invention. 5, parts that perform the same operations as in FIG. 1 are given the same numbers as in FIG. 1, and descriptions thereof are omitted. In addition, 21 is a fourth mixer.
[0048]
This embodiment is characterized by the channel selection method. That is, in the embodiment of FIG. 1, the PLL circuit 23 is used in the first local oscillator 10 to control the frequency of the local oscillation signal for converting the RF signal of the desired channel into the first IF signal, and the AFC voltage is used. In the present embodiment, the local oscillation signal from the first local oscillator 10 and the local oscillation signal from the second local oscillator 14 are converted by the fourth mixer 21. A signal indicating the difference frequency is mixed by the PLL circuit 23, the oscillation frequency of the first local oscillator 10 is controlled so that the difference frequency becomes constant when the desired channel is received, and the AFC voltage Is also performed by controlling the oscillation frequency of the first local oscillator 10 in accordance with the AFC voltage from the high-definition television signal demodulator 33 using the PLL circuit 23.
[0049]
In this embodiment, in addition to the effects described in the embodiment of FIG. 1, the frequency of the difference between the local oscillation signal from the first local oscillator 10 and the local oscillation signal from the second local oscillator 14 has received the desired channel. A simple channel selection means that performs frequency control only by the first local oscillator 10 by using the fact that it is sometimes kept constant can be obtained. Further, since the signal indicating the frequency of the difference between the local oscillation signal from the first local oscillator 10 and the local oscillation signal from the second local oscillator 14 is input to the PLL circuit 23, the signal is input to the PLL circuit 23. Therefore, an accurate PLL circuit can be easily created as the PLL circuit 23. Further, since only one PLL circuit and one low-pass filter are required, the number of parts may be small.
[0050]
FIG. 6 is a block diagram showing a receiving apparatus as a third embodiment of the present invention. 6, parts that perform the same operations as in FIG. 1 are given the same numbers as in FIG. 1, and descriptions thereof are omitted. In addition, 7 is a first variable attenuator, 8 is a second variable attenuator, and 60 is an RF amplifier.
[0051]
Among the RF signals input from the signal input terminal 1, the band including the desired channel is selectively passed by the input filter 5, and the variable attenuators 7, 8 and RF are set so that the desired signal level is obtained for the desired channel. The signal is amplified or attenuated as appropriate by the amplifier 60 and input to the first mixer 9. At this time, as the input signal level increases, the gain attenuation starts from the variable gain amplifier 31 for IF signal, and the second variable attenuator leaves the amount of attenuation to cope with the level change of the flutter and the like. 8 starts attenuation, and the first variable attenuator 7 starts attenuation when the amount of attenuation becomes maximum.
[0052]
Thus, by starting gain attenuation from the subsequent stage of the RF amplifier 60, the subsequent stage attenuation amount does not directly degrade the noise figure due to the gain of the RF amplifier 60, and the received signal-to-noise ratio is degraded at the time of gain attenuation. This can be reduced from the case of the first embodiment shown in FIG. In addition, by dividing the variable attenuator into the variable attenuators 7 and 8 and operating them with a delay, the sensitivity of the gain control amount with respect to the AGC voltage can be reduced, and a stable AGC operation becomes possible. Furthermore, by installing the variable attenuator 7 in front of the RF amplifier 60, the input level to the RF amplifier 60 is attenuated even at the maximum input level. The variable attenuator configured by the dual gate FET shown in FIG. Compared with the case where the gain amplifier 6 is used, it is possible to reduce the occurrence of intermodulation interference, intermodulation interference, and the like. When the variable gain amplifier 6 composed of a dual gate FET or the like is used, the intermodulation interference or the cross modulation interference may occur due to the nonlinearity of the element during gain control. Therefore, the occurrence of interference due to the nonlinearity of such elements can also be reduced.
[0053]
FIG. 7 is a block diagram showing a receiving apparatus as a fourth embodiment of the present invention. This embodiment is a receiving apparatus that can selectively receive a high-definition television signal and an NTSC signal of a desired channel.
[0054]
7, parts that perform the same operations as in FIG. 1 are given the same numbers as in FIG. 1, and descriptions thereof are omitted. In addition, 4 is an NTSC video and audio signal output terminal, 17 is an NTSC signal IF filter, 20 is an NTSC signal third IF amplifier, and 27 is an NTSC signal AFC voltage and a high-definition television signal AFC voltage. AFC voltage switching circuit for switching and outputting in accordance with a signal (ie, high definition television signal or NTSC signal), 34 an NTSC signal demodulator, 43 an NTSC signal AGC voltage and a high definition television signal AGC voltage Is an AGC voltage switching circuit that switches and outputs the signal according to a desired received signal (ie, high-definition television signal or NTSC signal).
