JPH0983903A - Front end used both for ntsc/pal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a front end used for NTSC/PAL capable of processing both television signals of NTSC and PAL systems by means of a simple circuit. SOLUTION: A high frequency television signal of an NTSC or PAL system inputted from an antenna 1 becomes a video signal of the NTSC or the PAL system and a second intermediate audio frequency signal of the system in a video demodulation circuit 33 through a tuner 2. The video signal of the NTSC or PAL system has the amplitude of a high band part emphasized by a video equalizer circuit consisting of a low-pass filter capable of changing the cut-off frequency and outputted from a video output terminal 36. On the other hand, the second intermediate audio frequency signal of the NTSC or PAL system has the video signal attenuated largely by a video signal blocking filter 37 consisting of a high-pass filter capable of changing the cut-off frequency and is made a third intermediate audio frequency signal of 6.0MHz by a mixer circuit 38, demodulated by a video demodulation circuit 41, and outputted from an audio output terminal 42.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an NTSC / PAL common front end which is used at an antenna input end of a TV, VTR or the like and which can receive and demodulate television high frequency signals of both NTSC and PAL systems.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing a conventional NTSC / PAL common front end.

As for the television high frequency signal of the NTSC or PAL system inputted from the antenna 1, only the high frequency signal of the desired channel is received and selected by the tuner 2. The tuner 2 converts the high frequency signal of the desired channel selected for reception into a video intermediate frequency signal and a first audio intermediate frequency signal and outputs the signal. If these signals are NTSC
If it is a signal of the system, these signals are
The signal is amplified by the NTSC SAW filter 4 to remove unnecessary signal components, and the NTSC video demodulation circuit 5 uses the NTSC video signal and the NTSC second signal.
Converted to an audio intermediate frequency signal.

Here, as shown in FIG. 5, the NTSC video signal includes a color subcarrier of 3.58 MHz and has a frequency component of 0 to 4.2 MHz, and the NTSC second audio intermediate frequency signal is F
It is an M-modulated carrier signal of 4.5 MHz.

The NTSC audio trap 6 traps the NTSC second audio intermediate frequency signal and passes the NTSC video signal. NTS
The NTSC video signal that has passed through the C audio trap 6 has its amplitude in the high frequency band enlarged by the NTSC video equalizer circuit 7 and is output from the NTSC video output terminal 8.

On the other hand, the second audio intermediate frequency signal of NTSC is
NTSC voice filter 9 which is a band pass filter with a center frequency of 4.5MHz
The signal is FM demodulated by the NTSC audio demodulation circuit 10, converted into an audio signal, and output from the NTSC audio output terminal 11.

Next, if the video intermediate frequency signal and the first audio intermediate frequency signal are PAL type signals, these signals are amplified by the PAL amplifier circuit 12, and the PAL SAW filter 13 removes unnecessary signal components. The signal is removed and converted by the PAL video demodulation circuit 14 into a PAL video signal and a PAL second audio intermediate frequency signal.

By the way, the PAL system is based on the B / G-PAL, IP
There are different types such as AL system, but as shown in Fig. 6, B / GP
The AL video signal is a signal that contains a color subcarrier of 4.43MHz and has a frequency component from 0 to 5.0MHz.The second audio intermediate frequency signal of B / G-PAL is an FM-modulated 5.5MHz subcarrier. It is a signal. Also, the I-PAL video signal is a signal that includes a color subcarrier of 4.43 MHz and has a frequency component of 0 to 5.5 MHz.
-PAL second audio intermediate frequency signal is FM modulated 6.0MHz
Is a subcarrier signal of.

The PAL audio trap 15 traps the second audio intermediate frequency signal of the PAL system and passes the PAL system video signal. The PAL video signal that has passed through the PAL audio trap 15 has its high frequency range amplitude enlarged by the PAL video signal equalizer circuit 16 and is output from the PAL video output terminal 17.

The PAL second audio intermediate frequency signal attenuates the PAL video signal frequency band, allows the PAL second audio intermediate frequency signal to pass, and has a video signal blocking filter 18 having a high-pass characteristic.
The signal is mixed with the local oscillation signal from the local oscillation circuit 20 in the mixing circuit 19 and converted into the PAL third audio intermediate frequency signal.

