JPH02219396A - Side lock prevention circuit - Google Patents

Abstract

PURPOSE:To prevent excess control from being applied to a VCO and to execute oscillation by a preceding control voltage by providing a horizontal synchronizing signal detection circuit and prohibiting it in correspondence to a detection output that the output signal of a frequency identification circuit is added to the VCO. CONSTITUTION:From a state that a desired carrier chrominance signal is obtained by the oscillation output signal of a stable VCO 4, a VTR is switched to a special reproducing state. When special reproducing is executed, the lack of a horizontal synchronizing signal or the fluctuation of a period is generated. A horizontal synchronizing signal detection circuit 17, to which the horizontal synchronizing signal is impressed from a terminal 6, detects that the horizontal synchronizing signal does not arrive in the normal period. Then, a switch 18 is opened in correspondence to the detection output. Thus, it is prohibited that an abnormal control signal is added from a frequency identification circuit 14 to the VCO 4. When the horizontal synchronizing signal is fluctuated, the operation of a phase comparator circuit 13 is stopped and an output terminal is opened. Then, an LPF 15 still holds the voltage before the fluctuation. Accordingly, the VCO 4 can be oscillated by the stable frequency before the fluctuation.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention relates to a sidelock prevention circuit used in a home VTR (video tape recorder) having a color video signal playback function, and particularly relates to a sidelock prevention circuit used in a home VTR (video tape recorder) having a color video signal playback function. The present invention relates to a sidelock prevention circuit that can stabilize the oscillation of a vCO (voltage controlled oscillator) used for frequency conversion during special reproduction.

(b) Conventional technology In a home VTR, a burst signal is extracted from the carrier color signal during playback, the extracted burst signal and the oscillation output signal of the oscillator are compared in phase, and frequency conversion is performed according to the phase difference. It controls the oscillation of vCO for Since the burst signal arrives intermittently, control over the CO is also intermittently.

Then, the oscillation frequency of the vCO deviates from its original value,
It has been known to accidentally lock onto abnormal frequencies. Therefore, as shown in Japanese Patent Application Laid-open No. 55-147892, for example, a so-called side system is used in which the oscillation output signal of the VCO is counted by a frequency discrimination circuit based on the horizontal synchronization signal, and the oscillation of the VCO is controlled according to the counted value. Methods of arranging anti-lock circuits have been used. By doing so, vCO is always controlled by the feedback signal, and a normal lock state can be maintained.

(c) Problems to be Solved by the Invention However, in the above method, if a horizontal synchronization signal is missing during special playback of a VTR, the frequency identification circuit will malfunction, and even if the oscillation frequency of the vCO is normal, it will become abnormal. There was a problem that the oscillation frequency of the above-mentioned CO would be disturbed.

(d) Means for Solving the Problems The present invention has been made in view of the above points, and includes a voltage controlled oscillator whose oscillation is controlled by phase comparison between an oscillation output signal of the oscillator and a burst signal; A sidelock prevention circuit having at least a frequency identification circuit that identifies an oscillation frequency of a voltage controlled oscillator based on a reference signal and controls oscillation of the voltage controlled oscillator according to the identification output,
A horizontal synchronization signal detection circuit for detecting the state of the horizontal synchronization signal is provided, and the output signal of the frequency identification circuit is prohibited from being applied to the voltage controlled oscillator in accordance with the detection output of the horizontal synchronization signal detection circuit. Features.

(E) Effect According to the present invention, a condition such as a missing horizontal synchronizing signal is detected, and the identification output of the frequency identification circuit is prohibited from being applied to vCO according to the detected output. ■CO
No extra control is applied to the oscillation, and oscillation can be performed using the previous control voltage.