[0055]
The high-definition television signal and the NTSC signal are frequency-multiplexed, transmitted as an RF signal, and input to the signal input terminal 1 shown in FIG. When the desired received signal is an NTSC signal according to the channel selection of the viewer, the demodulator 34 demodulates the signal and outputs it from the output terminal 4. On the other hand, when the desired received signal is a high-definition television signal, the signal is demodulated by the high-definition television signal demodulator 33 and output from the output terminal 3.
[0056]
At this time, the first IF filter 11 is shared by the high-definition television signal and the first IF signal of the NTSC signal. However, as described above, the high-definition television signal is more accurate than the case of the standard television signal such as the NTSC signal. Since the first IF filter 11 requires high demodulation, the first IF filter 11 is a bandpass having in-band flatness and low group delay deviation that does not deteriorate the demodulation characteristics of a high-definition television signal that requires demodulation with higher accuracy than the NTSC signal. Use a filter.
[0057]
Further, the second IF signal is amplified by the first IF amplifier 15 and then branched into two, which are respectively supplied to the high-definition television signal IF filter 16 and the NTSC signal IF filter 17 which are configured together with a SAW filter or the like. Only the band of the desired channel passes through each IF filter 16, 17.
[0058]
Thereafter, when a high-definition television signal is received, it is demodulated by the same operation as in the embodiment shown in FIG. On the other hand, when an NTSC signal is received, the signal of the desired channel is amplified by the third IFSC amplifier 20 for NTSC signal, input to the AM demodulator 34 for NTSC signal, AM demodulated, and baseband video signal and audio signal. Is output from the output terminal 4.
[0059]
The selection control unit 26 outputs a switching signal for the AGC voltage switching circuit 43 and the AFC voltage switching circuit 27 in accordance with the desired reception signal. As a result, when receiving a high-definition television signal, the AGC branches the third IF signal in the same manner as the embodiment shown in FIG. 1, and passes only a desired band in the third IF signal by the band-pass filter 39. The signal level of the portion is detected by the detector 40, an AGC voltage is generated by the low-pass filter 41 and the AGC voltage amplifier 42, and applied to the variable gain amplifier 6 through the AGC voltage switching circuit 43. When receiving an NTSC signal, it is performed inside the AM demodulator 34 and is applied by applying the AGC voltage generated in the AM demodulator 34 to the variable gain amplifier 6 through the AGC voltage switching circuit 43. Further, the AFC switches and outputs the AFC voltage from the high-definition television signal demodulator 33 and the NTSC signal AM demodulator 34 by the AFC voltage switching circuit according to the received signal, and the second local oscillator 14 oscillates. Adjust the frequency finely.
[0060]
As in the embodiment shown in FIG. 1, the channel selection control unit 26 sends a band switching voltage from the channel selection control unit 26 to the input filter 5 during channel selection, and further receives a high-definition television signal. Sends to the high-definition television signal demodulator 33 a signal indicating that the reception channel has been changed. From the viewpoint of reducing power consumption, it is also possible to supply power only to either the demodulator 33 or 34 according to the desired received signal.
[0061]
As described above, the receiving apparatus according to the present embodiment can not only receive the NTSC signal and the high-definition television signal but also share the first IF filter 11 with the NTSC signal and the high-definition television signal. Since the 2IF filters 16 and 17 and the demodulators 33 and 34 are separately provided for NTSC signals and high-definition television signals, the double superheterodyne dual frequency conversion unit can be shared for both signals. The receiver can be reduced in scale and excellent in operability.
[0062]
FIG. 8 is a block diagram showing a receiving apparatus as a fifth embodiment of the present invention. 8, parts that perform the same operations as in FIGS. 5 and 7 are given the same numbers as in FIGS.