Here, at the time of receiving the 5.5 MHz B / G-PAL second audio intermediate frequency signal, the local oscillation circuit 20 outputs an oscillation signal of 0.5 MHz, and the 6.0 MHz third audio intermediate frequency signal is output. Is generated. When the 6.0 MHz I-PAL second audio intermediate frequency signal is received, the local oscillation circuit 20 stops the oscillation and does not output the oscillation signal, so the 6.0 MHz I-PAL second audio intermediate frequency signal is Passes through the mixing circuit 19 without frequency conversion, and B / G-PAL
The same as when receiving the second audio intermediate frequency signal of
It becomes an audio intermediate frequency signal.

The PAL third audio intermediate frequency signal output from the mixing circuit 19 passes through a PAL audio filter 21 which is a band pass filter having a center frequency of 6.0 MHz, is demodulated by a PAL audio demodulation circuit 22, and outputs PAL audio. Output from terminal 23.

[0013]

However, the above-mentioned conventional NTSC / PAL common front end has a PAL system signal processing circuit and an NTSC system in order to process television signals of both the PAL system and the NTSC system. Since the signal processing circuit had to be provided separately, there was a problem in that the front end circuit was complicated.

An object of the present invention is to solve the above problems and to provide an NTSC / PAL shared front end capable of processing television signals of both PAL and NTSC systems with a simple circuit.

[0015]

In order to solve the above-mentioned problems, the NTSC / PAL common front end of the invention according to claim 1 is N
The TSC and PAL system television high frequency signals are received, and the NT
A tuner for converting a television and high frequency signal of SC and PAL system to an intermediate frequency signal, and a tuner connected after the tuner and demodulating the intermediate frequency signal to generate a video signal and a second audio intermediate frequency signal. A video demodulation circuit for generating, a video signal equalizer circuit connected to a subsequent stage of the video demodulation circuit for correcting the video signal amplitude and outputting the corrected video signal, and a post stage of the video demodulation circuit, A video signal blocking filter circuit that passes the second audio intermediate frequency signal and attenuates the video signal, and is connected to a subsequent stage of the video signal blocking filter circuit, and mixes the second audio intermediate frequency signal and the local oscillation signal. And a mixing circuit for generating a third audio intermediate frequency signal,
Connected to the mixing circuit, changing the local oscillation signal frequency or stopping the oscillation based on the reception system control signal so that the third audio intermediate frequency signal frequency becomes a constant predetermined value. A local oscillation circuit that outputs the local oscillation signal to the mixing circuit, and a voice demodulation circuit that is connected to a stage subsequent to the mixing circuit and that demodulates the third audio intermediate frequency signal and outputs the demodulated audio signal as an audio signal. Is characterized by. The NTSC / PAL shared front end of the invention according to claim 2 is the NTSC / PAL shared front end according to claim 1, wherein the video signal equalizer circuit is a low-pass filter that passes a video signal frequency band. Yes, based on the reception system control signal, by changing the cutoff frequency of the low-pass filter, when receiving the PAL system video signal, the cutoff frequency is more than when receiving the NTSC system video signal. Characterized by high price. The NTSC / PAL front end of the invention of claim 3 is the same as that of claim 1 or 2.
In the NTSC / PAL common front end, the cutoff frequency of the video signal blocking filter circuit is changed based on the reception system control signal, and the NTSC system is used when the second audio intermediate frequency signal of the PAL system is received. The cutoff frequency is higher than when the second audio intermediate frequency signal is received. The NTSC / PAL common front end of the invention of claim 4 is the N of claim 2.
In the TSC / PAL shared front end, the low-pass filter of the video equalizer circuit has an inductor inserted in series with the signal line and a plurality of capacitors inserted between the signal line and the ground. At least one of the plurality of capacitors is connected between the signal line and the ground via a first switch, and the first switch is turned on or off based on the reception system control signal. Characterize things. The NTSC / PAL shared front end according to claim 5 is the NTSC / PAL shared front end according to claim 3, wherein the high-pass filter of the video signal blocking filter circuit is connected in series to a signal line. It has a inserted capacitor and a plurality of interactors inserted between the signal line and the ground, and at least one of the interactors is connected between the signal line and the ground via a second switch. The second switch is connected and turned on or off based on the reception method control signal. Claim
The NTSC / PAL shared front end according to the invention described in 6 is the NTSC / PAL shared front end according to claim 4, wherein the first switch is composed of a switching diode or a transistor. . The NTSC / PAL common front end according to the invention of claim 7 is the same as the NTSC / PAL common front end according to claim 5,
The second switch is composed of a switching diode or a transistor.

