JPH0744694B2 - Color signal processing circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a color signal processing circuit such as a home VTR (video tape recorder), and particularly to an oscillation of an oscillation circuit that oscillates in synchronization with a burst signal. The present invention relates to a color signal processing circuit that stabilizes the frequency and can be used as a clock signal for an external circuit.

(B) Conventional technology A color signal processing circuit for a home VTR requires a frequency conversion circuit for converting the frequency of the color signal, but such a frequency conversion circuit oscillates in synchronization with the burst signal during recording. Do
Requires VXO (voltage controlled crystal oscillator circuit). FIG. 2 is a circuit diagram showing a color signal processing circuit equipped with such a VXO. In the recording mode (EE mode), the color image signal applied to the input terminal (1) is filtered by the bandpass filter (2). The chroma signal is extracted. Then, the chroma signal is made to have a constant amplitude by the ACC circuit (3) and then applied to the first frequency conversion circuit (4) and the burst gate circuit (5). The burst gate circuit (5) extracts the burst signal in the chroma signal according to the burst gate pulse from the terminal (6), and the extracted burst signal is applied to the phase comparison circuit (7). Then, the burst signal is phase-compared with the oscillation output signal of VXO (8), and an error voltage corresponding to the difference is applied to VXO (8). Therefore, the oscillation output signal of the VXO (8) is synchronized with the burst signal, and the synchronized oscillation output signal is applied to the second frequency conversion circuit (9). On the other hand, an output signal synchronized with the horizontal synchronizing signal is applied to the second frequency converting circuit (9) from the PLL (phase locked loop) multiplying circuit (10) at a frequency that is an integral multiple of the horizontal synchronizing signal.
Therefore, the second frequency conversion circuit (9) generates an output of the sum and difference of both signals, and only the sum component is extracted by the bandpass filter (11) and is output to the first frequency conversion circuit (4). Is applied. Therefore, the chroma signal applied to the first frequency conversion circuit (4) is converted to a low frequency based on the output signal of the bandpass filter (11) and is led to the output terminal (12). Therefore, according to the circuit of FIG. 2, it is possible to accurately obtain the low frequency conversion chroma signal synchronized with the chroma signal to be recorded. A color signal processing circuit as shown in FIG. 2 is described in Japanese Patent Publication No. 61-59594.

(C) Problems to be solved by the invention By the way, in the recording mode, the ACC circuit (3) extracts the burst signal in the chroma signal according to the burst gate pulse, detects the level of the burst signal, and detects the level of the chroma signal. It keeps the amplitude constant. Therefore, a signal at the time of a weak electric field or a signal at the time of detuning in which the level of the burst signal becomes extremely small, or a test pattern signal (monoscope signal) without the burst signal itself is applied to the ACC circuit (3). At this time, the output of the detection circuit for detecting the level of the burst signal in the ACC circuit (3) becomes the minimum, and the gain of the ACC circuit (3) is controlled to the maximum. Therefore, signals and noise due to unnecessary radiation from the VXO (8) are amplified during the burst signal period in the chroma signal. Then, an abnormal output signal (unnecessary component) in the burst signal period is applied to the phase comparison circuit (7) via the burst gate circuit (5). Then, the oscillation output signal of VXO (8) is
A frequency other than the free-run frequency is generated depending on the abnormal output signal. As a result, there arises a problem that other circuits using the oscillation output signal of VXO (8) are adversely affected. For example, recently, VTRs that have the function of recording video signals in memory and playing back still images and slow motions have appeared, but with such VTRs, when recording and playing back the oscillation output signal of VXO (8), It is used as a write clock and a read clock. In such a case, it is not preferable that the frequency of the oscillation output signal fluctuates.

(D) Means for Solving the Problems The present invention has been made in view of the above points, and a comparator that compares a gain control signal that controls the gain of the ACC circuit with a reference voltage, and an output signal of the comparator. And a means for switching the oscillation circuit to a fixed oscillation type according to the above, so that the oscillation of the oscillation circuit is stabilized.

(E) Operation According to the present invention, when it is detected that the level of the burst signal in the chroma signal has become equal to or lower than a predetermined value, the oscillation circuit in the APC circuit is switched to the oscillation frequency fixed type. The circuit will oscillate stably at a predetermined frequency.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which ( 13 ) is a burst gate circuit (14), an ACC detection circuit (15), a low pass filter (16) and a variable gain amplification circuit. An ACC circuit composed of (17), (18) a comparator for comparing the output signal of the low-pass filter (16) with a reference voltage of a reference power supply (19), and ( 20 ) a crystal oscillator (21) serving as an oscillation source. A first amplifier circuit (22) forming a fixed oscillation circuit together with the crystal oscillator (21), and a second amplification circuit (23) forming a variable oscillation circuit together with the crystal oscillator (21) (24) is the first and second amplifier circuits (22)
A mode selector switch for switching the operations of (23) and (23) according to a recording / reproducing control signal, (25) an output terminal for obtaining an oscillation output signal of the oscillation circuit ( 20 ), and ( 26 ) a phase comparison circuit (7). ) And the oscillator circuit ( 20 ), which is an APC (automatic phase control) circuit. In FIG. 1, the same circuit elements as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted.