[0063]
In this embodiment, the channel selection method in the embodiment of FIG. 5 is applied to the embodiment of FIG. That is, in the embodiment of FIG. 7, the PLL circuit 23 is used in the first local oscillator 10 to control the frequency of the local oscillation signal for converting the RF signal of the desired channel into the first IF signal, and the AFC voltage is used. In the present embodiment, the local adjustment signal from the first local oscillator 10 and the local oscillation from the second local oscillator 14 are the same as in the embodiment of FIG. The oscillation signal is mixed by the fourth mixer 21, the signal indicating the difference frequency is compared by the PLL circuit 23, and the difference frequency of the first local oscillator 10 is made constant when the desired channel is received. In the fine adjustment using the AFC voltage by controlling the oscillation frequency, the PLL circuit 23 is used to control the oscillation frequency of the first local oscillator 10 according to the AFC voltage from the high-definition television signal demodulator 33. It is carried out by.
[0064]
In this embodiment, in addition to the effects described in the embodiment of FIG. 7, the effects described in the embodiment of FIG. 5 can be obtained.
[0065]
7 and 8, the RF signal variable gain amplifier 6 is composed of an RF amplifier 60 and variable attenuators 7 and 8 provided in the preceding and succeeding stages as in the embodiment of FIG. For example, in addition to the effects described in the embodiment of FIGS. 7 and 8, the effects described in the embodiment of FIG. 6 can be obtained.
[0066]
【The invention's effect】
According to the present invention, a digital television signal can be received in a good state.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a receiving apparatus as a first embodiment of the present invention.
FIG. 2 is an explanatory diagram for explaining the operation of a bandpass filter 39 in FIG. 1;
3 is a circuit diagram showing an example of a circuit configuration of a bandpass filter 39 in FIG. 1. FIG.
4 is a block diagram showing a connection method when a recording / reproducing apparatus is connected to the receiving apparatus shown in FIG. 1. FIG.
FIG. 5 is a block diagram showing a receiving apparatus as a second embodiment of the present invention.
FIG. 6 is a block diagram showing a receiving apparatus as a third embodiment of the present invention.
FIG. 7 is a block diagram showing a receiving apparatus as a fourth embodiment of the present invention.
FIG. 8 is a block diagram showing a receiving apparatus as a fifth embodiment of the present invention.
FIG. 9 is a block diagram showing a conventional double superheterodyne television receiver.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Signal input terminal, 2 ... Channel selection signal input terminal, 3 ... High-definition television signal output terminal, 4 ... NTSC signal output terminal, 6 ... Variable gain amplifier, 7, 8 ... Variable attenuator, 60 ... RF amplifier, DESCRIPTION OF SYMBOLS 9 ... 1st mixer, 10 ... 1st local oscillator, 11 ... 1st IF filter, 12 ... 1st IF amplifier, 13 ... 2nd mixer, 14 ... 2nd local oscillator, 15 ... 2nd IF amplifier, 16 ... High-definition television IF filter for signal, 17 ... IF filter for NTSC signal, 19 ... 3rd IF amplifier, 20 ... 3rd IF amplifier for NTSC signal, 21 ... 4th mixer, 22, 25, 30, 41 ... Low pass filter, 23, 24 ... PLL Circuit, 28 ... 3rd mixer, 29 ... 3rd local oscillator, 33 ... High-definition television signal demodulator, 34 ... NTSC signal demodulator, 39 ... Bandpass filter, 40 ... High Fine television signal level detector, 42 ... AGC voltage amplifier, 43 ... AGC voltage switching circuit, the 1IF filter 72 ... NTSC signal.

Claims (4)

A receiving device for receiving a digital television signal,
The digital television signal is a signal obtained by data compression of a video signal and an audio signal, and a signal having substantially the same bandwidth as the bandwidth of the analog television signal,
First frequency converting means for converting a received digital television signal into a frequency substantially equal to an intermediate frequency signal used for demodulation of the analog television signal, and outputting the same;
Extraction means for extracting and outputting a digital television signal of a predetermined channel including a video signal and an audio signal from the output signal from the first frequency conversion means;
Second frequency conversion means for converting the output signal of the extraction means to a lower frequency than the output signal of the first frequency conversion means;
And a recording means for recording the output signal from the second frequency converting means in a compressed state without being demodulated.   2. The digital broadcast receiving apparatus according to claim 1, wherein the digitally modulated television signal is a signal modulated by any one of OFDM, QAM, and QPSK.   3. The digital broadcast receiving apparatus according to claim 1, wherein the intermediate frequency of the predetermined band is an intermediate frequency for a standard television signal.   The digital broadcast receiving apparatus according to claim 1 or 2, wherein the intermediate frequency of the predetermined band is a 45 MHz band or a 58 MHz band.

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