[0016]

The above technical means operates as follows.

That is, according to the NTSC / PAL common front end of the present invention, the video demodulation circuit and the video signal equalizer circuit are composed of NTSC and PA.
At the same time processing both L video signals, video demodulation circuit,
Since the video signal blocking filter circuit, the mixing circuit, the local oscillation circuit, and the audio demodulation circuit process both NTSC and PAL audio signals, the circuit configuration is simplified.

Further, since the video signal equalizer circuit can change the cutoff frequency in accordance with the reception system control signal, it is possible to provide optimum transmission characteristics to each of the NTSC and PAL video signals.

Further, since the video signal blocking filter circuit can change the cut-off frequency according to the reception system control signal, it is possible to suppress a bus fault.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an embodiment of an NTSC / PAL common front end according to the present invention. In FIG. 1, parts corresponding to those of the conventional example shown in FIG. 2 are denoted by the same reference numerals. The television high frequency signal of the NTSC or PAL system input from the antenna 1 is selected and received by the tuner 2, and only the high frequency signal of the desired channel is received.

The tuner 2 transmits the high frequency signal of the desired channel selected by reception to the video intermediate frequency signal of NTSC or PAL and NTSC or PAL.
The first audio intermediate frequency signal is converted and output. These NTSC or PAL video intermediate frequency signals and NTSC or PAL
1 The audio intermediate frequency signal is amplified by the amplifier circuit 31, the unnecessary signal component is removed by the SAW filter 32, and the NT
Converted to SC or PAL video signal and NTSC or PAL second audio intermediate frequency signal.

The audio trap circuit 34 traps the NTSC or PAL second audio intermediate frequency signal and passes the NTSC or PAL video signal. The video signal of NTSC or PAL that has passed through the audio trap circuit 34 has its amplitude in the high frequency band enlarged by the video signal equalizer circuit 35 and is output from the video output terminal 36.

The operation of the video signal equalizer circuit 35 will be described in detail.

FIG. 2A is a circuit diagram showing the video signal equalizer circuit 35. The NTSC or PAL video signal input from the input terminal 51 of the video signal equalizer circuit passes through the interactor 52 and the resistor 53, It is output from the output terminal 59 of the video equalizer circuit.
The capacitors 54 and 55, together with the interactor 52, form a low-pass filter, pass the video signal frequency band of NTSC or PAL, and
Attenuate the second audio intermediate frequency signal of TSC or PAL. The reception system control signal is applied to the reception system control signal input terminal 58, and the switching transistor 56 is turned on or off through the resistor 57, and the capacitor 55 is grounded or ungrounded.

When an NTSC video signal is input to the video signal equalizer circuit 35, a high level reception system control signal is applied to the reception system control signal input terminal 58 to switch the switching transistor.
56 is turned on, capacitor 55 is grounded, the low pass filter is set to a low cutoff frequency, and NTSC color subcarrier 3.5
The amplitude of the frequency signal in the vicinity of 8MHz is emphasized, and the pass characteristic is suitable for receiving the NTSC video signal.

When a PAL video signal is input to the video signal equalizer circuit 35, a low level reception system control signal is applied to the reception system control signal input terminal 58, and the switching transistor 56 is turned off. The low frequency filter of the video signal equalizer circuit 35 is set to a high cutoff frequency, and the amplitude of the frequency signal near the PAL color subcarrier 4.43 MHz is emphasized. The pass characteristics are suitable for receiving PAL video signals.

By the way, the above-mentioned reception system control signal is not shown in the figure, but from the SET of the TV or VTR in which this NTSC / PAL common front entry is built, the NTSC / PAL common front is in accordance with the instruction of the receiver. Is supplied to the end.

The circuit of FIG. 2 (b) uses a switching diode 60 in place of the switching transistor 56 in the circuit of FIG. 2 (a), and the operation is described in FIG. 2 (a). It is the same as the operation.

Now, the NTSC output from the video demodulation circuit 33
Alternatively, the PAL second audio intermediate frequency signal passes through a video signal blocking filter circuit 37 that attenuates the NTSC or PAL video signal and passes the NTSC or PAL second audio intermediate frequency signal.

The operation of the video signal blocking filter circuit 37 will now be described in detail.

