JPH0746466B2 - Frequency modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a frequency modulator of a recording device that frequency-modulates a video signal (hereinafter abbreviated as FM modulation) and records it on a recording medium.

[Technical background of the invention and its problems]

Since a video signal has a wide frequency band ranging from direct current to several megahertz, a method of recording an FM wavelength subjected to FM modulation is widely used in order to record this with good characteristics. Among them, the magnetic recording device for professional use adopts a method of recording FM waves with a wide carrier frequency and a higher carrier frequency than that for consumer use, from the viewpoint of emphasizing the characteristics at the time of recording and reproducing, for example, 525/60 NTSC system video signals. In recording, carrier center frequency called high band method
FM waves of 8.95MHZ and deviation of 2.2MHz are recorded, and sufficient characteristics are obtained at present for video signal band, S / N ratio, and video signal distortion.

However, in recent years, for example, in the case of high-definition television, the demand for realization of high-definition and high-quality image quality by making the video signal band several times wider than that of the current one has increased. Luminance signal,
The so-called component recording method, which divides into a narrow band chromaticity signal, performs FM modulation with different carrier frequencies and different deviations, and then records them in independent recording areas, is attracting attention as a method with less image quality deterioration.

In order to perform recording satisfying such technical requirements, the following conditions must be satisfied for the FM modulator.

(1) From several megahertz to over 50MHz, linear distortion is small, waveform symmetry is good, and high frequency response is good.

(2) In order to obtain a stable demodulated video signal, the pedestal frequency of the modulated FM wave should be kept stable and always at a constant value against disturbances such as fluctuations of the input signal and temperature changes that the device receives.

(3) When a plurality of FM modulators having different carrier frequencies and deviations are provided as in the case of performing the component recording, it is desirable that the same circuit be used in consideration of economical efficiency and maintainability. However, the conventional frequency modulator for the purpose of FM-modulating and recording the video signal is not sufficient to satisfy the above condition.

[Object of the Invention]

An object of the present invention is to provide a frequency modulator which has a small direct distortion in a wide frequency range and has good waveform symmetry and high frequency response.

Another object of the present invention is to provide a frequency modulator capable of arbitrarily setting the carrier frequency and the deviation within the above frequency range.

[Outline of Invention]

The present invention provides the following frequency modulator.

(1) By providing an input amplifier that can change the DC component and gain of the input video signal, the pedestal frequency of the FM wave can be automatically controlled and the deviation can be set arbitrarily.

(2) A simple frequency modulator consisting of only one constant current source, a capacitor, a voltage comparator and a switch circuit connected to the output of the input amplifier is oscillated at twice the carrier frequency, and the frequency is divided by two. By dividing the frequency by 1, it is possible to perform frequency modulation with little linear distortion up to high frequencies and good waveform symmetry.

(3) In addition, the pedestal frequency of the FM wave is always stable and constant by using a digital counting type automatic frequency controller (hereinafter abbreviated as AFC) that is stable against environmental changes such as temperature changes in the equipment. It was possible to retain the value.

This AFC consists of a gate circuit, a counter, a latch, a DA converter, a pulse generator, an inverting integrator amplifier, and a presetting switch for presetting the data value of the above counter, which is accurate in synchronization with the video signal generated by the pulse generator. By counting and counting a part of the FM signal with a gate pulse with a wide width, it is possible to perform accurate and stable frequency counting, and by switching the switch connected to the preset input of the counter, It is possible to set the carrier frequency arbitrarily.

〔The invention's effect〕

(1) According to the present invention, in a recording device for FM-modulating a video signal to record it on a recording medium, it is possible to perform FM modulation with less linear distortion and good waveform symmetry and high-frequency response up to 50 MHz, and a wide band for high-definition television signals. It is possible to cope with recording of various video signals.

(2) With the digital counting AFC of the present invention, the pedestal frequency of the FM wave can be always kept stable against disturbances such as temperature changes.

(3) According to the present invention, it is possible to arbitrarily set the carrier frequency and the deviation within the frequency range of (1) above by a simple operation, and it is possible to easily support a wide range of systems.

Example of Invention

An example in which the present invention is applied to a frequency modulator of a magnetic tape recording device will be described with reference to the drawings.

FIG. 1 shows a video signal input terminal. The video signal input to this terminal is subject to band limitation, DC clamp, etc. as necessary.

The video signal input to the input terminal 1 is amplified by the input amplifier 2. This amplifier is equipped with a volume 3 for adjusting the deviation of FM waves and a DC control terminal 4 for determining the carrier frequency so that the deviation can be set arbitrarily, and AF
The voltage from C allows the pedestal frequency to be automatically maintained at a constant value.

The constant current source 5 converts the output of the input amplifier 2 into a constant current proportional to the video signal voltage, and charges the capacitor 6. By charging with a constant current, the time required to charge to a constant voltage becomes a value that is inversely proportional to the current value, and as described later, the frequency of the FM wave changes linearly with changes in current, that is, changes in video signal voltage. It is supposed to do.

When the voltage charged in the capacitor 6 becomes higher than the reference voltage of the voltage comparator 7, the output of the voltage comparator 7 is delayed by the delay time of the element and becomes high level. The output of the voltage comparator 7 becomes high level with a delay of the delay time of the element.
When the output of the voltage comparator 7 becomes high level, the switch circuit 8 is closed with a delay of the delay time of the element, and the charge of the capacitor 6 is discharged.

