JPH01106590A - Automatic gain controlling circuit - Google Patents

Abstract

PURPOSE:To improve response at the time of special reproduction, such as speed search, etc., without spoiling the S/N at the time of standard reproduction, by changing the detecting sensitivity of an amplitude detector in plural stage in accordance with the amplitude of input to be detected. CONSTITUTION:An amplitude detector 2 can independently change current values I2-I4 of constant-current sources I2-I4. When this automatic gain controlling circuit is applied to, for example, the chrominance components processing circuit of a VTR, the part having a low detecting sensitivity with a gentle inclination can be used at the time of standard reproduction and the part of a high detecting sensitivity with a steep inclination can be used when the input amplitude to be detected extremely changes due to special reproduction, such as speed search, etc., if the output signal of a voltage-controlled amplifier 1 is set so that it can be set in the middle of V1 and V2 as standard input to be detected. Therefore, the response to special reproduction can be improved without spoiling the S/N at the time of standard reproduction.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention relates to automatic gain control circuits used in a wide range of fields such as VTRs, TVs, and audio, and particularly relates to improvement of responsiveness when handling various input signals. .

[Conventional technology]

FIG. 3 is a block diagram showing a schematic configuration of a conventional automatic gain control circuit. The voltage controlled amplifier 1 amplifies an input signal to a magnitude corresponding to a control voltage and outputs the amplified signal. The output signal is amplitude-detected by an amplitude detector 2, and a negative feedback loop is constructed by applying a suppressing voltage corresponding to the detected amplitude to the voltage-controlled amplifier 1.

FIG. 4 is a circuit diagram showing details of the amplitude detector 2, and FIG. 5 is a diagram showing waveforms of various parts of the circuit of FIG.

From the voltage controlled amplifier 1, the polarity is opposite (i.e. the phase is 1).
Two outputs (shifted by 80°) are taken out, and the transistors Q1. are applied to the base of Q2, respectively. Transistor Q1. Q2 constitutes a differential amplifier, which amplifies the signal received at the base and transmits the signal to the transistor Q3. Give each to the base of Q4.

Figure 5(a), (bl is this transistor Q3.Q4
The base input waveform of is shown.

Transistor Q3. The emitters of Q4 are commonly connected, and a capacitor +JC is further connected to form a peak detector. Its output voltage is the transistor Q
Applied to the base of 5. FIGS. 5C and 5D show the base input waveforms of transistor Q5 without and with capacitor C, respectively. That is, transistor Q3. The common emitter output voltage of Q4 is the output voltage of transistors Q3. Since Q4 conducts alternately every half cycle of the waveforms (a) and (b), if there is no capacitor C, the fifth
As shown in Figure (C), the waveforms (a) and (b)' are full-wave rectified with the center level shown by the dotted line as the reference,
Due to the presence of the capacitor C, this becomes a voltage signal proportional to the amplitude ΔV of the signals (a) and (b), that is, the output signal of the voltage-controlled amplifier 1, as shown in FIG. 5(d). The higher the amplitude of the output signal is, the higher this voltage is.

Transistor Q5.06 constitutes a differential amplifier,
The power supply voltage voo is connected to the base of the transistor Q6 through a resistor R.
The voltage divided by R1 and R2 is obtained as a reference voltage. The voltage input to the base of transistor Q5 is compared with this reference voltage, and depending on the difference, transistor Q5. The current flowing through Q6 varies linearly. The current flowing through transistor Q5 is the current flowing through transistor Q7. The control voltage that is supplied to the resistor R3 through the current mirror formed by Q8 and therefore appears on the resistor R3 is at a minimum of O( The current of transistor Q5 is zero)
to maximum R3X11 (current of transistor Q5 is 11
). FIG. 6 illustrates this situation. However, R3 and 11 are the resistance value of the resistor R3 and the current value of the constant current source (1), respectively.

On the other hand, the voltage ff-a, lI control type amplifier 1 is configured such that when the control positive pressure increases, the voltage decreases by 1%. Therefore, the following negative feedback is applied, and the amplitude of the output signal of the voltage controlled amplifier 1 is held constant.

The amplitude of the output signal increases → the detection voltage increases and the base voltage of transistor Q5 increases → transistor Q5
The current increases → control! l! The pressure increases → the amplitude of the output signal decreases.

