JPH048068A - digital image processing circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a digital image processing circuit for digitizing a video signal, expanding the video signal, and increasing contrast.

[Conventional technology]

Generally, the cathode current I, l of a picture tube (CRT) used to display a video signal as an image and the voltage Eax between the grid and the cathode have a relationship as shown in Fig. 2.
Also, the relationship between cathode current and brightness is proportional. In the figure, Ec is the cut-off voltage, and if the voltage is below this, cathode current will not flow through the picture tube. Also, E8 is the maximum allowable driving voltage of the picture tube. If the voltage is higher than E$, the cathode current will flow excessively and the spot diameter of the electron beam will become large, making it impossible to express fine parts of the image and causing an increase in the amount of X-rays generated from the picture tube.

For this reason, the voltage applied to the picture tube is from Ec to E. That is, Ec to Es is the dynamic range of the picture tube, and the contrast ratio (ratio from black to white) of the picture tube is determined from this value.

In other words, in order to obtain an image with sufficient contrast, it is sufficient to drive the picture tube by adding the amplitude of the video signal at a sufficient level within this range.

However, the video signal always falls within this range of 100
%, but there are also portions close to the black level as shown in FIG. 3(a), that is, times when the level of the video signal is low. In order to increase the contrast by utilizing the dynamic range of the picture tube, it is possible to obtain sufficient contrast by particularly enlarging and driving the changing portion of the signal as shown in FIG. 3(b). Based on this basic idea, 1. Autopedestal This lowers the blackest part of the image to the pedestal level. This situation is shown in FIG.

2. Increase in gain Since the operating range of the picture tube is not fully used as it is, the gain of the amplifier is increased (expanded as shown in Figure 3 (b)). A method of effectively using the operating range has been devised and put into practical use.

[Problem to be Solved by the Invention] However, in the above-mentioned conventional example, when autopedestal is performed, the content of the image, for example, an image that is bright and has a uniform screen with little change, such as a solid white image, is lowered toward black, so it is not sufficient to At the same time that the brightness is no longer felt, the amplification factor becomes excessive, resulting in an unpleasant image. Also, because the gain of the amplifier is increased, the contrast is higher than before for ordinary images, but sometimes for signals containing white peaks, the E
The drawback was that the GK was higher than E8.

There are limiters (such as ABL) to prevent this, but they have the disadvantage that a sufficient white peak (white color) cannot be obtained because they limit the white peak.

[Means for Solving the Problems (and Effects)] According to the present invention, a circuit processes a digital image signal by obtaining a maximum value, a minimum value, an intermediate value between the maximum value and the minimum value, and a gain coefficient of the digital image signal. By providing a shift means for shifting the intermediate value based on the comparison between the maximum value, the minimum value and a predetermined level, it is possible to increase the contrast without losing each texture even in bright and black images. This is how it was done.

〔Example〕

FIG. 1 is a diagram showing an embodiment of the present invention, where 1 is an input terminal to which a video signal is input, and 2 is a sampled signal after passing through a low-pass filter (not shown) that performs band limitation in a desired band. , an analog/digital converter that performs quantization and converts an analog signal into a digital signal; 3 is a memory for storing and delaying the digitized video signal data; 4 is a memory for multiplying the video signal data by a coefficient, etc. An image processing circuit that performs processing; 5 is a digital/analog converter; 6 is a maximum value detection circuit, a minimum value detection circuit, and an arithmetic circuit that calculates an intermediate value between the maximum value and the minimum value.

7 is a coefficient calculation circuit for determining the amplification factor, 8 is a coefficient calculation circuit for multiplying the coefficient from 7 by the coefficient from 9, and 9 is a coefficient for generating a coefficient for varying the amplification factor to obtain a desired contrast ratio. The generating circuit 10 is a control section that variably controls the coefficients of the coefficient generating circuit.

In the above configuration, the video signal input from the input terminal 1 is band-limited by passing through a low-pass filter (not shown) that performs band-limiting in a desired band.
Sampling is performed by analog/digital converter 2 (sampling clock generation circuit, power supply, etc. are not shown as they are well known) and quantization is performed, and the digital signal (digital data) is
and is sequentially supplied to the memory circuit 3, for example,
After being stored for one horizontal period (one line memory), it is sent to the image processing circuit No. 4.

Further, the digital signal from 2 is sent to the maximum value detection, minimum value detection, and intermediate value calculation circuit of 6.

