JPS6136426B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video modulator used in video equipment such as a VTR, and its purpose is to easily switch between multiple channels (5 to 10 channels) among television channels, and to・
The purpose of the present invention is to provide a video modulator that uses a locked loop (hereinafter referred to as PLL) to stabilize the output frequency with the accuracy of a crystal oscillator and is free from unnecessary spurious signals.

In a conventional video modulator, as shown in FIG. A VHF band video carrier signal generated by a crystal oscillator 3 and a multiplier 4 is input, and the video signal is modulated and output to a mixer 9.

The audio signal from the video equipment is subjected to processing such as pre-emphasis in the audio circuit 5, and is input to the intercarrier oscillator 6, where the intercarrier oscillation signal is FM-modulated with the audio signal. Note that the intercarrier oscillator 6 oscillates a frequency corresponding to the difference between the video carrier signal and the intercarrier signal, for example,
In Japan and the United States, the frequency is 4.5MHz, so the oscillator is an LC oscillator that uses a variable capacitance diode or the internal capacitance of a transistor as a variable reactance element for FM modulation.

The FM modulation signal from the intercarrier oscillator 6 is input to a mixer 7, where it is frequency-converted with the video carrier signal from the multiplier 4, which is input to the mixer 7, and the sum component of the FM modulation signal and the video carrier signal is converted into a component. The audio carrier signal is taken out from the bandpass filter 8, and the audio carrier signal is input to the mixer 9.

The mixer 9 mixes the video carrier signal and the audio carrier signal to obtain a VHF band television signal.
This VHF band television signal is input to the UHF mixer 10 and mixed with the output signal of the UHF oscillator 11,
Bandpass filter 12 for UHF TV signals
Take it out. Here, when switching the UHF band TV signal to a desired channel among multiple channels (5 to 10 channels), the UHF oscillator 1 is used.
1 and a bandpass filter 12 are interlocked using a paricon or a variable capacitance diode to change the tuning frequency.

In the video modulator described above, the accuracy of the output frequency is determined by the performance of the UHF oscillator 11, and the accuracy of the oscillation frequency is determined by the performance of the UHF oscillator 11. The accuracy was poor, and the drift due to temperature changes was generally about 100 KHz, which was not completely satisfactory.

Also, since the intercarrier oscillator 6 is an LC oscillator, the accuracy is generally 10 ^-3 , that is, 4.5M.
There is a fluctuation of ±4 to 5 KHz with respect to Hz, and if the intercarrier frequency is switched using a switching diode or the like, the accuracy and drift will further deteriorate.

Furthermore, when creating a VHF band television signal, the component of the difference between the unnecessary video carrier signal and the intercarrier signal generated by the mixer 7 cannot be sufficiently attenuated by the bandpass filter 8 and has an adverse effect as an unnecessary signal. Further, there was a drawback that unnecessary components generated by the UHF mixer 10 and components of the UHF oscillator 11 could not be sufficiently attenuated by the bandpass filter 12.

The present invention solves the above-mentioned drawbacks, and an embodiment of the present invention is shown in FIG. , a signal from a video carrier oscillator 13 that directly oscillates a UHF band video carrier signal is input to the video modulator 2.
A UHF band video carrier signal is output. The output of the video carrier oscillator 13 is input to the video modulator 2 and also to the frequency divider 14, and after being frequency-divided, the output is input to the variable frequency divider 15 in the subsequent stage and is divided at a desired frequency division ratio. The output of the variable frequency divider 15 is input to a phase comparator 16, which divides the output of a reference oscillator 19 equipped with a crystal oscillator by a frequency divider 18, and further divides the frequency-divided signal by a frequency divider 17 into a phase comparator 16. Input to comparator 16. Therefore, the phases of the frequency-divided signal from the video carrier oscillator 13 and the frequency-divided signal from the reference oscillator 19 are compared, and the phase control DC voltage output from the phase comparator 16 is fed back to the video carrier oscillator 13. to configure the first PLL and operate the video carrier oscillator 13 stably. For example, to operate the video carrier oscillator 13 at the frequency of US channels 30 to 38, the video carrier frequency must be 567.25MHz to 615.25M.
Hz, and if the output frequency of the frequency divider 18 is 0.25MHz and the frequency dividers 14 and 17 are of the same frequency division ratio, the frequency division ratio of the frequency divider 14 and the variable frequency divider 15 combined is When is 2269, the lock frequency is 567.25MHz
When the division ratio is 2461, there are 38 channels with a lock frequency of 615.25MHz. The frequency division ratio of the frequency divider 14 is preferably selected to a value that allows the variable frequency divider 15 to operate sufficiently.

