JPH09252442A - Demodulation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To widen the range capable of being made into an IC and to facilitate application to many television systems by using an FIR filter as an intermediate frequency filter provided with selectivity characteristics for the residual side wave band signals of video intermediate frequency signals. SOLUTION: The FIR filter 5c is used as the intermediate frequency filter provided with the selectivity characteristics for the residual side wave band signals of the video intermediate frequency signals. That is, the FIR filter 5c is newly inserted between a variable gain amplifier 5a and a demodulation circuit 5b, clocks CK are supplied from the demodulation circuit 5b to the FIR filter 5c and a part including the FIR filter 5c in addition to the variable gain amplifier 5a and the demodulation circuit 5b is added to the range capable of being made into the IC further. Thus, demodulation by the FIR filter 5c is realized, Also, since signals are hardly attenuated in the FIR filter 5c the need of a pre-amplifier required in the case of using a SAW filter is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to demodulation of a television signal, and more particularly to a demodulator for demodulating an intermediate frequency signal.

[0002]

2. Description of the Related Art Generally, demodulation of a television broadcast signal is performed by a system as shown in FIG.

First, an antenna 1 captures a broadcast radio wave transmitted as a high frequency signal (RF signal), and a tuner 2 performs R
Frequency conversion is performed from the F signal to the intermediate frequency signal (IF signal). The IF signal output from the tuner 2 is amplified by the preamplifier 3 and input to the IF filter 4. The output signal of the IF filter 4 is adjusted in signal level by the variable gain amplifier 5a and demodulated into a baseband signal by the demodulation circuit 5b.

Since the IF signal output from the tuner 2 is a residual side wave signal, if it is demodulated as it is, it becomes a signal in which the low frequency component is emphasized. Therefore, the IF filter 4 limits the signal component near the video carrier frequency,
The frequency component of the video signal demodulated by the demodulation circuit 5b is made flat.

As the IF filter 4, a surface acoustic wave filter (SAW filter) is used because it is easy to obtain good frequency characteristics. Generally, SAW
The signal attenuation in the filter is as large as about 10 dB to 20 dB. Therefore, by installing the preamplifier 3 in the preceding stage of the SAW filter and performing sufficient signal amplification, S / N deterioration due to signal attenuation in the SAW filter is prevented.

Further, the demodulation circuit 5b includes an automatic gain control (AGC) circuit for controlling the gain of the system on the basis of the demodulation level, from which the tuner 2 and the variable gain amplifier 5a are connected to the RF circuit. A gain control signal RFAGC for controlling the gain of the signal and a gain control signal IFAGC for controlling the gain of the IF signal are respectively sent to control so that the demodulation signal level is kept constant.

Further, a portion surrounded by a broken line in FIG. 6 indicates an integrated circuit (IC) realizable range 5, and the SAW filter cannot be integrated into an IC. Therefore, the parts which can eventually be integrated into the IC are the variable gain amplifier 5a and the demodulation circuit 5b. .

As described above, according to the system shown in FIG. 6, good television signal demodulation can be performed and IC can be formed to some extent. However, in recent years, as the tuner unit and the video demodulation circuit have been integrated into ICs, problems peculiar to the SAW filter have become a problem. That is, the SAW filter when viewed as a peripheral component needs to have a certain size due to its structure. Moreover, the SAW filter is usually an IC
Since it cannot be made with silicon used for, it is difficult to embed it in an IC. Further, as already mentioned, since the signal attenuation in the SAW filter is large, the preamplifier 3 which is not originally necessary is provided.
Must be provided.

Further, in the case of the split carrier system and the quasi parallel system, which are demodulation systems for realizing high image quality and high sound quality as compared with the inter carrier system, they are used for video IF signals (VIF) and audio IF signals (SIF). Two SA
There is a problem that the W filter is necessary and the cost becomes high. There are many broadcasting systems, including the M system used in Japan, but when supporting multiple television systems, a large number of SAW filters were also required according to the number of compatible systems.

As described above, the conventional method has many problems, but recently, attempts have been made to solve the problems. For example, IE, a US magazine published in 1989.
In EE's "ICCE WPM6.7, P90-91",
A technique has been reported in which a SAW filter is laminated on a monolithic IC and integrated. According to the contents introduced here, there is an advantage that the SAW filter and the monolithic IC can be integrated into one chip and the size can be reduced.

However, since only one chip is used, the signal attenuation in the SAW filter remains unchanged and cannot be improved. Moreover, there is a demerit that the chip cost becomes very high because the process is very special. Therefore, it is difficult to apply it to consumer equipment where cost is important.

As another method, an IF signal is directly converted into a digital signal by an A / D converter, and an SA signal is converted by a digital filter.
Although it is considered to perform IF signal filtering equivalent to the W filter, it is very difficult to realize a circuit for A / D converting a high frequency signal such as an IF signal, and the cost is high, which is not practical. Absent.

