JPH10136287A - Multi-system audio IF processing circuit - Google Patents

Abstract

(57) [Summary] In a multi-system audio IF processing circuit, a BPF I
Built-in C is possible, there is no need to switch depending on the system, and the improvement of buzz disturbance is also performed. A mixer circuit (15) for converting an audio IF frequency into a low frequency and a local oscillation signal generator (17) for generating a carrier for conversion are provided, and a reference signal not affected by a video synchronization signal is inputted. Is used to control the oscillation frequency of the local oscillation signal from the reference signal to a desired value. With this, the frequency of the local oscillation signal can be changed in accordance with the system, and control can be performed so that the low-frequency conversion frequency is always constant. Therefore, the center frequency of the BPF 18 may be fixed. Even if it is built-in, one BPF characteristic may be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-system audio IF processing circuit which is optimally used for, for example, a television receiver.

[0002]

2. Description of the Related Art IF of a conventional television (TV) receiver
The signal processing block is shown in FIG. 4 and described. An intermediate frequency (IF: Inte
rWave signal) is input from an input terminal 41, and a SAW (Surface Acoustic Waves) filter 42 separates only the IF band signal. The video IF (PIF) processing circuit 43 reproduces the video modulation carrier and demodulates the video signal. At this time, the audio signal is a signal that is demodulated to baseband but is still modulated. There are various types of baseband audio modulation signals, and the carrier frequency also differs depending on the type. Excluding voice multiplexing, 4.5, 5.5,
There are four systems of 6.0 and 6.5 MHz. Television receivers that can receive these methods employ an intercarrier system that demodulates the PIF demodulated output modulated signal.

[0003] In order to extract only the audio modulation signal, band pass filters (BPF) 44 to 47 corresponding to each frequency are provided to remove video signal components. Except for France, FM is adopted as the modulation method, so FM will be described as a representative. When performing FM demodulation, it is complicated to prepare demodulation circuits corresponding to four types of carriers.
The three methods of 0 and 6.5 MHz employ a method of converting the signals into 6.0 MHz and demodulating them specially in the two methods of 4.5 MHz and 6.0 MHz.

The BPF outputs of the three systems of 5.5, 6.0 and 6.5 MHz are input to a 6.0 MHz conversion circuit 48, respectively, where the signal converted to 6.0 MHz and a 4.5M signal are output.
The output of the Hz BPF filter 44 is selected by the switch 49, the SIF is demodulated by the SIF demodulation processing circuit 50, and the demodulated audio IF signal is output from the output terminal 51.
The 4.5 MHz or other system is determined by the 4.5 MHz detection circuit 52, and the switch 49 and the SIF processing circuit 5
Control 0.

In this way, it is possible to cope with four types of audio modulation systems and to realize a multi-system receiver. However, this system requires four BPFs, which causes a problem of high cost. Most of the BPFs in the market are realized by external components, and the system cost cannot be reduced even if the cost of other processing circuits is reduced by the use of ICs.

[0006] Recently, a TV receiver dedicated to the NTSC in Japan, not a multi-system, in which a BPF filter is built in an IC has been realized. As the BPF filter characteristics, PI
In order to extract only the audio modulation signal superimposed on the video signal after F detection, a signal having a very high Q is required.
When built in, it is inferior to external components in terms of S / N and band characteristics. Therefore, there is also a problem that the performance is inferior if the characteristics of the external parts are realized even if the IC is built in.

[0007] Further, in a multi-system TV receiver, since the IF frequency varies depending on the destination, there is a problem that the inter-carrier system has to be adopted and is easily affected by buzz interference. In the PIF processing circuit, the input IF
Since the video carrier is reproduced from the signal, the reproduction carrier frequency is easily modulated by the synchronization signal, and the audio modulation signal is also modulated by the synchronization signal during PIF demodulation. This modulation causes the synchronization frequency signal to leak to the SIF processing output, causing annoying vertical and horizontal frequencies. Recently, the PLL type is often used for carrier regeneration, and efforts have been made to reduce this interference, but it cannot be fundamentally eliminated.

