JPH0127330Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement of a 3RF modulator installed in players such as VTRs and video disc players.

The video and audio signals played by the player are input via the antenna terminal of a television (TV) receiver, so that they are placed on a carrier wave of an empty channel of the TV receiver by an RF modulator. This is to facilitate connection between the two. The number of available channels on a TV receiver varies depending on the region where it is installed.
A general RF modulator has two types of carrier waves, and one of them is used. for example
In the Kanto region, the vacant channel in the VHF band is the 2nd channel (video and audio carriers are
97.25MHz, 101.25MHz), the first channel in the Kansai region (video and audio carriers are 91.25MHz, respectively)
95.75MHz). Incidentally, since an RF modulator mixes and outputs a signal obtained by modulating a carrier wave with a video signal and an upper sideband of a signal obtained by converting the frequency of the carrier wave with a frequency-modulated audio signal, the converted signal It is necessary to remove the lower sideband of the signal, so a trap circuit is provided for this purpose. FIG. 1 shows a partial configuration diagram of a conventional RF modulator. Terminal 1 inputs a video carrier modulated by a video signal (modulated video carrier), terminal 2 inputs a signal (converted signal) obtained by converting the frequency of an FM audio signal by a video carrier, and terminal 3
outputs a mixed signal (RF signal) of a modulated video carrier and a converted signal obtained by removing the lower sideband. The converted signal from the terminal 2 is applied to a trap circuit 4 including first and second trap circuits 4a and 4b, and the first trap circuit 4a converts the unnecessary lower sideband F ₁₂ (near 86.75MHz) of the first channel. , and the second trap circuit 4b removes unnecessary lower sideband F ₂₂ (near 92.75MHz) of the second channel. The transmission characteristics of this trap circuit 4 are as shown in FIG. 2a. Here, the first
When a channel is selected, the frequency characteristics of the video signal obtained at the output terminal 3 deteriorate as shown in FIG. 3a. This is because _F22 exists within the transmission band of the first channel, so when selecting the first channel,
This is recognized to be due to absorption or reflection of the video signal occurring near _F22 . On the other hand, the vicinity of F ₁₂ is the first and second
There is no such deterioration because it is outside the video band of the channel. Therefore, a flat frequency characteristic can be obtained for the entire video band as shown in FIG. 3b. Note that FIGS. 3a and 3b show the output characteristics for all video frequency inputs based on the output level appearing at the output terminal 3 when a single frequency (video signal) of 100 KHz is input. FIG. 2b is a pass characteristic diagram of the bandpass filter 5, showing that _F12 is outside the video band and _F22 is inside the band. Although it is effective to reduce the influence of the trap circuit 4b by increasing the resistance value of the resistor 6 in order to alleviate the above-mentioned drawbacks, a new problem arises in that the audio signal level decreases due to this resistor.

The present invention attempts to improve such deterioration of frequency characteristics that occurs when selecting the first channel, and instead of always providing trap circuits for the first and second channels in the transmission circuit as in the conventional example, The trap frequency of the trap circuit is changed according to the channel selection, thereby preventing the adverse effects caused by unnecessary trap circuits (for example, the second trap circuit when using the first trap circuit in the conventional example) and improving the frequency characteristics of the video signal. The aim is to provide an RF modulator that improves the

Next, the present invention will be explained according to the embodiment shown in FIG. In the figure, 10 is a video signal source, which outputs a standard television signal (video signal) in, for example, a video reproduction system of a VTR. 1
Reference numeral 1 denotes an audio signal source, which outputs a baseband audio signal in, for example, an audio reproduction system of a VTR. 12 is an IC (TA7673P) designed for an RF modulator, and the first input terminal 1 inputs the video signal from the video signal source 10.
3, a second input terminal 14 for inputting the audio signal from the audio signal source 11, and first and second output terminals 15 and 16 for outputting the signal-processed video signal and audio signal, respectively. A carrier wave generation source 17 selectively outputs a first or second carrier wave, and a video signal from the first input terminal 13 is preprocessed as appropriate by a clamp circuit 18 and a clip circuit 19, and then the video signal is output from the first or second carrier wave. a modulator 20 that modulates with two carrier waves, and the second input terminal 14
a second modulator 22 which amplifies the audio signal from the carrier wave generator 17 with an amplifier 21 and then frequency-modulates the frequency-modulated audio signal;
A converter 24 that converts the frequency of a carrier wave by attenuating it with an attenuation circuit 23 is provided. The carrier wave generation source 17 includes first and second surface acoustic wave resonators 25 and 26.
By connecting the switch 27 to the terminal A, the resonator 25 is selected, and by connecting the switch 27 to the terminal B, the resonator 26 is selected to output the first or second carrier wave. I have to.

Figure 5 shows the first and second output terminals 1 of this IC12.
5 and 16 are shown. 1st
From the output terminal 15, when the first carrier wave (first channel) is selected, an AM video signal represented by the first carrier wave _F1 is output as shown in FIG. ) is selected, AM video signals represented by the second carrier wave _F2 are output as shown in FIG. 5b, while from the second output terminal 16, when the first carrier wave is selected, As shown in FIG. 5C, when the first carrier wave _F1 and the upper and lower sideband signals represented by frequencies _F11 and _F12 are selected, and when the second carrier wave is selected, as shown in FIG. 5D, The above second carrier wave F ₂ and frequencies F ₂₁ and F ₂₂
Upper and lower sideband signals represented by are output respectively.

