JPH06153113A - Video signal processor and automatic gain control circuit - Google Patents

Abstract

PURPOSE:To accurately control an amplitude level of an output signal to a prescribed value through only the slight revision of the circuit. CONSTITUTION:A resistor 81 as a bypass circuit is connected in parallel between a base and an emitter of a current mirror circuit 80. Thus, a detection signal E corrected by a current via the resistor 81 is generated with respect to a detection signal D generated by differential transistor(TR) pair to detect a level of an analog output signal B. Thus, an output in the nonlinear operating range of the differential TR pair 7 is not included in a rising portion of the detection signal E. Thus, the level of the output signal is accurately controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal processing device and an automatic gain control circuit, and more particularly to a video signal processing device provided with an automatic gain control circuit for keeping the amplitude level of a video signal constant. The present invention relates to an automatic gain control circuit for keeping the amplitude level of analog signals such as video signals and audio signals constant.

[0002]

2. Description of the Related Art FIG. 2 (a) shows a circuit diagram of a preceding stage of a reproducing circuit in a video tape recorder (hereinafter referred to as VTR) having a conventional automatic gain control circuit. Here, 1 is a recorded tape, 2 is a reproducing head for reading a recording signal from the tape 1, and 3 is a first-stage amplifier for amplifying a video signal read by the reproducing head and outputting it as an analog signal A. Reference numeral 4 is a variable resistance circuit, and 5 is a final stage amplifier, which operate as a variable amplifier and further constitute an automatic gain control circuit together with a gain control signal generation circuit (7, 8 etc.). Then, the analog signal A is amplified to generate a video signal B whose amplitude is limited to have a constant amplitude.
Reference numeral 6 denotes an FM demodulation circuit for performing an FM demodulation process, which is an inverse process of the FM modulation at the time of recording, on the video signal B to generate a demodulated video signal.

The gain control signal generation circuit is generally a detection circuit which receives a video signal B which is its analog output signal and generates a detection signal, and a smoothing which smoothes the detection signal to generate a gain control signal. It is composed of a circuit. A so-called diode detection is famous as such a detection circuit. However, in the case of a video signal, its amplitude is about 300
It is as small as mV. Therefore, diode detection is not suitable.
Therefore, in order to detect the level of the video signal, a detection circuit including the differential transistor pair 7 and the current mirror 8 is usually used.

With this detection circuit, the value of the video signal B is compared with the reference value C for determining the amplitude of the video signal B, and the difference is detected to generate the detection signal D.
Then, the inverted detection current E ′ of the detection signal D is output to the smoothing circuit. This smoothing circuit is composed of a capacitor C1 and a transistor Q1, and smoothes the detection current E ′ before converting it into a control current G. In the variable amplifiers (4, 5) that receive the control current G generated by such a gain control signal generation circuit as a gain control signal, the amplitude of the video signal B corresponds to the reference voltage C, for example, 300 mV (pp). The amplification factor is controlled so that the analog signal A is amplified to generate the video signal B as an analog output signal.

Therefore, for example, in the reproduction mode, this VT
The R reads the signal recorded on the tape 1 via the reproducing head 2. Then, the read signal is primary-amplified by the first-stage amplifier 3 and sent as an analog signal A to the automatic gain control circuit (4,5, etc.). The automatic gain control circuit (4,5, etc.) amplifies the analog signal A to generate a video signal B, which is sent to the FM demodulation circuit 6, for example. As described above, regarding the video processing circuit in the subsequent stage for inputting such a video signal and performing the video signal processing by performing the processing so that the amplitude of the video signal becomes constant in the pre-stage portion of the reproduction circuit of the VTR. , Its design conditions are eased.

[0006]

As described above, in the conventional automatic gain control circuit and video signal processing device, the differential transistor pair is adopted as the detection circuit for detecting the amplitude of the video signal. However, since the differential transistor pair exceeds the range of the original differential amplification operation and is used as a detection circuit, its detection characteristic is not always ideal. More specifically, the video signal B is compared with the reference voltage C by the differential transistor pair (FIG. 2B).
(See the waveform example in FIG. 2C), and the detection current D flows almost corresponding to the portion where the video signal B exceeds the reference voltage C (see the waveform example in FIG. 2C).

