JPH0140554B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an ACC for a color signal processing circuit of a home VTR.
The present invention relates to a detection circuit suitable for use in a circuit.

Currently, in home VTRs, the luminance signal is FM modulated, the color signal is band-converted to a low frequency, and recorded.
Two-head helical scan VTRs with playback have become mainstream. In such VTRs, an ACC circuit that stabilizes the level of color signals is essential. In other words, the low-frequency converted color signal is recorded superimposed on the FM signal, so if the color signal is too large, the cross-modulation between the color signal and the FM signal will increase, and this cross-modulation component will interfere with the luminance signal. It appears. Furthermore, if the color signal is too small, the S/N of the color signal will be poor. In this way, during recording, an ACC circuit is required to keep the level ratio with the FM signal at an optimal constant value. Also, since it is a two-head helical scan system, each field is recorded and reproduced alternately in the two heads. Therefore, due to the uneven characteristics of the two heads, a level difference occurs between the fields. If the saturation level of the color signal differs from field to field, flickering occurs on the TV screen, making it difficult to view. Therefore, an ACC circuit is still required to absorb the level difference between fields.

The ACC circuit detects the level of the burst signal and
This detection output is used to control a variable gain amplifier, and peak detection is generally used to detect the level of this burst signal. Emitter follower type detection, in which a large capacitor is connected to the emitter of a transistor, is often used for this peak detection. Usually, in the ACC circuit of a TV, etc.
With this detection circuit, the problem was the charging time constant, and as long as the discharging was slow enough, it was hardly a problem.
The same is true for VTRs during recording, but the discharge time constant also poses a problem during playback. That is, as described above, this is because the color signal levels differ between the fields.

This will be explained using the signal waveform shown in FIG. When the color signal level differs between fields as shown in Figure 1a, when the color signal level changes from large to small, the DC voltage decreases due to discharge as shown in Fig. 1b, and the gain of the variable gain amplifier increases. Control is exercised. Therefore, the ACC circuit output becomes as shown in c, and the level is the same as when the level is high. Next, when the color signal level changes from low to high, the detection circuit starts charging, and the DC voltage increases, as shown in b.
Control is performed to lower the gain of the variable gain amplifier, and the levels are aligned as shown in c.

In this way, the levels will become equal after a certain period of time has passed since the field is switched, but the levels will differ between the fields during transient periods of discharging and charging.

The TV screen will appear 20 hours after the field is switched.
Since the discharge starts at approximately 1.3 ms, the transient state of discharge (period of time τ ₁ shown in FIG. 1b) must also be completed within this time period.

For this reason, a method is used in which a discharge resistor is connected in parallel with a capacitor in the detection circuit during reproduction, and the discharge time constant is determined using this resistor and the capacitor.

An example of this case is shown in FIG. In FIG. 2, resistor R6 is the resistor for this discharge, and τ ₁ shown in FIG.
is approximately determined by the values of this resistor R6 and the detection capacitor C2. In addition, resistor R5 is connected to capacitor C2.
The charging time constant is approximately determined by the values of this resistor R5 and capacitor C2. This resistor R5 is used to prevent the capacitor C2 from being charged by impulse noise.

The operation shown in FIG. 2 will be briefly explained.

A burst signal is input from terminal 1. Q1 is a transistor for clamping the burst signal, and as shown in Figure 3a, transistor Q2
The negative level of the burst signal is approximately (E ₁ −
A signal clamped to ube (approximately 0.7V) is input. Therefore, when the positive peak of the burst signal reaches approximately E2, transistor Q2 turns ON.
Then, the signal shown in FIG. 3b is output to its collector. This is further amplified by transistor Q4, and a burst signal shown in FIG. 3c is outputted to the emitter of transistor Q5. This burst signal shown in FIG. 3c charges the capacitor C2.
In this part where there is no burst signal, the capacitor C
2 is discharged through the resistor R6, and at a point where the amount of this discharged charge and the amount of charged charge are equal,
The ACC circuit enters a steady state. As shown in Figure 1a, when the color signal level suddenly changes from high to low, the burst signal also changes in the same way, so the peak value on the positive side of the burst signal becomes lower than E2, and the transistor No burst signal appears on the emitter of Q5, and charging is no longer performed.
Therefore, discharge only occurs through the resistor R6, the DC potential decreases, and the gain of the variable gain amplifier is controlled to be increased. Therefore, the burst signal level input from terminal 1 increases again, and the peak value on the positive side becomes approximately E2. Then, charging from transistor Q5 starts again, and the DC voltage becomes constant.

When the color signal level suddenly changes from small to large, the peak value on the positive side of the burst becomes much higher than E2, the burst signal appearing at the emitter of transistor Q5 also becomes large, and the amount of charge becomes larger than the amount of discharge. Increase the DC voltage. For this reason,
The gain of the variable gain amplifier is lowered, the burst signal level input from terminal 1 becomes small again, the positive peak value becomes approximately E2, charging and discharging are balanced, and the DC voltage becomes constant.

Here, the color signal level difference between fields is 6dB
About that happens. This 6dB needs to be compensated for for about 20 hours, which requires fairly fast discharge. Therefore, DC due to discharge within 1H period
The voltage varies considerably as shown in Figure 3d. As a result, the color signal level gradually increases within the 1H period. In other words, on a television screen, the color saturation gradually increases from left to right.
This increase is 0.5 due to variations in the parts used, etc.
It can be as high as ~1 dB and cannot be ignored.