[Problem that the invention seeks to solve]

Conventional automatic gain υ control circuits are configured as described above, and the detection characteristics of the amplitude detector 2 change linearly in accordance with changes in the input signal to be detected, as shown in FIG. It was only. That is, the amplitude detector 2 had only a single detection sensitivity determined by the slope of the straight line in FIG.

Incidentally, the VTR, which is one of the devices to which the above-mentioned automatic gain control circuit is applied, has special playback modes such as slow, sprue, and speed search in addition to the standard playback mode. A particular problem arises in the case of speed search. Taking color signals as an example, the problem is how sharp and normal the color is after the speed search noise bar. FIG. 7 is a diagram showing the input/output waveforms of the color signal of the automatic gain control circuit during speed search, and as indicated by the symbol N, the portion with no output becomes a noise bar on the screen. As is clear from the comparison of FIGS. 7(b) and 7(c), the higher the detection sensitivity of the amplitude detector 2, the sharper the response, which is advantageous for rapid coloring after the noise bar.

However, it is known as a fact that if the detection sensitivity is made too high, the S/N in standard reproduction deteriorates. This is because the reproduced signal from the head is subject to high frequency amplitude fluctuations, so the response overreacts to ψ and its high frequency components, resulting in a state similar to oscillation in the feedback loop, causing amplitude fluctuations. It is thought that this will increase the S/N ratio as a result. For this reason, in the past, the detection sensitivity of the amplitude detector was set by taking into account the convexity of the S/N during standard playback and the response during speed search, and making a compromise at an appropriate point.

This invention was made to solve the above problems, and the detection sensitivity of the amplitude detector changes depending on the situation. For example, when applied to the color signal processing circuit of a VTR, the An object of the present invention is to provide an automatic gain control circuit that can improve response during special playback such as speed search without sacrificing S/N.

[Means for solving problems]

In the automatic gain control circuit according to the present invention, the detection sensitivity of the amplitude detector is changed in multiple stages depending on the amplitude of the input signal to be detected.

[Effect]

The detection sensitivity of the amplitude detector in this invention changes in multiple stages depending on the amplitude of the input signal to be detected. For example, when applied to a color signal processing circuit of a VTR,
During standard 'U4 live and during special playback such as speed surge,
Optimal responsiveness can be achieved for each.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of an automatic gain control circuit according to the present invention. In this example, transistor Q3
．． Three differential amplifiers are provided that receive the output voltage of the peak detector constituted by Q4 and capacitor C. That is, the tenth differential amplifier includes transistors Q9. Q10
, the second differential amplifier includes transistors Q11 . Q12, the third differential amplifier is a transistor Q13. Q14 and one transistor Q9 of each differential amplifier.
．． Ql 1. The base of Ql3 is the transistor Q3. Q4
is connected to the connection point between the common emitter of the capacitor C and the capacitor C. The other transistor QIO, Q1 of each differential amplifier
2. The power supply voltage vCC is connected to the base of Q14 through a resistor R4,
The first, second . A third reference voltage is provided respectively. Transistor Q9. Ql 1. Ql3
The sum of the currents flowing through transistor Q7. The control voltage is supplied to the resistor R3 via a current mirror formed by Q8, and a control voltage is generated to be applied to the voltage controlled amplifier 1. The other configurations are similar to the conventional circuit shown in FIG.

Figure 2 shows the amplitude detection shown in Figure 1! FIG. 2 is a diagram showing the detection characteristics of No. 2, that is, the relationship between the input amplitude of the detected wave to the amplitude detector 2 (Ti voltage, the amplitude of the output signal of the controlled amplifier 1) and the control voltage. The operation of the circuit shown in FIG. 1 will be explained below with reference to FIG.

When the input amplitude is from 0 to Vl, transistor Q9.
Only the first differential amplifier constituted by Q10 operates,
The current of transistor Q9 varies from 0 to 12 depending on its base voltage. However, 12 is the current value of the constant current source I2. At this time, the second. In the third differential amplifier, transistor Q12.014 is on, transistor Q11.
Ql3 is off. Therefore, transistor Q7
The current supplied to the resistor R3 via the current mirror formed by Q8 is only the current of the transistor Q9, and the control voltage varies between O and RxI2.