However, the range of the original differential amplification operation of the differential transistor pair, that is, the range in which it operates linearly (the range in which the transconductance, which is the differential of the detected output current value with the differential input current value, hardly changes) is set. In the input range exceeding the range, the detection characteristic becomes non-linear (mutual conductance changes (see D'part of detection characteristic in FIG. 2D)). Then, the detection current D and the detection current E ′ obtained as an inversion current thereof are generated in accordance with this non-linear characteristic at the rising and falling portions thereof. Therefore, this influence reaches the gain control characteristic of the automatic gain control circuit through the control current G, and it becomes difficult to control the amplitude of the video signal B accurately and accurately.

For example, when the amplitude of the analog signal A fluctuates, the amplitude of the video signal B fluctuates within the range of 300 to 330 mV with respect to the target value of 300 mV (see the graph of gain control characteristic in FIG. 2 (e)). Since there is no other good circuit due to cost constraints and the like, the above circuit has been put to practical use by paying close attention to such variations, but it is not fully satisfactory. An object of the present invention is to solve the above problems of the prior art, and a video signal processing having a configuration capable of accurately controlling the amplitude level of an output signal to a constant value with a slight circuit modification. Circuit and an automatic gain control circuit.

[0009]

The structure of the video signal processing apparatus of the present invention which achieves such an object is provided with the following automatic gain control circuit. Further, the structure of the automatic gain control circuit of the present invention which achieves such an object is such that the analog input signal is received, and the analog input signal is amplified by the variable gain amplifier circuit so that a predetermined amplitude level is maintained. An automatic gain control circuit that outputs a converted analog output signal, has a differential transistor pair, receives the analog output signal as one input signal of the differential transistor pair, and determines the predetermined amplitude level. Is received as the other input signal of the differential transistor pair, a current corresponding to the collector current of the one transistor of the differential transistor pair is set as a detection current, and the detection current is smoothed to obtain a gain. A gain control signal generating circuit for generating a control signal, before generation according to a difference between the analog output signal and the reference signal By providing a bypass circuit that does not generate the detection current for the collector current in a non-linear region where the collector current rises, and the variable gain amplifier circuit amplifies the analog input signal with an amplification factor according to the gain control signal, whereby the analog Even if the amplitude level of the input signal changes, the amplitude level of the analog output signal is maintained at the predetermined amplitude level with high accuracy.

[0010]

In the video signal processing circuit and the automatic gain control circuit of the present invention having such a configuration, the collector current generated according to the difference between the analog output signal and the reference signal, that is, the signal corresponding to the conventional detection signal. In the non-linear region where R rises, the detection current of the present invention is corrected by the action of the bypass circuit and is not output. As a result, the detection signal rises in the range where the differential transistor pair operates linearly. Therefore, by generating the gain control signal based on this detection signal, it is possible to remove the adverse effect caused by the non-linear operation of the differential transistor pair from the gain control signal. As a result, the amplitude level of the output signal can be accurately controlled to a constant value.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A shows a circuit diagram of a front stage portion of a VTR reproducing circuit as an embodiment of an apparatus adopting an automatic gain control circuit of the present invention. Here, 1 is a recorded tape, 2 is a reproducing head that reads a recording signal from the tape 1, and 3 is a first-stage amplifier that amplifies the signal read by the reproducing head and outputs it as an analog signal A. Reference numeral 4 is a variable resistance circuit, and 5 is a final-stage amplifier. These constitute an automatic gain control circuit together with a gain control signal generation circuit (7, 80, etc.), and amplify the analog signal A so that the amplitude becomes constant. The limited video signal B is generated as an analog output signal. Reference numeral 6 denotes an FM demodulation circuit for generating a demodulated video signal by performing an FM demodulation process, which is a reverse process of the FM modulation at the time of recording, on the video signal B. Also,
Reference numeral 81 is a resistor provided as a bypass circuit in the gain control signal generation circuit (7, 80, etc.).

The variable resistance circuit 4 is a circuit which is constructed by using, for example, a gyrator or the like, and whose resistance value changes according to the control current G. The final stage amplifier 5 receives and amplifies the signal generated by the voltage-current conversion of the analog signal A at a ratio according to the resistance value of the variable resistance circuit 4. As a result, the variable resistance circuit 4 and the final stage amplifier 5 operate as a variable gain amplifier whose amplification factor is controlled by using the control signal G as a gain control signal.

The control signal G is generated from the video signal B which is an analog output signal of the variable gain amplifier by the gain control signal generating circuit. The gain control signal generation circuit includes a differential transistor pair 7 that operates as a comparison and detection circuit,
A current mirror circuit 80 that operates as a detection current inverting circuit, a capacitor C1 and a transistor Q1 that operate as a smoothing and current-voltage conversion circuit are connected in this order. However, in the current mirror circuit 80, a resistor 81 that operates as a bypass circuit is provided in parallel.