Also, the charging time is at most 1μs in 1H period (63ms)
Since the discharge period is the entire 1H period, the charging and discharging times differ by about two orders of magnitude. For this reason,
If you try to make this detection circuit work smoothly,
The resistor R6 requires a resistor with a value of several tens of kilohms.
Therefore, if you try to create a detection circuit using an IC,
In order to create the resistor R6 with a resistor inside the IC, a large chip size is required, and it is difficult to provide it inside the IC.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the prior art, eliminate changes in color saturation within a 1H period,
Another object of the present invention is to provide a detection circuit suitable for IC implementation.

The present invention reduces the detection output during the video period by limiting the discharge of the detection circuit to the burst signal period.
This prevents DC changes and reduces the resistance value of the discharge resistor.

FIG. 4 shows one embodiment of the present invention. In FIG. 4, Q1 to Q5 and Q7 are transistors;
Q6 is a diode, E ₁ and E ₂ are constant voltage sources, R1 ~
R8 is a resistor, C1 and C2 are capacitors, 1 is a burst signal input terminal, 2 is an output terminal, and 3 is a burst gate pulse input terminal.

When a burst signal is input from input terminal 1,
As in the case of FIG. 2, a burst signal as shown in FIG. 3c appears at the emitter of transistor Q5. Therefore, capacitor C2 is charged with the burst signal of FIG. 3c. On the other hand, from the burst gate pulse input terminal 3, a pulse of the burst signal period is inputted as shown in FIG. 5b. At the high level of this pulse, that is, during the burst signal period, the transistor Q7 is turned on, and the resistor R7 is grounded via the transistor Q7. Therefore, during the burst signal period, the charge in the capacitor C2 is discharged through the resistor R7 and the transistor Q7. During the video signal period, the transistor Q7 is turned off and is no longer discharged. Since charging is not performed at this time, the DC voltage at the output terminal 2 is kept constant. Therefore, when the discharge by the resistor R7 and the transistor Q7 during the burst signal period and the charge by the burst signal become equal, the output becomes balanced as shown in FIG. 5a.

In this way, in the video signal section, the output is always a constant voltage regardless of the springiness of components, so no change in color saturation occurs on the television screen.

When using the circuit shown in FIG. 4, when the color signal level changes from high to low as shown in FIG. 1a, a burst signal is sent to the emitter of transistor Q5 as explained in the case of FIG. will no longer appear. Therefore, during the burst signal period, the resistance R
A discharge occurs through resistor R7, the DC voltage decreases, this voltage is maintained during the video period, and during the next burst signal period, a discharge occurs again through resistor R7, the DC voltage decreases, and so on. The DC voltage gradually decreases during the signal period, increasing the gain of the variable gain amplifier, and the burst signal level input from terminal 1 increases again, causing transistor Q5 to increase.
Charging starts from , and charging and discharging become the same and a steady state is reached.

When the color signal level suddenly changes from low to high, the burst signal appearing at the emitter of transistor Q5 increases, the amount of charge becomes greater than the amount of discharge during the burst signal period, and the DC voltage increases. As a result, the burst signal level input from terminal 1 becomes small again, and charging and discharging become the same, resulting in a steady state.

As can be easily understood from the above explanation, in order to make the characteristics of the transient response when there is a level difference between fields the same as in the case of FIG. 2, the amount of discharge over the 1H period in FIG. The amount of discharge during the burst signal period in FIG. 4 may be made the same.

Since the burst gate pulse width is about 5 μs at most, it is less than 1/10 of the 1H period. Therefore, the resistance value of the resistor R7 can be reduced by this ratio with respect to the resistor R6. In other words, a value of several kilohms is sufficient for the resistor R7, and even if it is integrated into the IC, the
It can be realized with the same size as the internal transistor,
Even if transistor Q7 and resistor R8 are included, the chip size can be made smaller than when resistor R6 is integrated.

Furthermore, in FIG. 4, only a part of the burst signal is amplified and supplied to the transistor Q5, but it is easily understood that the same operation can be performed even if the burst signal is supplied as is. be. It is thus clear that the present invention does not depend on the method of amplifying the burst signal. Further, in the present invention, for convenience of explanation, the signal supplied to the base of the transistor Q5 has been described as having no color signal during the video period, but it is sufficient that the detection by the transistor Q5 operates as a burst signal when it is normal. Therefore, it is clear that there is no problem even if the video signal is supplied to the base of the transistor Q5, as long as the color signal in the video period is attenuated to a level smaller than the burst signal.

As explained above, the detection circuit according to the present invention
Even when used in a VTR reproducing circuit, there is no change in color saturation, making it possible to reproduce good color signals. Furthermore, when integrated into an IC, the chip size can be made smaller than before, resulting in lower costs.

[Brief explanation of drawings]

Figures 1 a to c are waveform diagrams for explaining the operation of the ACC circuit of the VTR reproduction circuit, Figure 2 is a circuit diagram showing a conventional example of a detection circuit used in the ACC circuit of the VTR reproduction circuit, and Figure 3 a to d are waveform diagrams showing the waveforms of each part in Fig. 2, Fig. 4 is a circuit diagram showing an embodiment of the present invention, and Figs. 5a and b are waveform diagrams showing the waveforms of each part in Fig. 4. be. 1...Burst signal input terminal, Q5, Q7...
Transistor, C2...Capacitor, R5, R7
...Resistor, 3...Burst gate pulse input terminal.

Claims (1)

[Claims] 1 Connect one end of a first resistor to the emitter of the first transistor, connect a capacitor to the other end of the first resistor, and connect a second resistor to the connection point between the capacitor and the first resistor. one end of the second resistor, and a second resistor to the other end of the second resistor.
, the emitter of the second transistor is grounded, a burst gate pulse is supplied to the base of the second transistor, and a burst signal is supplied to the base of the first transistor. Detection circuit.