Next, when the input amplitude is between ■ and v2, in the first differential amplifier, transistor Q9 is on and transistor QIO is on.
is off, and in the 20th differential amplifier, the current of the transistor Q11 changes between O and I3 according to its base voltage, and in the third differential amplifier, the transistor Q13 is off and the transistor Q14 is on. However, I3 is the current value of constant current source 3. Therefore, transistor Q7
The current supplied to resistor R3 via the current mirror formed by Q8 is the sum of the currents of transistors Q9 and 011, and this sum varies between ] and (12+13).

Therefore, the control wJ voltage is from R3×■ to R×(I2+
13).

Next, when the input amplitude is between v2 and V3, the first differential amplifier and the second transistor Q9. Qll is on, transistor Q10. Ql2 is turned off and in the third differential amplifier the current of transistor Q13 varies between 0 and 14 depending on its base voltage. However, I4 is a constant current source I
The current value is 4.

Therefore, the current supplied from the l-transistor Q 7 +' Q 8 to the resistor R3 via one current mirror is transferred to the transistor Q9. It is the sum of the currents of Qll and Ql3, and this sum is from (■ 113) to (I2 + I3 + 14
) varies between. Therefore, the control voltage is RX(14
It will change between I> and R3×(I2+I3+14).

In the amplitude detector 2 configured as described above, the constant current source +
2.13.14 current values I and I3°■4 can be changed independently, for example, by setting l2=14>I3, the detection characteristics shown in FIG. 2 can be obtained. When this automatic gain control circuit is applied to, for example, a VFR color signal processing circuit, if the output signal of the voltage-controlled amplifier 1 is set to be exactly between ■1 and v2 as the standard test wave input, the standard reproduction Sometimes, a point with a small slope and low detection sensitivity can be used, and when the input amplitude of the detected wave changes drastically during special playback such as speed search, a point with a large slope and high detection sensitivity can be used. /
Response to special playback can be improved without sacrificing N.

In the above embodiment, a case where the automatic gain control circuit of the present invention is applied to a color signal processing circuit of a VTR has been described, but the automatic gain control circuit of the present invention can be applied to any case where it is necessary to change the sensitivity of an amplitude detector according to the input amplitude of a test wave. Can be widely applied.

Further, in the above embodiment, the sensitivity of the amplitude detector is changed in three steps, but it goes without saying that the sensitivity may be changed in two steps or four or more steps.

〔Effect of the invention〕

As explained above, according to the present invention, the detection sensitivity of the amplitude detector is changed in multiple stages according to the input amplitude of the detected wave, so that the detection sensitivity of the amplitude detector changes depending on the situation, for example. To obtain an automatic gain control circuit that can improve response during special playback such as speed serve without sacrificing S/N during standard playback when applied to a color signal processing circuit of a VTR. be able to.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of an automatic gain control circuit according to the present invention, and FIG. 2 is a diagram showing detection characteristics of an amplitude detector.
FIG. 3 is a block diagram showing a schematic configuration of a conventional automatic gain control circuit, FIG. 4 is a circuit diagram showing a conventional self-effect gain control circuit, and FIG. 5 is a diagram showing waveforms of various parts of the circuit in FIG. 4. FIG. 6 is a diagram showing the detection characteristics of a conventional amplitude detector, and FIG. 7 is a diagram showing the input and output waveforms of the color signal of the automatic gain 19 control circuit during speed search. In the figure, 1 is a voltage controlled amplifier and 2 is an amplitude detector. Note that the same reference numerals in each figure indicate the same or corresponding parts. Agent Masuo Oiwa Figure 2 Figure 3 Figure 4 Figure 5

Claims (2)

[Claims] (1) A voltage-controlled amplifier that amplifies an input signal with a gain corresponding to a control voltage, receives the output signal as a test wave input, performs amplitude detection on this, and controls the control voltage so that negative feedback is applied. An automatic gain control circuit equipped with an amplitude detector for applying a signal to a shaped amplifier, characterized in that the automatic gain control circuit is provided with means for changing the detection sensitivity of the amplitude detector in a plurality of stages according to the amplitude of the input wave to be detected. control circuit. (2) The automatic gain control circuit according to claim 1, wherein the detection sensitivity is set relatively low at the stage corresponding to the standard input of the test wave, and the detection sensitivity is set relatively high at the other stages.