The differential transistor pair 7 receives at one of the input pairs a reference signal C having a predetermined value for determining the amplitude level of the video signal B. The other of the input pair receives the video signal B. Then, the collector current of the transistor on the side receiving the video signal B is obtained as the detection current D,
It is used as an input signal of the current mirror circuit 80. here,
The amplitude of the video signal B is usually about 300 mV (pp), which exceeds the input range in which the differential transistor pair 7 can perform differential amplification.

Therefore, the value of the video signal B is compared with the value of the reference signal C by the differential transistor pair 7, and the value of the video signal B is different from that of the reference signal C by the differential transistor pair 7. When it is within the input range in which the amplification operation is possible and when the value of the reference signal C is exceeded, that is, intermittently,
The detection current D is passed. In this way, since the differential transistor pair 7 is used even outside the differential operable range, the detection current D is non-linearly differentially amplified by the differential transistor pair 7 at the rising and falling edges thereof. Is generated (see the D ′ portion of the detection characteristic graph D in FIG. 1B).

The detection current D is inverted by the current mirror circuit 80 having the resistor 81 to generate the detection current E. The resistor 81 is a current mirror circuit 80.
Is connected in parallel between the base and the emitter of the transistor pair forming the current mirror. Therefore, the detection current D, which is the input signal of the current mirror, is supplied through this resistor 81 until the voltage between the base and emitter of the transistor pair forming the current mirror reaches 1 VF. Further, after the voltage reaches 1 VF, the current flowing through the resistor 81 is fixed to be substantially constant under the substantially constant voltage of 1 VF and is also supplied as a part of the detection current D. As a result, the resistance 81
Supplies the current at the rising and falling portions of the detection current D by bypassing the current mirror.

Therefore, the detection current E is not a mere reverse current of the detection current D, but is a current bypassed by the resistor 81, that is, a current obtained by subtracting the bypass current. The resistance value of the resistor 81 is, for example, 10
0 KΩ is adopted, and the maximum value of bypass current is about 7 μA. This is as small as possible within a range in which the value of the detection current D linearly changes with respect to the difference (BC) between the value of the video signal B and the value of the reference signal C. This is because the transition point of and is selected, and the value of the detection current D corresponding to this point is about 7 μA in this example.

The detection current E obtained by subtracting the bypass current from the detection current D flows out only when the value of the detection signal D exceeds the maximum value of the bypass current. That is, from the time of the rise, the differential transistor pair 7 linearly differentially amplifies and generates (see the detection characteristic graph E in FIG. 1B). The detection signal E is converted into a control current G after being made into a smoothed voltage signal F by the capacitor C1 and the transistor Q1.

As described above, the detection current E, which is corrected by the action of the bypass circuit, is converted into the control current G, not the mere inversion current of the detection current D, and the gain control signal for the variable gain amplifiers (4, 5) is generated. Is generated. Then, the variable gain amplifiers (4, 5) respond to this gain control signal, so that when the amplitude of the video signal B exceeds 300 mV, the amplification factor of the variable gain amplifier (4, 5) decreases and the amplitude of the video signal B changes. Below 300 mV, the amplification factor of the variable gain amplifiers (4, 5) increases. By such feedback control, the amplitude of the video signal B is controlled to a constant target value regardless of the amplitude of the analog signal A.

Moreover, since the detection current E is modified so that it rises in the linear operation region of the differential transistor pair 7, this automatic gain control circuit exhibits a substantially constant gain control characteristic within its operation range ( See FIG. 1 (c) graph of gain control characteristic). Therefore, the amplitude of the video signal B is controlled to be substantially constant regardless of the amplitude of the analog signal A. To give concrete numerical examples, the conventional value is 300 to 330 m.
Although it fluctuated with V and could be controlled only with the fluctuation range of 30 mV, it became possible to control within a fluctuation of 300 to 310 mV and with a slight fluctuation range of 10 mV. Moreover, linear detection characteristics can be obtained by a slight change in the circuit in which the resistor 81 is connected in parallel, and there is almost no increase in cost due to this change in the circuit.

Such an automatic gain control circuit (4,5, 7, 8)
0, etc.), when the VTR starts the operation in the reproduction mode, the VTR outputs the signal recorded on the tape 1 to the reproduction head 2
Read through. The read signal is primarily amplified by the first-stage amplifier 3 and sent as an analog signal A to the automatic gain control circuit (4,5, 7, 80, etc.). The automatic gain control circuit (4,5, 7, 80, etc.) amplifies the analog signal A so that the amplitude becomes constant, generates a video signal B as an analog output signal, and outputs it as a circuit in a subsequent stage.
For example, it is sent to the FM demodulation circuit 6.