When the voltage of the capacitor 6 becomes lower than the reference voltage of the voltage comparator 7, the output of the voltage comparator 7 is delayed by the delay time of the element and becomes the low level as in the case of the high level, and the switch circuit 8 delays the element. Opens after a delay. Since the switch circuit 8 is opened, the capacitor 6 becomes the constant current source 5
After being charged again with the current from, the above operation is repeated and oscillation continues.

FIG. 2 shows the above series of operations, (a) shows the terminal voltage of the capacitor 6, (b) shows the reference voltage waveform of the voltage comparator 7, and (c) shows the switch circuit operating waveform. There is. In the figure (a), A to C are charging time, C-
A'is a discharge time, T ₁ and T ₃ in FIG. 7B are operation delay times of the voltage comparator 7, and FIG. 7C is T ₂ and T ₄ are operation delay times of the switch circuit 8.

As can be seen from FIG. 2, if the discharge time constant is sufficiently small, one cycle time A to A required for charging and discharging is the charging time A to
B is the sum of the operation delay time (T ₁ + T ₂ + T ₃ + T ₄ ), and the frequency f is the charging current ic, the capacitance of the capacitor is C, the reference voltage of the voltage comparator is V _REF , and the voltage when discharging is V _D Then, f = ic / {C (V _REF −V _D ) + (T ₁ + T ₂ + T ₃ + T ₄ ) ie}, and the frequency is f _o = for the change of the charging current ic as shown in FIG. It has a characteristic of being saturated at 1 / (T ₁ + T ₂ + T ₃ + T ₄ ), and in the range shown by O to A in the figure, 31 has the characteristic that the frequency linearly changes with respect to the charging current.

Although the upper limit of the frequency is determined by the delay time of the element, as shown in FIG. 1, the frequency modulator of the present invention has a simple structure in which only one voltage comparator and one switch circuit have a delay time. Therefore, it is possible to easily oscillate with a small linear distortion up to a high frequency, and the frequency modulator of this embodiment has a saturation frequency of 140 MHz and a maximum frequency of 110 MHz with a small linear distortion.
Has been achieved.

The output signal of the voltage comparator 7 is equal to the carrier frequency 2
This is an asymmetrical pulse waveform set to a doubled frequency, and by being divided by half by the frequency divider 9, it becomes a rectangular wave with good symmetry and is output from the output terminal 10 as an FM wave.

The above is the description of the operation of the first frequency modulator. Second
The operation of the frequency modulator, especially AFC, will be described.

The gate pulse 12 generated by the pulse generator 11 in FIG. 1 is generated at the timing of the back porch of the input video signal, and its pulse width is determined by means such as counting the pulses of an oscillator having strict frequency accuracy such as a crystal oscillator. It has an exact width.

The gate circuit 13 gates the FM signal corresponding to the back porch of the video signal with the gate pulse 12.

The counter 14 is provided to count the frequency of the gated FM signal, and the frequency corresponding to the blanking level of the video signal can be accurately counted by the gate and the counter.

The counter 13 takes in the data value specified by the preset switch 16 by the preset pulse 15 from the pulse generator immediately before starting the count, determines the starting point of the count, and sets the AFC operating point, that is, the pedestal frequency by the preset switch 16. It can be set to.
(If it is necessary to change the pedestal frequency more precisely, fine adjustment is possible by changing the reference voltage of the DA converter described later, but it is not shown in the figure.) The counter 14 is a gated FM. When the wave counting is completed, the count value is held until the next preset pulse.

The latch 17 latches the counter data by the hold pulse 18 from the pulse generator and simultaneously DA converter 19
Converts the latched data into an analog voltage.

In this embodiment, the timing of the hold pulse 18 is the timing of the sync tip of the video signal.

FIG. 4 shows the timing of the gate pulse 12, preset pulse 15, and hold pulse 18 with respect to the input video VS.

The analog voltage obtained from the DA converter 19 is a voltage including an error in the FM signal frequency corresponding to the blanking level of the video signal, amplified by the reaction integration amplifier 20 and applied to the DC control terminal 4 of the input amplifier 2 described above. As a result, the pedestal frequency can always be kept constant.

As described above, in the AFC of the present invention, the analog part is minimized and the main part is digitally processed, so that stable operation is guaranteed against environmental changes such as temperature changes.

[Other Embodiments of the Invention]

Although the AFC of the present invention counts the FM signal frequency corresponding to the blanking level at the back porch timing every H cycle of the video signal, it is also possible to count at the sync chip timing or every V cycle. It is possible.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 and FIG.
FIG. 4 and FIG. 4 are operation explanatory views. In the figure, 1 is a video signal input terminal, 2 is an input amplifier, 3 is a volume control, 4 is a DC control terminal, 5 is a constant current source, 6 is a capacitor, 7 is a voltage comparator, 8 is a switch circuit, and 9 is a frequency divider. , 10 is an FM wave output terminal, 11 is a pulse generator, 12 is a gate pulse, 13 is a gate circuit, 14 is a counter, 15 is a preset pulse, 16 is a preset switch, 17 is a latch,
18 is a hold pulse, 19 is a DA converter, 20 is an inverting integration amplifier.

Claims (1)

[Claims] 1. A frequency modulator used in a recording device for frequency-modulating a video signal and recording it on a recording medium, comprising: an input amplifier capable of changing a gain and a DC component of the video signal; and an output voltage of the input amplifier. A constant current source that generates a proportional constant current, a capacitor that charges the constant current from this constant current source, a voltage comparator that compares the voltage charged in this capacitor and a reference voltage, and this voltage comparator A frequency modulator controlled by the output voltage of 1 above to discharge the capacitor, and a frequency divider that divides the output of the voltage comparator into halves.