(v) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention in the main circuit of an NTSC VTR. (3) is an oscillator that oscillates at carrier color signal frequency esc, and <4> is 160f++ (
fo is a VCO that oscillates at the horizontal synchronization signal frequency (5)
is a 4φ phase shift circuit that divides the frequency of the 16 OfH oscillation output signal of the VCO (4) according to the horizontal synchronization signal from the terminal (6) and shifts the phase by 90 degrees, (
7) converts the 4Of signal from the 44 phase shift circuit (5) into the f signal from the oscillator (3). (8) is a band-pass filter that passes only the 4.21 MHz component from the output signal of the sub-converter (7), and (9) is a band-pass filter that converts the frequency according to the signal from the low-pass filter (1). A main converter converts the frequency of a 629 KHz carrier color signal according to the output signal of the band pass filter (8), and a main converter (10) passes only the 3.58 MHz component from the output signal of the main converter (9) and outputs it. A bandpass filter (12) is connected to a terminal (11) to which the carrier color signal derived from the output terminal (11) is applied, and only the burst signal in the signal is applied to the horizontal synchronization signal from the terminal (6). A burst extraction circuit (13) is extracted according to the burst extraction circuit (12).
A phase comparison circuit (14) performs a phase comparison between the burst signal obtained from the oscillator and the oscillation output signal of the oscillator (3), and a phase comparison circuit (14) converts the oscillation output signal of the VCO (4) into a horizontal synchronization signal from the terminal (6). A frequency identification circuit (15) adds the output signal of the phase comparison circuit (13) and the output signal of the frequency identification circuit (14), and passes the added output to a low-pass filter. (16) to the V CO (4), and (1
Reference numeral 7) is a horizontal synchronizing signal detection circuit that detects a state change such as a loss of the horizontal synchronizing signal from the terminal (6) according to the oscillation output signal of the oscillator (3) and switches the switch (18).

First, reproduction in the normal mode of the VTR will be explained. Low-frequency converted 6 2 of the reproduced signal from the input terminal (2)
The 9 KHz carrier color signal is extracted via a low pass filter (1) and applied to the main converter (9). On the other hand, since a 4.21MHz signal is applied from the bandpass filter (8) to the main converter (9),
Frequency conversion of the carrier color signal is performed. The frequency-converted carrier color signal is passed through a bandpass filter (10).
, and only the 3.58 MHz carrier color signal is led out to the output terminal (11). The carrier color signal is applied to a burst extraction circuit (12), and only the burst signal is extracted based on the horizontal synchronization signal from the terminal (6). The phase of the extracted burst signal is compared with the oscillation output signal of the oscillator (3) by the phase comparison circuit (13) during the burst period. On the other hand, the oscillation output signal of the VCO (4) is compared with the oscillation output signal of the oscillator (3) by the frequency discrimination circuit (14). ) is identified based on the horizontal synchronization signal from the terminal (6), and when it is outside the predetermined range, the error output is applied to the adder circuit (15) via the switch (18). ), the two input signals are added, and the addition result is sent to the low-pass filter (16).
It is applied to VCO (4) via VCO (4), and the oscillation of VCO (4) is stably controlled.

Therefore, the stable oscillation output signal of the V CO (4>
By applying the signal to the main converter (9) via the φ shift circuit (5), sub-converter (7) and band-pass filter (8), a desired carrier color signal can be obtained.

Assume that the VTR is switched from this state to the special playback state. When special playback is performed, the horizontal synchronization signal may be missing or the period may fluctuate. Then, the horizontal synchronization signal detection circuit (17) to which the horizontal synchronization signal from the terminal (6) is applied detects that the horizontal synchronization signal does not arrive at a normal cycle, and switches the switch ( 18) Open. Therefore, an abnormal control signal from the frequency identification circuit (14) is transmitted to V CO (
16) is prohibited.

Also, when the horizontal synchronization signal fluctuates, the phase comparator circuit 13
Since its operation is stopped, its output terminal becomes open, and the low-pass filter (16) continues to hold the voltage before fluctuation.