Here, the amplitude of the analog signal A which is the analog input signal of the automatic gain control circuit fluctuates in a considerably wide range due to the recording state of the tape 1 and the like. On the other hand, the amplitude of the video signal B, which is the analog output signal of the automatic gain control circuit, is more accurately kept substantially constant. Therefore, the video signal transmitted to the video signal processing circuit in the subsequent stage such as the FM demodulation circuit 6 is stable, and the load on the video signal processing circuit in the subsequent stage is lightened, and the operation of this VTR is also stabilized accordingly. Although the video signal processing device has been described in detail above as an example, the application of the present invention is not limited to a video signal. For example, the automatic gain control circuit of the present invention is also useful for FM radio audio signals and the like.

[0023]

As can be understood from the above description, the present invention relates to the rising portion of the detection signal generated by the differential transistor pair to detect the amplitude level of the analog output signal. , A modification is made so that the output in the non-linear operation range of the differential transistor pair is not included. Moreover, this is realized by a slight circuit change such as the addition of one resistor. As a result, the amplitude level of the output signal can be controlled to a constant value more accurately than before with a slight change in the circuit.

[Brief description of drawings]

FIG. 1 shows a front stage portion of a reproducing circuit of a VTR as an embodiment of an apparatus adopting an automatic gain control circuit of the present invention. (A) is the circuit diagram,
(B) is a detection characteristic graph of the detection circuit, and (c)
Is a gain control characteristic graph of the automatic gain control circuit.

FIG. 2 is a diagram illustrating a V which adopts a conventional automatic gain control circuit.
This is for the front stage part of the TR reproducing circuit.
(A) is a circuit diagram thereof, (b) and (c) are waveform examples of the signal, (d) is a detection characteristic graph of the detection circuit, and (e) is a graph of the automatic gain control circuit. It is a gain control characteristic graph.

[Explanation of symbols]

 1 tape 2 reproducing head 3 first stage amplifier 4 variable resistance circuit 5 final stage amplifier 6 FM demodulation circuit 7 differential transistor pair 8 current mirror 80 current mirror circuit 81 resistance

Claims (2)

[Claims] 1. An automatic gain control circuit for receiving an analog input signal and amplifying the analog input signal by a variable gain amplifier circuit to output an analog output signal controlled to maintain a predetermined amplitude level. In the video signal processing device, having a differential transistor pair, receiving the analog output signal as one input signal of the differential transistor pair,
A reference signal having a predetermined value for determining the predetermined amplitude level is received as the other input signal of the differential transistor pair, and a current corresponding to the collector current of the one transistor of the differential transistor pair is used as the detection current. , A gain control signal generating circuit for smoothing the detected current to generate a gain control signal, wherein the collector current is generated in a non-linear region in which the collector current generated according to a difference between the analog output signal and the reference signal rises. Regarding the above, a bypass circuit that does not generate the detection current is provided, and the variable gain amplifier circuit amplifies the analog input signal with an amplification factor according to the gain control signal, so that the amplitude level of the analog input signal varies. An automatic gain control in which the amplitude level of the analog output signal is maintained at the predetermined amplitude level with high accuracy. Video signal processing apparatus characterized by comprising a control circuit. 2. An automatic gain control circuit for receiving an analog input signal and amplifying the analog input signal by a variable gain amplifier circuit to output an analog output signal controlled to maintain a predetermined amplitude level, A differential transistor pair, and receives the analog output signal as one input signal of the differential transistor pair,
A reference signal having a predetermined value for determining the predetermined amplitude level is received as the other input signal of the differential transistor pair, and a current corresponding to the collector current of the one transistor of the differential transistor pair is used as the detection current. , A gain control signal generating circuit for smoothing the detected current to generate a gain control signal, wherein the collector current is generated in a non-linear region in which the collector current generated according to a difference between the analog output signal and the reference signal rises. Regarding the above, a bypass circuit that does not generate the detection current is provided, and the variable gain amplifier circuit amplifies the analog input signal with an amplification factor according to the gain control signal, so that the amplitude level of the analog input signal varies. The amplitude level of the analog output signal is maintained at the predetermined amplitude level with high accuracy. Automatic gain control circuit for the butterflies.