Now, assume that the VTR is in the recording mode and the mode selector switch (24) is as shown in the figure. Input terminal (1) in this state
When a color video signal having a normal level burst signal is applied to, after the chroma signal in the color video signal passes through the bandpass filter (2) and is amplified by the variable gain amplifier circuit (17), It is applied to the burst gate circuit (14). Then, only the burst signal in the chroma signal is extracted according to the burst gate pulse from the terminal (6) and applied to the ACC detection circuit (15) for level detection. Therefore, an output signal corresponding to the level of the burst signal is applied as a gain control signal to the variable gain amplifier circuit (17) via the low pass filter (16). Therefore, at the output end of the variable gain amplifier circuit (17), a chroma signal with a constant burst signal level can be obtained.

Now, when the burst signal level is low, the ACC circuit ( 13 ) in FIG. 1 adds a small gain control signal from the low-pass filter (16) to the variable gain amplifier circuit (17) to control its gain to a large level. Conversely, when the level of the burst signal is high, a large gain control signal is added to the variable gain amplifier circuit (17) to control the gain to a small level. Also, when a normal level burst signal arrives,
A gain control signal of an intermediate level is generated, and the gain control signal is also applied to one end of the comparator (18). Here, if the reference voltage of the reference power source (19) connected to the other end of the comparator (18) is set to a predetermined value, when a burst signal larger than the predetermined value is applied, the comparator (18) An "H" level control signal is generated at the output end and applied to the mode changeover switch (24). However, the mode changeover switch (24) is insensitive to the "H" level control signal and holds the state shown in the figure. Therefore, the first amplifier circuit (22) of the oscillator circuit ( 20 )
The second amplifier circuit (23) maintains the operating state while the operation is stopped. As a result, a variable-type oscillation circuit determined by the crystal oscillator (21) and the second amplification circuit (23) is formed, and its oscillation output signal is applied from the output terminal (25) to the phase comparison circuit (7). . Further, the burst signal from the burst gate circuit (5) intermittently arrives at the phase comparison circuit (7), the phases of both signals are compared, and the error voltage corresponding to the phase difference is detected by the second amplification circuit (23). ) Is applied. Therefore, the oscillation output signal of the oscillation circuit ( 20 ) synchronized with the burst signal is applied from the output terminal (25) to the second frequency conversion circuit (9).

Next, when the level of the burst signal in the chroma signal becomes extremely small due to a weak electric field or during detuning, or a signal in which the burst signal itself does not exist is a bandpass filter (2).
Is applied to the burst gate circuit (14) through the variable gain amplifier circuit (17), the signal of the burst gate period in the chroma signal is level-detected by the ACC detection circuit (15). At this time, the level of the signal during the burst gate period is very small, and the level of the output gain control signal of the ACC detection circuit (15) becomes small, and control for maximizing the gain of the variable gain amplification circuit (17) is added. Therefore, the burst signal amplified with a very large gain is used in the burst gate circuit (5).
And is applied to the phase comparison circuit (7). On the other hand, the low level gain control signal applied to the variable gain amplifier circuit (17) is simultaneously applied to one end of the comparator (18). Here, since the level of the gain control signal is very small, an "L" level control signal is generated at the output end of the comparator (18) and applied to the mode changeover switch (24). Then, the mode selector switch (24) is "L".
In accordance with the level control signal, the state is switched to the opposite of the one shown.
Therefore, the first amplifier circuit (22) of the oscillator circuit ( 20 ) is in the operating state and the second amplifier circuit (23) stops operating. As a result, the crystal unit (21) and the first amplifier circuit (22)
A fixed type oscillation circuit determined by and is formed, and its stable oscillation output signal is applied from the output terminal (25) to the second frequency conversion circuit (9).

At this time, the oscillation output signal of the oscillation circuit ( 20 ) is not synchronized with the incoming burst signal, but more accurate oscillation can be performed as compared with the case of oscillating at an abnormal frequency due to an unnecessary component. The reason why the oscillation frequency of the oscillation circuit ( 20 ) is switched between the fixed type and the variable type is that it is originally switched between recording and reproduction. During VTR playback, the oscillation output signal of the oscillation circuit ( 20 ) is used as a reference signal for frequency inverse conversion, so a very accurate one is required. Therefore, the structure shown in FIG. 1 is used to stabilize the oscillation operation during reproduction. Therefore, if the oscillation circuit ( 20 ) is used in a fixed type, its oscillation output signal is very stable and can be sufficiently used as a clock for other circuits.

(G) Effect of the Invention As described above, according to the present invention, a test pattern signal in which a burst signal level is extremely small and a signal is not present during a weak electric field and during detuning or a burst signal is applied. Since the oscillation circuit is switched to the fixed oscillation frequency type, the oscillation output signal is more stable than when it is oscillating due to an unnecessary component, and an external circuit using the oscillation output signal can be stably operated.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional color signal processing circuit. ( 13 ) …… ACC circuit, (18) …… Comparator, ( 20 )
…… Oscillator circuit, ( 26 ) …… APC circuit.

Claims (1)

[Claims] 1. An AP having an ACC circuit for controlling the amplitude of a recording chroma signal in a VTR to a constant value and an oscillation circuit for switching between an oscillation frequency fixed type and an oscillation frequency variable type according to a control signal.
In a color signal processing circuit having a C circuit, a comparator that compares a gain control signal that controls the gain of the ACC circuit with a reference voltage, and switches the oscillation circuit to a fixed oscillation frequency type according to the output signal of the comparator. Means for stabilizing the oscillation of the oscillation circuit.