[0008]

In order to realize the above-mentioned conventional multi-system TV receiver, a plurality of B-types corresponding to the systems are required.
There is a problem that PF is required, which leads to high cost.
F has a problem that most of the market receivers are realized by external components, and that even if the cost of the processing circuit is reduced by using an IC, the system cost cannot be reduced.

An object of the present invention is to provide a multi-system audio IF processing circuit which can incorporate a BPF into an IC, does not need to switch according to the system, and improves buzz interference.

[0010]

According to the present invention, there is provided a mixer circuit for converting an audio IF frequency into a low frequency, a local oscillator for generating a conversion carrier, and a video synchronizing signal. Means for inputting a reference signal which is not affected by the above, and controlling the oscillation frequency of the local oscillation signal to a desired value from the reference signal.

If the frequency of the local oscillation signal is changed in accordance with the system and controlled so that the low-frequency conversion frequency is always constant, the center frequency of the BPF may be fixed, and the IC is built in even if it is externally mounted. Even in this case, one BPF characteristic can be realized.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a system diagram for explaining an embodiment of the present invention. A video and audio IF signal detected by a tuner circuit (not shown) is input from an input terminal 11.
This signal is input to the SAW filter 12 to extract only the desired IF band, and the AGC circuit 13 stabilizes the output level. The output of the AGC circuit 13 is input to a PIF detection circuit 14 and a mixer 15. In the PIF detection circuit 14, the video I
The F carrier is reproduced to detect a composite video signal, and a video signal is output from a video output terminal 16.

The converted carrier from the local oscillation signal generator 17 is input to the mixer circuit 15, and the output low-frequency converted voice modulated signal is selected by the BPF 18. The output of the BPF 18 is FM-demodulated by the SIF detection circuit 19 and the audio output terminal 20
Outputs more audio signals.

Here, the local oscillation signal generator 16 will be further described. As a means for generating an arbitrary frequency using digital technology, a circuit called DDS (Direct Digital Synthesizer) has recently been actively used in measuring instruments and the like. An overflow cycle generated by full addition of digital data is taken out, and an arbitrary frequency can be generated from a clock by controlling addition data. The oscillation signal of the local oscillation VCO 171 is input to the DDS 172 as a clock, and the added data is changed by the frequency control signal. The signal generated by the DDS 172 and the reference signal are input to the phase comparator 173, and the output is fed back to the VCO 171 via the low-pass filter 174.
LL.

The signals generated by the DDS 172 are switched according to the method to obtain VCO signals of different frequencies. If the number of bits of the full addition is increased, necessary and sufficient frequency accuracy can be obtained. In order to obtain a desired VCO frequency, a reference signal frequency is obtained from the VCO frequency.
What is necessary is just to determine the addition value of DDS. If this is calculated for each system and the added value of each system is switched by the frequency control signal, the PLL drives the oscillation frequency of the VCO 174 to a desired value.

Most tuner units of the current TV set are provided with a 4 MHz oscillation circuit for a synthesizer, which may be used as a reference clock, or a 3.579545 MHz oscillation signal for reproducing a color subcarrier. May be used. The DDS is directly driven by the VCO signal, and a frequency signal equal to the reference signal frequency is output. Since the DDS is a logic circuit, it is generally configured by a CMOS circuit.
In the current semiconductor process, it is conceivable that the DDS does not operate with a clock of about 60 MHz. In this case, a frequency divider is inserted between the local oscillator VCO and DDS,
What is necessary is just to reduce to the frequency which can operate DS.

Since the SAW filter is an external component of the IF processing IC and is expensive, FIG. 1 is based on the assumption that one SAW filter uses an inter-carrier type SAW filter that passes a certain amount of audio IF. Therefore, even in the output of the AGC circuit, the audio IF signal can be taken out although the video IF signal is suppressed. This signal is converted by the conversion carrier from the local oscillator,
Convert to low frequency. In contrast to the above four methods, the frequency of the local oscillation signal changes by tracking, and always outputs the same low frequency. In the BPF, there is no need to switch the center frequency according to the method, and it is only necessary to mount one regardless of whether it is externally mounted or built-in IC. If this output signal is subjected to FM (AM in France) demodulation by the SIF detection circuit, an audio signal is finally obtained. The frequency of the local oscillation signal is created by a reference signal that does not depend on the video synchronization signal.