FIGS. 3A and 3B are electric circuit diagrams showing the first and second embodiments of the video signal blocking filter circuit 37. In FIG. 3 (a), first, when the second audio intermediate frequency signal of NTSC is input from the input terminal 71 of the video signal blocking filter circuit,
The second audio intermediate frequency signal of SC passes through the capacitor 72 and is output from the output terminal 75 of the video signal blocking filter circuit.

The interactors 73 and 74 form a high-pass filter together with the capacitor 72, attenuate the video signal frequency band of NTSC or PAL, and pass the second audio intermediate frequency signal of NTSC or PAL. Receiving method control signal, receiving method control signal input terminal
The switching transistor 76 is turned on or off and the interactor 74 is grounded or ungrounded through the resistor 77.

First, when the NTSC second audio intermediate frequency signal is input to the video signal blocking filter circuit 37, a low-level receiving method control signal is applied to the receiving method control signal input terminal 78, The switching transistor 76 is turned off via the resistor 77, and the interactor 74 is turned off. Therefore, the interactor 73 constitutes a high-pass filter together with the capacitor 72, sets the cut-off frequency of this high-pass filter to a low value, attenuates the NTSC video signal, and simultaneously transmits the 4.5 MHz NTSC second audio intermediate signal. The pass characteristic is suitable for passing a frequency signal.

Next, when the second audio intermediate frequency signal of PAL is input from the input terminal 71 of the video signal blocking filter circuit, the second audio intermediate frequency signal of PAL passes through the capacitor 72 and becomes the video signal. It is output from the output terminal 75 of the blocking filter circuit. The high level reception method control signal input from the reception method control signal input terminal 78 is sent through the resistor 77 to the switching transistor.
76 is turned on and the inductor 74 is grounded. As a result, the cutoff frequency of the high-pass filter 37 composed of the interactors 73 and 74 and the capacitor 72 is set to a high value, the PAL video signal is attenuated, and the 5.5 MHz or 6.0 MHz PAL first signal is attenuated. 2 It has a transmission characteristic suitable for passing an audio intermediate frequency signal.

If the cutoff frequency of the above high pass filter is not changed by the received signals of NTSC and PAL, the following problems will occur.

That is, when the cutoff frequency is set to a value suitable for passing the second audio intermediate frequency signal of NTSC, PA
At the time of L reception, the PAL second audio intermediate frequency signal and a part of the PAL video signal pass through the high-pass filter, and a demodulation audio signal of PAL suffers a bus fault.

On the contrary, when the cutoff frequency is set to a value suitable for passing the PAL second audio intermediate frequency signal,
Upon reception of NTSC, the NTSC second audio intermediate frequency signal is greatly attenuated, and sensitivity failure occurs in the NTSC demodulated audio signal.

For the above reasons, when the video signal blocking filter circuit is shared between NTSC and PAL reception,
It becomes necessary to change the cutoff frequency.

FIG. 3B shows the second part of the video signal blocking filter circuit 37.
However, the difference from FIG. 3A showing the first embodiment is that the switching transistor 76 in FIG. 3A is switched to the switching diode 79.
However, the operation is exactly the same.

The second signal passed through the video signal blocking filter circuit 37
The audio intermediate frequency signal is mixed with the oscillation frequency from the local oscillator 39 in the mixer 39 to become a 6.0 MHz third audio intermediate frequency signal.

The second audio intermediate frequency signal passed through the video signal blocking filter circuit 37 is NTSC, B / G-PAL, I-PAL.
In the above case, the frequencies are 4.5, 5.5, and 6.0 MHz, respectively. In these cases, the local oscillation circuit 39 corresponds to each of the cases as follows. In other words, 1.5M for NTSC and B / G-PAL systems
It outputs the transmission signal of Hz and 0.5MHz, and when it is I-PAL, it stops the transmission and does not output the transmission signal. This local oscillator circuit 39
Although not shown, the operation of is switched by a reception method control signal input from the outside of the front end.

Since the transmission signals as described above are input to the mixing circuit 38, any second audio intermediate frequency signal is converted into a third audio intermediate frequency signal of 6.0 MHz.

The third audio intermediate frequency signal output from the mixing circuit 38 passes through an audio filter 40 which is a band pass filter having a center frequency of 6.0 MHz, is demodulated by an audio demodulation circuit 41, and is output from an audio output terminal 42. To be done.