The audio signal from the video equipment is subjected to frequency characteristic processing such as pre-emphasis in the audio circuit 5.
The signal is input to an audio carrier oscillator 20 that directly oscillates a UHF band audio carrier, and the oscillation frequency is FM-modulated with the audio signal. The FM modulated audio carrier signal is sent from the audio carrier oscillator 20 to the mixer 9.
and mixed with the video carrier signal.
A television signal in the UHF band is obtained and output, and is also input to the mixer 21, where it is mixed with the output from the video carrier oscillator 13, and the difference between the output of the audio carrier oscillator 20 and the output of the video carrier oscillator 13 is The signal (same frequency as the intercarrier frequency, 4.5MHz in Japan and the United States) is extracted and input to the frequency divider 22, and after being divided by the frequency divider 22, it is input to the phase comparator 23. is the reference oscillator 19 that generated the reference signal for the first PLL.
A signal obtained by dividing the output from the frequency divider 18 by the frequency divider 18 is input, and the phase comparator 23 compares the phases of the output from the frequency divider 22 and the output from the frequency divider 18, and outputs the phase-controlled DC voltage to the audio carrier oscillator 20. The second PLL is configured by controlling the oscillation frequency by providing feedback to the second PLL.

In this way, the second PLL is the video carrier oscillator 1
3 and the output of the audio carrier oscillator 20 is always a constant intercarrier frequency. (reference signal) is 0.25 MHz, and the output of the mixer 21 is 4.5 MHz, the frequency division ratio of the frequency divider 22 is 18, and the lock frequency of the audio carrier oscillator 20 is 4.5 MHz equal to the oscillation frequency of the video carrier oscillator 13.
The frequency is the sum of .

Note that the output of the audio carrier oscillator 20 is an audio signal that has been FM modulated, and in order to lock it at its center frequency, the response time constant of the phase control DC voltage of the phase detector 23 is changed from the synchronization of the FM modulated signal. Also choose long enough. Furthermore, in addition to controlling the frequency of the audio carrier oscillator 20 using the second PLL, the output of the phase comparator 16, which is the output of the first PLL, is added, and the outputs of the phase comparators 16 and 23 are superimposed. It can also be controlled accordingly (as shown by dotted lines).

As mentioned above, since the oscillator of the present invention is controlled by a PLL, an output with the same accuracy as a reference oscillator using a crystal can be obtained, and the frequency drift due to deviation from the frequency setting value and temperature change is ±
A very stable output can be obtained at around 10PPM, and when changing the intercarrier frequency (for example, when using it differently between Japan, the United States, Germany, and the United Kingdom),
The intercarrier frequency can be easily changed by changing the frequency division ratio of the frequency divider 22 using a switch or the like. If the division ratio of frequency divider 22 is 22, the intercarrier frequency is 5.5MHz, and the division ratio is 24.
When set to 6MHz.

Furthermore, since oscillation is directly performed in the UHF band in order to obtain the video carrier signal and the audio carrier signal, a mixer is not required and unnecessary spurious components are not generated, resulting in extremely large effects.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional video modulator, and FIG. 2 is a block diagram showing an embodiment according to the present invention. 1... Video circuit, 2... Video modulator, 5... Audio circuit, 9, 21... Mixer, 13... Video carrier oscillator, 14, 17, 18, 22... Frequency divider, 15... Variable frequency divider, 16, 23... Phase comparator, 19... Reference oscillator, 20... Audio carrier oscillator.

Claims (1)

[Claims] 1. In a video modulator that modulates a video signal and an audio signal to obtain a television signal, controlling a video carrier oscillator that generates a signal for video signal modulation by a first phase locked loop,
controlling the frequency of the difference between the output of the video carrier oscillator and the output of the audio carrier oscillator to be constant in a second phase locked loop; A video modulator characterized in that it also serves as a reference oscillator.