Therefore, the VCO (4) can oscillate at a stable frequency before fluctuations.

FIG. 2 shows a specific circuit example of the horizontal synchronization signal detection circuit (17) in FIG. When N signals are counted, an output signal is generated, and an N-ary counter (20) is reset in response to a horizontal synchronizing signal from a terminal (6>).
) is an AND gate whose conduction is controlled by a control signal indicating special reproduction from the terminal (21), and (22) is a horizontal synchronization signal from the terminal (6) and the AND gate (
20) is a flip-flop that is inverted in accordance with the output signal of 20). The clock signal f'sc frequency is 3.58MH
z and the reset signal f.

Since the frequency of is 15.734KHz, the count value N of the N-ary counter (19) is N > f sc/
o:227.

It is set as 5. Therefore, N is set to 230, for example. Assume now that a horizontal synchronizing signal with a regular period arrives from terminal (6). Then, the N-ary counter (19) becomes 22
Since it is reset every 7.5 counts, no output signal is generated. Therefore, the output signal of the AND gate (20) remains at r L level and the output signal of the flip-flop (22
) is in the reset state and the output terminal (23) is at the rLJ level. Next, assume that special playback is performed and the horizontal synchronization signal is missing.

Then, the N-ary counter (19) continues counting the clock signal even after 227.5 counts, and at 230 counts, rH
, is applied to the AND gate (20). The other input of the AND gate (20) has a terminal (21)
Since the control signal obtained from the horizontal synchronization signal is applied, its output becomes 'HJ level, and the flip-flop (22) is set. Therefore, the output terminal (23) has rH, which indicates the lack of horizontal synchronization signal, and the level of It is possible to obtain output.

In the example shown in Figure 2, in order to operate the horizontal synchronizing signal detection circuit (17) only during special playback, the AND gate (2
0), but if it is desired to operate it also in normal playback, the output of the N-ary counter (19) may be applied directly to the flip-flop (22).

(G) Effects of the Invention As described above, according to the present invention, when reproducing a VTR equipped with a frequency identification circuit for controlling the oscillation frequency of the vCO, a change in the state of the horizontal synchronizing signal is detected. Since the operation of the frequency discrimination path is prohibited according to the detected output, the VCO no longer receives an abnormal control voltage and can oscillate at a predetermined frequency before fluctuation. Therefore, the frequency of the carrier color signal can be stably converted even during special reproduction.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, and FIG. 2 is a circuit diagram showing a specific circuit example of the horizontal synchronization signal detection circuit (17) of FIG. 1. (3)...Oscillator, (4)...VCO, (13)
...Phase comparison circuit, (14) ...Frequency discrimination circuit, (17) ...Horizontal synchronization signal detection circuit, (18)
···switch.

Claims (2)

[Claims] (1) A voltage-controlled oscillator whose oscillation is controlled by phase comparison between an oscillation output signal of the oscillator and a burst signal, and a voltage-controlled oscillator that identifies the oscillation frequency of the voltage-controlled oscillator based on a reference signal and adjusts the voltage according to the identified output. In a sidelock prevention circuit having at least a frequency identification circuit for controlling oscillation of a controlled oscillator, a horizontal synchronization signal detection circuit for detecting the state of the horizontal synchronization signal is provided, and the sidelock prevention circuit includes a horizontal synchronization signal detection circuit for detecting the state of the horizontal synchronization signal, and A side lock prevention circuit characterized in that the output signal of the frequency identification circuit is prohibited from being applied to the voltage controlled oscillator. (2) The horizontal synchronization signal detection circuit uses a signal of a predetermined frequency as a clock pulse, and includes a counter that is reset according to the horizontal synchronization signal, and a flip-flop that is inverted according to the output signal of the counter and the horizontal synchronization signal. 2. The sidelock prevention circuit according to claim 1, wherein the sidelock prevention circuit detects the state of a horizontal synchronizing signal according to the output of the flip-flop.