The switch circuit 28 has first and second contact pieces 29 and 30 which are controlled in conjunction with each other, and first terminals 31 and 32 to which each contact piece is connected when selecting the first channel.
The second channel to which each contact piece is connected when the second channel is selected
It is equipped with terminals 33 and 34, and the switch 2
7 to select the first or second carrier wave from the carrier wave generation source 17 and to control the trap frequency of a trap circuit to be described later. That is, the first contact piece 29 is connected to the reference potential point 31', the second contact piece 30 and the second terminal 33 are each connected to the power supply terminal 35, and the second terminal 3
3 and the first terminal 32 are connected to a switch 27 and a trap circuit (described later), respectively.

A trap circuit 40 connects the output of the converter 24 to a resistor 36.
and a first input terminal 41 for input via a series circuit of a capacitor 37, an output terminal 42 for providing an audio signal to the mixing circuit 55, a trap coil 43 connected between these two terminals, and an intermediate terminal between this coil 43. a resistor 44 connected between the point and the reference potential point, and a second resistor 44 coupled to the coil 43.
A coil 45, a movable iron core 46, a capacitor 47 connected in parallel to the second coil and selected so that the resonance frequency is approximately the second carrier frequency, and a trimmer connected in parallel to this capacitor via a switching diode 48. A capacitor 49, a second input terminal 50 for applying a control signal from the switch circuit 28 to the diode 48, and a bias resistor 5.
1,52. In this trap circuit 40 , the second contact piece 30 of the switch circuit 28 is connected to the second terminal 34.
Since the supply of via power to the diode 48 is cut off when the second channel is selected, the frequency around F ₂₁ determined mainly by the second coil 43 and the capacitor 47 is trapped. On the other hand, since the diode 48 is turned on when the second contact piece 30 is connected to the first terminal 32 as shown, the trap circuit 40 mainly connects the second coil 4
5. Trap around the frequency _F11 determined by the capacitor 47 and trimmer capacitor 48.

The mixing circuit 55 connects the output of the modulator 20 to the capacitor 3.
8 and a resistor 39, and the output obtained by level-adjusting the output of the trap circuit 40 with a resistor 56 are inputted to form a bandpass filter 57 having a pass characteristic as shown in FIG. 2b. Output to. The output of this filter 57 is connected via a terminal 58 to an antenna input terminal of a television receiver. FIGS. 5e and 5f show the outputs of the terminal 58 when the first and second channels are selected, respectively.

In this way, the trap circuit in the RF modulator of the present invention is designed to change the trap frequency when the first channel is selected and when the second channel is selected, so that, for example, when the first channel is selected, Since a trap frequency for selecting the second channel is not provided, absorption or reflection of the video signal as described above does not occur, and the level is constant over the entire video band for measurement at the input terminal 13. When a certain video signal was applied to the output terminal 58, it was observed that an output having the tendency shown in FIG. 3b (that is, without deterioration of frequency characteristics) was obtained when either the first channel or the second channel was selected.

[Brief explanation of the drawing]

Figure 1 shows a partial configuration diagram of a conventional RF modulator,
FIGS. 2a and 2b are transfer characteristic diagrams of the trap circuit 4 and bandpass filters 5 and 57. Figure 3a,
b is a frequency characteristic diagram of the RF modulator. FIG. 4 is a block diagram of an embodiment of the present invention, and FIGS. 5 a to 5 f are waveform diagrams of outputs of various parts when the first and second channels are selected. Explanation of main figure numbers, 10...Video signal source, 11
...Audio signal source, 17...Carrier wave generation source,
20...Modulator, 22...Second modulator, 24...
Converter, 40 ...trap circuit.

Claims (1)

[Claims for Utility Model Registration] Video signal source 10 and audio signal source 11
, a carrier wave generation source 17 that selectively outputs the first or second carrier wave, and a first carrier wave from the carrier wave generation source 17.
Alternatively, a first modulator 20 that modulates a second carrier wave with the video signal from the video signal source 10, a second modulator 22 that modulates the audio signal from the audio signal source 11, and an output of the second modulator. a converter 24 that performs frequency conversion using a first or second carrier wave from the carrier wave generation source 17; a first lower sideband for the first carrier wave of the output of the converter; and a first lower sideband for the first carrier wave from the converter output;
a trap circuit 40 for changing the trap frequency in response to selection of the first or second carrier to selectively remove a second lower sideband for the carrier;
In an RF modulator comprising a circuit 55 for mixing the output of the first modulator and the output of the trap circuit, the trap circuit 40 has an input terminal 41 that receives the output of the converter 24, and a circuit 55 that mixes the output of the converter 24.
Output terminal 42 that outputs a signal to 5, and both terminals 4
A trap coil 43 connected between 1 and 42;
A second coil 45 coupled to this trap coil 43, a first capacitor 47 connected in parallel with this second coil 45, and this first capacitor 4
7, a second capacitor 49 connected in parallel with the switching diode 48, a second input terminal 50 to which a control signal for changing the trap frequency is applied, and the second input terminal 50 and the switching diode. 48; and a second resistor 52, which connects the ground potential point and the other pole of the switching diode 50. The trap circuit 4 is controlled to turn on and off.
It is characterized by changing the trap frequency of 0.
RF modulator.