JPH077896B2 - Digital automatic gain controller - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control such as an ACC (automatic color signal controller) used to keep a color signal level constant in a VTR (video tape recorder) or the like. It relates to the device.

2. Description of the Related Art Conventionally, an automatic gain control device has been used to keep the level of an input signal constant. A conventional automatic gain control device will be described below with reference to the drawings.

FIG. 2 is a block diagram showing a conventional configuration. The gain of the signal input from the input terminal 10 is controlled by the multiplier 11,
Output to output terminal 16. The output of the multiplier 11 is sent to the level detection circuit 15 to detect the level. The detected level is compared with the reference level input from the input terminal 18 by the comparison circuit 14, the error with the reference level is integrated by the low pass filter 17, multiplied by a constant coefficient by the coefficient circuit 13, and the multiplier 11 Be returned to.

As a result, the level of the signal output to the output terminal 16 becomes equal to the reference level. In a control system using such a feedback loop, it takes time for the output level to become constant when the input level changes stepwise. The state of the response will be described with reference to FIG.

In FIG. 3, a is an input signal to the input terminal 10 and b is an output terminal 16.
The example of the output signal from is shown. The horizontal axis represents time and the vertical axis represents level. When the input changes stepwise at time t1 as shown in a, the output once rises in level and then returns to the original level as shown in b, and the output level is kept constant.

However, in the above configuration, as can be seen from FIG. 3b,
Output amplitude finally settles to a level greater than L1
It grows up to L2. Therefore, if the same control loop as the conventional one is configured by digital signal processing, L1 is sufficient for the dynamic range of the signal, but L2 is required for the dynamic range for calculation, and an extra bit number is required, resulting in a circuit scale. It had the drawback of becoming large.

Means for Solving the Problems To solve the above problems, a digital automatic gain control device of the present invention is an A / D converter for A / D converting an input signal, and an A / D conversion by the A / D converter. A limiter for controlling the amplitude of the limited data, and limiting the amplitude when the amplitude of the output data of the multiplier exceeds a certain value, and also outputting a determination signal indicating that the amplitude is limited. A circuit, a level detection circuit for detecting the amplitude of the output signal of the limiter circuit, a comparison circuit for comparing the value of the output of the level detection circuit and a reference level, a coefficient times the output of the comparison circuit, and its coefficient Comprises a coefficient circuit which switches to a large value when the limiter circuit performs the amplitude limitation in response to the determination signal of the limiter circuit, and is configured to control the multiplication factor in the previous period by the output of the coefficient circuit. is there.

Action The present invention has the above-mentioned configuration, and can limit the amplitude by using the limiter circuit, reduce the output dynamic range to reduce the circuit scale, and improve the response time.

Example Hereinafter, an example of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

The signal input from the input terminal 20 is converted into a digital signal by the A / D converter 1, and the amplitude is controlled by the multiplier. The output of the multiplier 2 has its input amplitude limited by the limiter circuit 3,
It is output to the output terminal 21. The limiter circuit 3 limits the amplitude and switches the coefficient of the variable coefficient circuit 4 when the input level exceeds a certain value. The variable coefficient circuit 4 is composed of coefficient circuits 6 and 7 having different coefficients and a switch 5, and the coefficient is changed by switching the coefficient circuits 6 and 7 with the switch 5. On the other hand, the level detection circuit 9 detects the level of the output of the limiter circuit 3, and the comparison circuit 8 compares it with the reference level input from the input terminal 22,
The error is integrated by the low pass filter 23 and sent to the variable coefficient circuit 4. The output of the variable coefficient circuit 4 is fed back to the multiplier 2. As a result, the level of the output terminal 21 is controlled to be the same level as the reference level.

The above operation will be described in more detail with reference to FIG. The input terminal to the input terminal 20 is shown in FIG.
In the case of the change shown in (1), if the limiter circuit 3 is not provided, the output thereof has the same response as the conventional example. However, since the limiter circuit 3 suppresses the dynamic range of the output, if the level is limited to L3, the output becomes as shown in FIG. 3c. In this case, if there is no limiter, as in the case of the conventional example, the amplitude of the response of FIG. Become.
That is, the dynamic range is kept small by controlling the amplitude, but the error signal fed back becomes small, and as a result, the response time becomes long. In the conventional example of FIG. 3b, time t3
While the response was established in Fig. 3c as it is.
As shown in, it takes until time t2. So limiter circuit 3
When the amplitude is limited by, the coefficient of the variable coefficient circuit 4 is switched to the larger one to increase the loop gain. As a result, the output of the limiter circuit 3 can have the same pull-in time as in the conventional case as shown in FIG.

As is apparent from the above description, the present invention can reduce the dynamic range of signal processing without deteriorating the transient response, so that the number of bits for digital signal processing can be reduced and the circuit scale can be reduced. .

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a digital automatic gain control device according to an embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of a conventional automatic gain control device, and FIG. It is a characteristic view which showed the response of invention. 1 ... A / D converter, 2 ... Multiplier, 3 ... Limiter circuit, 4 ... Variable coefficient circuit, 13 ... Coefficient circuit, 17, 23 ...
Low pass filter.

Claims (1)

[Claims] 1. An A / D converter for A / D converting an input signal, a multiplier for controlling the amplitude of data A / D converted by the A / D converter, and an output data of the multiplier. When the amplitude becomes a certain value or more, while limiting the amplitude, a limiter circuit that outputs a determination signal indicating that the amplitude is limited, a level detection circuit that detects the amplitude of the output signal of the limiter circuit, A comparison circuit that compares the output value of the level detection circuit with a reference level, and a coefficient times the output of the comparison circuit, the coefficient receiving the determination signal of the limiter circuit and limiting the amplitude by the limiter. Is a coefficient circuit that switches to a large value, and the multiplier is controlled by the output of the coefficient circuit.