[0044]

As described above, according to the NTSC / PAL common front end of the present invention, the video demodulation circuit and the video signal equalizer circuit, the video signal blocking filter, the mixing circuit, which is connected to the subsequent stage,
Since the voice demodulation circuit is shared by the NTSC system signal and the PAL system signal, the circuit configuration of the front end is simplified.

Since the cut-off frequency of the high-pass filter of the video signal equalizer circuit can be changed according to the NTSC and PAL system video signals, the NTSC and PAL system
The amplitude of the color subcarrier included in each video signal of the system is expanded, and the pass characteristic is suitable for receiving each video signal.

Further, since the cutoff frequency of the video signal blocking filter circuit can be changed according to the second audio intermediate frequency signal of NTSC and PAL system, it is possible to prevent the bus fault.

[Brief description of drawings]

FIG. 1 is a block diagram showing an NTSC / PAL common front end according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an embodiment of the video signal equalizer circuit shown in FIG.

FIG. 3 is a circuit diagram showing an embodiment of the video signal blocking filter circuit shown in FIG. 1.

FIG. 4 is a block diagram showing a conventional NTSC / PAL common front end.

FIG. 5 is a diagram showing an NTSC video signal.

FIG. 6 is a diagram showing B / G-PAL and I-PAL system video signals.

[Explanation of symbols]

 2 tuner 5 NTSC video demodulation circuit 14 PAL video demodulation circuit 33 video demodulation circuit 7 NTSC video equalizer circuit 16 PAL video equalizer circuit 35 video equalizer circuit 19, 38 mixing circuit 20, 39 local oscillation circuit

Claims (7)

[Claims] 1. A tuner for receiving a television high frequency signal of NTSC and PAL system and converting the television high frequency signal of NTSC and PAL system to an intermediate frequency signal, and a tuner connected after the tuner. A video demodulation circuit that demodulates the intermediate frequency signal to generate a video signal and a second audio intermediate frequency signal; and a video demodulation circuit that is connected to the latter stage of the video demodulation circuit, corrects the video signal amplitude, and corrects the video A video signal equalizer circuit for outputting a signal, a video signal blocking filter circuit connected to the latter stage of the video demodulating circuit, passing the second audio intermediate frequency signal, and attenuating the video signal, and the video signal blocking filter circuit A mixing circuit that is connected to the latter stage and that mixes the second audio intermediate frequency signal and the local oscillation signal to generate a third audio intermediate frequency signal; and a mixing circuit connected to the mixing circuit. Voice intermediate frequency signal frequency is changed based on the reception system control signal so that the local oscillation signal frequency is changed or the oscillation is stopped, and the local oscillation signal is sent to the mixing circuit. An NTSC / PAL common front end, comprising a local oscillation circuit for outputting and a voice demodulation circuit connected to the latter stage of the mixing circuit to demodulate the third voice intermediate frequency signal and output as a voice signal. . 2. The video signal equalizer circuit is a low-pass filter that passes a video signal frequency band, and changes a cutoff frequency of the low-pass filter based on the reception system control signal. , When receiving PAL video signals, NTS
2. The NTSC / PAL shared front end according to claim 1, wherein the cutoff frequency is higher than that when a C type video signal is received. 3. The cutoff frequency of the video signal blocking filter circuit is changed based on the reception system control signal,
NTSC / PAL according to claim 1 or 2, characterized in that the cutoff frequency is higher when receiving the second audio intermediate frequency signal of the L system than when receiving the second audio intermediate frequency signal of the NTSC system. Shared front entry. 4. The low-pass filter of the video equalizer circuit comprises an inductor inserted in series with a signal line and a plurality of capacitors inserted between the signal line and the ground, and the plurality of capacitors are provided. At least one of which is connected between the signal line and the ground via a first switch, and the first switch is turned on or off based on the reception system control signal. Claim to be
NTSC / PAL common front entry described in 2. 5. The high-pass filter of the video signal blocking filter circuit has a capacitor inserted in series with a signal line, and a plurality of interactors inserted between the signal line and the ground. At least one of the interactors is connected between the signal line and the ground via a second switch, and the second switch is turned on or off based on the reception system control signal. The NTSC / PAL shared front end according to claim 3, characterized in that 6. The N according to claim 4, wherein the first switch is composed of a switching diode or a transistor.
TSC / PAL shared front entry. 7. The N according to claim 5, wherein the second switch comprises a switching diode or a transistor.
TSC / PAL shared front entry.