As a result, the synchronizing signal does not leak into the low-frequency converted voice modulation signal, and the buzz interference can be improved. In order to obtain the same performance as the conventional one using the BPF with a built-in IC, it is desirable that the low-frequency conversion frequency is lower than 4.5 MHz. When the frequency conversion is performed, the carrier frequency changes without changing the frequency shift of the FM modulation, so that the modulation becomes apparently deeper. When extracting low frequency conversion frequency from mixer output by BPF, Q of filter is lowered,
Also, the SIF detection sensitivity can be reduced. Therefore S /
N and selectivity can be improved, and the deterioration due to the incorporation of the BPF into the IC can be offset.

The carrier frequency of the local oscillator is the audio IF
It is desirable to arrange the signal carrier on the opposite side to the video carrier on the frequency axis. If there is a carrier leak inside the IC, there is a possibility that a low-frequency beat will occur in the video if it is arranged on the video IF carrier side. The IF signal which is the input signal of the SAW filter includes not only a desired channel but also an adjacent channel signal. Since the video carrier of the adjacent channel is also converted to a low band by the mixer, it interferes with the audio signal. Although suppressed by the SAW filter, the level of the video carrier is large, so it is desirable to remove this with the BPF. For example, in the case of a domestic system adopting 4.5 MHz, the video IF is 58.75 MHz.
However, the audio IF is 54.25 MHz. The lower adjacent channel carrier frequency is 52.75 MH
z and only 1.5 MHz apart from the audio IF.
When the low band conversion frequency is selected to be 500 KHz, the emission frequency of the local oscillation signal is 53.75 MHz. Since the beat with the adjacent channel carrier is 1 MHz, the BPF
May be added with 1 MHz TRAP. This beat frequency differs depending on the method, but 1 MHz is the minimum value,
If the TRAP is configured to be attenuated even in a band higher than this, the same improvement can be achieved.

Therefore, if the low-band conversion frequency is selected between the audio IF frequency and the adjacent channel carrier (except for the intermediate channel carrier), it is possible to avoid interference by inserting TRAP. Therefore, it is preferable to select a low-frequency conversion frequency of 1.5 MHz or less.

In the above-mentioned domestic system, the low frequency conversion frequency is 500
If you choose KHz, the modulation factor will be enlarged 9 times,
The S / N can be simply improved 9 times. Q of BPF is 1/9
And the sensitivity of SIF demodulation is also reduced to 1/9, which can further improve the sensitivity. If a BPF is to be installed externally, an existing ceramic filter for AM radio can be used if the low-frequency conversion frequency is set to 455 KHz.
It can be improved over a system with an external PF.

Another specific example of the local oscillation signal generator will be described with reference to FIG. The local oscillation signal generator divides the frequency of the oscillation signal of the VCO 171 for generating the local carrier by the variable frequency divider 175 and compares the frequency with the reference signal with the phase comparator 17.
3 is compared with the VCO via LPF 174.
Return to 171. It is a synthesizer PLL.

By controlling the frequency dividing ratio of the variable frequency dividing circuit 175, an oscillation signal that is an integral multiple of the reference signal can be obtained. If the frequency division ratio is changed by the method, a desired frequency of the local oscillation signal can be obtained. This frequency depends not only on the system but also on which frequency the IF processing is to be performed (destination), and the frequency steps controlled by the synthesizer are case-by-case. The system of FIG. 1 is the most versatile system in that digital data can be switched to all destinations. However, when the destination is limited and the variable range of the dividing ratio is narrow, It can be configured with a smaller circuit scale than the DDS.

FIG. 3 is a system diagram for explaining another embodiment of the present invention. In the embodiment of FIG. 1, the case where only one inter-carrier SAW filter is used from the viewpoint of cost has been described. However, this embodiment is a case where the present invention is applied to a split carrier IF processing system.