[0013]

In the above-described conventional demodulation device, I generated by using the SAW filter, I
There are problems that it is difficult to convert to C, that the signal is greatly attenuated, and that the cost becomes very high when trying to support a high-quality / high-quality sound system or a multi-system television system.

The present invention expands the range of possible IC integration,
An object of the present invention is to provide a demodulation device that can be easily applied to a multi-system television system.

[0015]

In order to solve the above-mentioned problems, in the demodulation device of the present invention, an FIR filter operating with a clock obtained from a demodulation circuit for a video intermediate frequency signal is provided on the residual side of the video intermediate frequency signal. It is characterized by being used as an intermediate frequency filter having selectivity characteristics for waveband signals.

By adopting such a configuration, demodulation by the FIR filter can be realized. Further, since there is almost no signal attenuation in the FIR filter, the preamplifier which is required when using the SAW filter up to now becomes unnecessary. As a result, the cost can be reduced and the IC, particularly the one-chip IC, can be easily realized.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram for explaining the first embodiment of the present invention. In this embodiment, the preamplifier 3 and the IF of FIG.
The filter 4 is deleted and the variable gain amplifier 5a and the demodulation circuit 5 are removed.
The FIR filter 5c is newly inserted between b and the FI
The clock CK is supplied from the demodulation circuit 5b to the R filter 5c, and further the IC-enabled range 51 is set to the variable gain amplifier 5a.
The difference from the configuration of FIG. 6 is that a portion including the FIR filter 5c is added to the demodulation circuit 5b.

The variable gain amplifier 5a is a FIR filter 5c.
Although it may be installed in the latter stage, by placing it in the front stage of the FIR filter 5c, the input signal level of the FIR filter 5c can be made constant, and the signal level is sufficiently high with respect to the noise generated in the FIR filter 5c. It is possible to prevent S / N deterioration in the output signal.

Further, if the demodulation circuit 5b is composed of a PLL synchronous detection circuit, the oscillation frequency of the VCO is N times the carrier frequency, and the PLL feedback loop is applied by a signal obtained by dividing the frequency by N. A clock CK that supplies the oscillation output of the VCO to the FIR filter 5c
By doing so, it becomes unnecessary to newly provide a clock circuit for the FIR filter 5c. This structure, which is called an inter-carrier system, will be specifically described with reference to FIG.

The demodulation circuit 5b shown in FIG.
The signal from c is input to one input of the video detection circuit ID and the phase detection circuit QD. The output of the phase detection circuit QD is input to the VCO via the loop filter LF. V whose oscillation frequency is controlled by the output of the loop filter LF
The output of CO is input to the frequency dividing circuit Da to divide the output of the VCO by N, and the divided output of φ0 is fed to the video detection circuit ID and φ9.
The frequency-divided output of 0 is input to the other input of the phase detection circuit QD. A demodulation output can be obtained from the output end of the video detection circuit ID. Furthermore, the output of the VCO is
It is used as the clock signal CK of the FIR filter 5c.

The phase detection circuit QD, the loop filter LF,
The VCO and the frequency dividing circuit Da have a so-called PLL configuration, and the VCO is controlled so that the frequency dividing output φ90 of the frequency dividing circuit Da has a phase relationship of 90 degrees with respect to one input of the phase detecting circuit QD. It Therefore, the other divided output φ, which has a phase relationship of 90 degrees with the divided output φ90 of the divider circuit Da.
By setting 0 as the other input of the video detection circuit ID, a demodulation output can be obtained from the output end of the video detection circuit ID. The demodulation output is also input to the AGC circuit AD, from which the gain control signal RF of the tuner 2 and the variable gain amplifier 5a is input.
Outputs AGC and IFAGC.

VC of the demodulation circuit 5b thus constructed
By oscillating O at N times the carrier of the IF signal, a carrier for detection having the same frequency as the carrier of the IF signal can be obtained from the output of the frequency dividing circuit Da. Especially N =
4, the frequency detectors Da are exactly 90 degrees out of phase with the frequency division carrier Da and the phase detection carriers φ0 and φ9.
0 is obtained, and the phase shift circuit for making the phase difference of 90 degrees required for a general PLL synchronous detection circuit becomes unnecessary.

Further, by supplying the oscillation output of the VCO as the clock of the FIR filter 5c, accurate oversampling can be performed by the FIR filter 5c, and extremely high precision filtering can be performed without adjustment.

FIG. 3 is a block diagram for explaining the second embodiment of the present invention. This embodiment corresponds to a quad parallel type demodulation circuit, and can independently demodulate video and audio signals.

That is, the FIR filter 5c outputs two signals, that is, a video IF signal VIF and a sound IF signal SIF which are respectively subjected to optimum filtering for video and audio. The video IF signal VIF is input to one of the input ends of the video detection circuit ID and the phase detection circuit QD, and the audio IF signal SI is input.
F is input to one input end of the mixer Ma. The φ90 frequency-divided output of the frequency dividing circuit Da is connected to the other input end of the mixer Ma, and a frequency-converted audio signal is obtained from the output of the mixer Ma.