6, the maximum value detection, minimum value detection, and intermediate value calculation circuit detects the maximum level data in the effective video signal data excluding the synchronization part and pedestal part of the video signal as shown in FIG. 6(a). Da is calculated and sent to the coefficient calculation circuit 7. In the coefficient calculation circuit of 7, if the maximum value obtained in 6 is Da, the minimum value is Db, and the intermediate value is Dc, the maximum value that can be obtained as digital data is F, and the pedestal level G is Da- If Db □ is Dd, calculate and select the smaller value.

This means that by determining the difference between the maximum value of the data or the pedestal level and the intermediate value, it is possible to determine which direction has less margin for amplification, and dividing by Dd means the amplification rate. This means that you have asked for . This coefficient calculating circuit is a circuit that calculates the intermediate value of the video signal and calculates the maximum possible amplification factor that does not clip the video signal.

However, when such calculations are performed, the result shown in Fig. 7 (a
), for example, if the maximum value Da of the video signal is the same value as the maximum value F that can be taken as digital data, the contrast will not be improved (Similarly, if the minimum value Db
(The same applies even if Dc is the same value as the pedestal level G.) In such a case, in the arithmetic circuit 6, if the other one becomes a large value, the intermediate value Dc is moved to improve the contrast. Even in this case, the amount of movement is set at a level of ±15 to 20% at which no discomfort is felt.

In this way, for a signal with a small contrast ratio near the white peak or black level, a level shift of the intermediate value is performed, and white expansion or black expansion is performed to improve the contrast. This situation is shown in Figure 7 (b). Shown below.

The coefficient generation circuit No. 9 is a circuit that generates coefficients to freely vary the effect of the present invention depending on the image preference.
Necessary data is input from the control section and a coefficient of 1 or less is generated. 1, the present invention has a 100% effect (improvement in contrast ratio).

Note that the coefficients obtained in step 7 range from a minimum value of 1 to a maximum value close to infinity. Further, the coefficients generated at 9 are less than 1, and these coefficients are input to the coefficient arithmetic circuit at 8 to perform multiplication processing.

However, as shown in Fig. 5(a), when the signal level is medium and the level difference between the maximum and minimum values is small, that is, when the contrast ratio is small, the signal level shown in Fig. 5(b) is
), an image with a large amplification factor (coefficient) will be displayed, giving a sense of discomfort.

For this reason, in this coefficient calculation circuit, it is necessary to determine the maximum value of the amplification factor (coefficient), that is, a limiter, for example, if the value is 2, the signal in FIG. It does not become (b), but is similar to (c) in the same figure, and does not cause any discomfort.

For example, if the coefficient of the coefficient generation circuit is 1, the coefficients obtained by this coefficient calculation circuit will be greater than or equal to 1 and less than or equal to 2, and will be supplied to the image processing circuit 4.

This image processing circuit takes the intermediate value obtained in step 6 as the center and multiplies it by the coefficient (amplification factor) obtained in the coefficient calculation in step 8, resulting in video signal data as shown in FIG. 6(b). processing is performed.

This digital video signal data is converted into an analog signal by a digital/analog converter 5 and is then provided later. After being band limited by a low pulse filter, it is supplied to the picture tube drive circuit.

[Effects of the Invention] As explained above, by providing a means for determining the maximum value, minimum value, or adaptive intermediate value of the digital image signal and varying the gain coefficient, it is possible to obtain sufficient contrast depending on the image. This results in a significant improvement in image quality.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment according to the present invention, Fig. 2 is a drawing showing the characteristics of the picture tube, Fig. 3 is a drawing explaining contrast, Fig. 4 is a drawing explaining the autopedestal, and Fig. 5 is a drawing showing the characteristics of the picture tube. FIG. 6 is a diagram for explaining the amplification factor, FIG. 6 is a diagram for explaining the maximum value, minimum value, and intermediate value, and FIG. 7 is a diagram for explaining the intermediate value shift. 1 is an input terminal, 2 is an analog/digital converter, 3 is a memory, 4 is an image processing circuit, 5 is a digital/analog converter, 6 is a maximum value detection, minimum value detection, path, 7 is a coefficient calculation circuit, 8 The coefficient calculation circuit 9 is a coefficient generation circuit, and 1o is a control section. Intermediate value calculation times

Claims (1)

[Claims] (1) Calculating means for calculating the maximum value, minimum value, and intermediate value of the digital image signal, coefficient calculating means for calculating coefficients for the digital image signal based on the output of the calculating means, coefficient generating means, and coefficients in the coefficient generating means a control section for varying the coefficient, a coefficient calculation means for calculating the coefficient obtained by the coefficient calculation means and a coefficient generated by the coefficient generation means, and a digital image signal processed by the coefficient obtained by the coefficient calculation means. A digital image processing circuit comprising: image processing means; means for shifting the intermediate value based on a comparison between the maximum value, the minimum value and a predetermined level.