In FIG. 3, the same components as those in FIG. 1 will be described with the same reference numerals. The input IF signal is converted to a video IF P-SAW filter 121 and an audio IF
The signal is input to the SAW filter 122 to separate each signal. The output of the P-SAW filter 121 is passed through a P-AGC circuit 131 for a video signal, and PIF detection is performed by a PIF detection circuit 14. The output of the S-SAW filter 122 is stabilized in amplitude by an S-AGC circuit 132 for an audio signal, and is input to the mixer circuit 15.

In the case of this embodiment, the S-SAW filter 122 is a filter for extracting an audio IF signal, and suppresses a video signal including a signal of an adjacent channel. As a result, the interference from the video signal and the adjacent video carrier is reduced as shown in FIG.
It can be further reduced.

Although some systems do not have the S-AGC circuit 132, either system may be used regardless of the effect of the present invention. This system was originally adopted to improve buzz.
Better performance can be obtained.

According to the present invention, it is conceivable to change the system by selecting the effects in order to have another effect on top of the prerequisite effects. If the only purpose is to incorporate a four-system BPF, a video IF carrier can be used as the reference signal in FIGS. In this case, the buzz disturbance is not effective, but the cost is reduced. Further, in the present invention, the purpose is to convert the audio IF signal into a low frequency band by a local oscillation signal, and the difference between the SAW filters shown in FIGS. 1 and 2 is not an essential problem, and the effect can be obtained in either system. Can be

[0029]

As described above, the use of the multi-system audio IF processing circuit of the present invention not only allows the BPF to be built into an IC, but also eliminates the need for switching between systems and improves buzz interference. be able to.

[Brief description of the drawings]

FIG. 1 is a system diagram for describing an embodiment of the present invention.

FIG. 2 is a block diagram for explaining another specific example of the local oscillation signal generator of the present invention.

FIG. 3 is a system diagram for explaining another embodiment of the present invention.

FIG. 4 is a system diagram for explaining a conventional audio processing system.

[Explanation of symbols]

12 ... SAW filter, 13 ... AGC circuit, 14 ... PI
F detection circuit, 15 mixer, 17 local oscillation signal generator, 18 BPF, 19 SIF detection circuit 19, 171
... VCO, 172 ... DDS, 173 ... Phase comparator, 17
4 low-pass filters, 121, P-SAW filters,
122 ... S-SAW filter, 131 ... P-AGC circuit, 132 ... S-AGC circuit.

Claims (4)

[Claims] 1. An oscillation circuit capable of controlling an oscillation frequency by a control voltage or a current, a DDS circuit for selecting a predetermined switchable addition data by a frequency control signal and processing the oscillation signal, and an output of the DDS circuit. A phase comparison circuit that compares the phase of a signal and an external fixed frequency reference signal; a low-pass filter that passes only a low-frequency component of a phase comparison output of the phase comparison circuit; and a control voltage or current form that outputs the low-pass filter. A local oscillator for generating a signal capable of switching the oscillation frequency in accordance with the TV audio intermediate frequency, and a mixer circuit for inputting the audio intermediate frequency signal and the local oscillation signal. The audio intermediate frequency having a different frequency is down-converted by a mixer output to a predetermined frequency equal to or lower than an audio modulation frequency, and the frequency is converted into one frequency regardless of the system. Multimodal sound IF processing circuit, characterized in that the down-conversion frequency. 2. An oscillation circuit capable of controlling an oscillation frequency by a control voltage or a current, a frequency divider circuit dividing an oscillation signal by a frequency division ratio switchable to a predetermined value by a frequency control signal, and a frequency divider. A phase comparison circuit that compares the phases of the output signal and the reference signal; a low-pass filter that allows only a low-frequency component of the phase comparison output to pass; and a local oscillation that feeds back the output of the low-pass filter to the oscillation circuit in the form of a control voltage or current. And a multi-system audio IF processing circuit. 3. The multi-system audio IF processing circuit according to claim 1, wherein the local oscillation frequency is selected so that the low-frequency conversion frequency is 1.5 MHz or less. 4. The reference signal according to claim 1, wherein one of crystal oscillation of a tuner unit, color subcarrier oscillation of a chroma processing unit, and a video IF carrier is used as the reference signal. A multi-system audio IF processing circuit as described in the above.