The output of the mixer Ma is the output of the mixer M.
b is connected to one input terminal, and the other input terminal of the mixer Mb is connected to the output of the frequency dividing circuit Db for dividing the output of the VCO by M. From the output of mixer Mb to mixer M
A signal obtained by frequency-converting the output signal of a is obtained, the image is further removed by the band pass filter BPF, and the audio signal is demodulated by the FM demodulator FD.

According to this embodiment, since the FIR filter 5c performs optimum filtering for video and audio, there is a feature that interference due to beat interference between the video signal and the audio signal is reduced.

From the output of the mixer Ma, an audio signal frequency-converted into the frequency difference between the video carrier and the audio carrier is obtained, which is further selected by appropriately selecting the frequency division ratio M of the frequency dividing circuit Db. The output frequency of the mixer Mb for frequency conversion can be made sufficiently low, and especially S / N improvement in FM demodulation can be performed.

Further, by changing the frequency division ratio M of the frequency dividing circuit Db according to the broadcasting system, it is possible to obtain a predetermined frequency output signal from the mixer Mb for different broadcasting systems. It is easy to support television broadcasting.

To support multi-channel television broadcasting, it is necessary to change the filtering characteristic of the FIR filter 5c. Strictly speaking, the output of the mixer Ma requires a bandpass filter or a lowpass filter for image removal, or the image frequency is sufficiently high and the image component is attenuated by parasitic capacitance. It is unnecessary. The bandpass filter BPF may be a lowpass filter, but since the DC component is transmitted as it is, attention must be paid to the circuit design.

FIG. 4 is a block diagram for explaining the third embodiment of the present invention. In this embodiment, instead of the video detection circuit ID and the phase detection circuit QD of FIG. 2 which use a mixer circuit, a video detection circuit DI and a phase detection circuit DQ which use a sample and hold circuit are used.

Since the influence of the non-linearity is eliminated by performing the detection by the video detection circuit DI and the phase detection circuit DQ using the sample and hold circuit, the linearity, distortion and dynamic range are different from those in the case of the mixer circuit. Will be advantageous.

FIG. 5 is a block diagram for explaining a configuration example of the FIR filter 5c used in the present invention. That is, the unit delay circuits D1 to Dn that operate by the supply of the clock signal CK are connected in cascade, and the input signal is input to the first unit delay circuit D1. The outputs of the unit delay circuits D1 to Dn are weighted by the tap coefficient circuits a1 to an, respectively, and added by the adder circuit Add to obtain an output signal.

[0034]

As described above, according to the demodulation device of the present invention, the IF filter characteristic can be obtained without using the SAW filter, the amount of attenuation generated by using the SAW filter is large, and the IC is built-in. It is possible to solve the problem that it is difficult to realize, and it is possible to perform filtering suitable for the video IF signal and the audio IF signal.

[Brief description of drawings]

FIG. 1 is a block diagram for explaining a first embodiment of the present invention.

FIG. 2 is a block diagram for explaining FIG. 1 more specifically;

FIG. 3 is a block diagram for explaining a second embodiment of the present invention.

FIG. 4 is a block diagram for explaining a third embodiment of the present invention.

FIG. 5 is a block diagram for explaining a configuration example of an FIR filter used in the embodiment of the present invention.

FIG. 6 is a block diagram for explaining a conventional demodulation device.

[Explanation of symbols]

1 ... Antenna, 2 ... Tuner, 5a ... Variable gain amplifier,
5b ... Demodulation circuit, 5c ... FIR filter, 51 ... IC possible range, ID, DI ... Video detection circuit, QD, DQ ... Phase detection circuit, LF ... Loop filter, Da, Db ... Dividing circuit, Ma, Mb ... Mixer , BPF ... band pass filter, FD ... FM demodulator.

Claims (6)

[Claims] 1. An FIR filter operating with a clock obtained from a video intermediate frequency signal demodulation circuit is used as an intermediate frequency filter having a selectivity characteristic for the residual sideband signal of the video intermediate frequency signal. Demodulator. 2. The demodulator according to claim 1, wherein a gain control amplifier is provided in a stage preceding the FIR filter, and the gain control of the gain control amplifier is performed based on a demodulation output obtained by demodulating the output of the FIR filter. . 3. The demodulation circuit comprises a VCO that oscillates at a frequency N times that of a video carrier, and a PLL synchronous detection circuit that includes an N divider circuit that divides the output of the VCO by N.
2. The demodulation device according to claim 1, wherein the output of is used as a clock signal of the FIR filter. 4. The MCO frequency dividing circuit for dividing the VCO output by M is used, and the output of the M frequency dividing circuit is used as a frequency conversion carrier for an audio intermediate frequency signal.
The demodulator described. 5. The demodulator according to claim 3, wherein the sample-and-hold circuit that operates with the output of the frequency dividing circuit is used for demodulation or frequency conversion. 6. A demodulator according to claim 4, further comprising means for switching a frequency division ratio M of the M frequency dividing circuit, for frequency-converting the audio intermediate frequency signal into a predetermined frequency.