JPS61224571A - Horizontal afc circuit - Google Patents

Abstract

PURPOSE:To obtain a picture without jitter of VTR etc. of fluctuation to the horizontal direction even with weak inputting at on-air by providing loop filters with considerably large and small time constant within a PLL loop and switching these two filters according to an AGC voltage. CONSTITUTION:When a function other than a television receiving is selected at on-air, a function interlocking switch is opened and an alteration switch 5 switches the loop filter to a loop filter 4 having a considerable small time constant. On the other hand, when the function of the television is selected, The function interlocking switch is closed and the alteration switch 5 is controlled by the control signal of a switching signal generating circuit 6. In other words, an RFAGC voltage responded to the intensity of electric field at the on-air is compared with a reference voltage and in normally inputting, the filter is switched to the loop filter 4a that has a considerable small time constant and it is switched to a loop filter 4b that has a considerable large time constant in weak inputting.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a horizontal AFC circuit that performs horizontal scanning in a television receiver.

[Background technology of the invention]

As a conventional circular horizontal AFC circuit, there is one shown in FIG. In FIG. 2, 1 is a synchronization separation circuit for obtaining a reference frequency, 2 is a phase comparator, 3 is a horizontal oscillation circuit including a deflection yoke, and 4 is a loop filter, which constitutes a PLL loop.

In the configuration shown in FIG. 2, the video signal is input to the sync separation circuit IK, and the horizontal sync signal is taken out.

This horizontal synchronization signal is input to the phase comparator 2, as well as the oscillation signal from the horizontal oscillation circuit 3, which compares the phases of the two signals and outputs a voltage based on the phase difference. This output voltage is supplied to the horizontal oscillation circuit 3 through the loop filter 4, and a horizontal oscillation signal locked to the horizontal synchronization signal of the video signal is obtained.

[Problems with background technology]

Since the conventional horizontal AFC circuit is configured as described above, the time constant of the loop filter 4 is fixed:
If the time constant is made small to follow the jitter of a VTR, etc., it will follow the noise and cause horizontal fluctuations on the screen when there is a temporary weak input on air.If the time constant is made large as a countermeasure for weak input on air, jitter becomes noticeable.

[Purpose of the invention]

This invention was made in order to eliminate the above-mentioned drawbacks of the conventional technology.Horizontal AF allows you to obtain a screen with suppressed horizontal fluctuations even during the jitter of ζ-VTRs and weak inputs on air.
The purpose is to provide a C circuit.

[From #IA1! The horizontal AFC circuit of this invention has a relatively large loop filter and a small loop filter in the PLL loop, and the two loop filters are switched by the AGC voltage.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention, and the same parts as in FIG. 2 are denoted by the same reference numerals.

In FIG. 1, the PLL loop includes a loop filter 4at! having a relatively small time constant. -1 loop filter 4b having a relatively large time constant, and these two loop filters 4a and 4b are switched by a changeover switch 5. The changeover switch 5 is changed over by a changeover signal generation circuit 6 which outputs a control signal based on the AGC voltage input.

The operation as a PLL loop with this configuration is similar to that shown in FIG. 2, but the fabrication is different during on-air reception. When a function other than television reception is selected while on-air, the function interlocking switch is opened and the selector switch 5 is switched to the loop filter 4a, which has a relatively small time constant.

On the other hand, when television is selected, the function interlocking switch is closed, and the changeover switch 5 is controlled by the control signal from the changeover signal generation circuit 6.

The RFAGC voltage corresponding to the on-air electric field strength is compared with the reference voltage, and Vζ is switched to the loop filter 4a with a relatively small time constant during normal input, and the loop filter 4b with a relatively large time constant during weak input. A control signal is output from the switching signal generating circuit 7 to the changeover switch 6 so as to switch to the switch 6.

Therefore, when there is a weak input in 1 and TV functions other than the television function, the PLL loop operates through the loop filter 4b with a relatively large time constant, and the PLL loop operates through the loop filter 4a with a relatively small time constant. By setting the level to a level that can track the jitter of VTRs, video discs, etc., and setting the loop filter 4b, which has a relatively large time constant, to a level that cannot track noise during weak on-air input, the level can be maintained even when the stop function and weak air/air input are input. A stable screen with no directional fluctuations can be obtained.

Furthermore, by switching the loop filter using the RFAGC voltage, a stable screen without noticeable jitter can be obtained even when an RF signal from a VTR, video disc, etc. is input.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain a screen with no horizontal fluctuation even when there is jitter in a VTR or weak input on air.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a horizontal AFC circuit showing an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional horizontal AF'C circuit. 1...Synchronization separation circuit 2...Phase comparator 3...Horizontal oscillation circuit 41L, 4b...Loop filter 5...
・・Choice switch

Claims (1)

[Claims] A first loop filter having a relatively small time constant and a second loop filter having a relatively large time constant are provided in the PLL loop, and the first and second loop filters are controlled by an AGC voltage. A horizontal AFC circuit characterized by switching.