JPS63191489A - Nonlinear quantization processing system for television luminance signal - Google Patents

Abstract

PURPOSE:To make quantization noise inconspicuous even when the number of quantization levels is decreased by utilizing logarithmic variation in human sensation with brightness, and determining electric quantization characteristics of a video signal so that steps of the brightness of a reproduced image are at logarithmically equal intervals. CONSTITUTION:The electric quantization characteristics of the video signal are so determined that reproduced brightness steps are at logarithmically equal intervals including the influence of tube surface reflection caused by background light, etc. Namely, when a luminance signal after gamma correction is quantized by (m) gradations, the quantization characteristics are so determined that D0>Dm-1 holds and there are >=1 (i)th quantization step width (0<=i<m) which satisfies Di<=Dm-l (inot equal to m-1). Consequently, visual quantization noise reduced and the capacity of a frame memory may be small when an image of the same quality is recorded.

Description

[Detailed description of the invention] <Industrial technology field> The present invention quantizes a television video signal.

The present invention relates to quantization characteristics when quantizing a video signal in a system that performs digital processing such as storage or transmission on the quantized video signal.

<Conventional technology> First, the terminology used.

E: Video (luminance) signal voltage (before gamma correction) Eo:
Video (luminance) signal voltage (after gamma correction) T: Picture tube gamma value E,: Black level video signal voltage E8: White level video signal voltage ■: Picture tube input voltage B: Tube surface brightness B,: Tube surface minimum Brightness B8: Maximum brightness on the tube surface S: Video signal compression voltage C: Contrast ratio: Defined as a constant.

Under ideal conditions in which reflections on the picture tube surface due to background light etc. can be ignored, there is a difference between the picture tube surface brightness B and the input voltage of the video signal.

B=kOV' (2) There is a nonlinear relationship given by: Since the gamma value γ is generally about 2.2, in order to reproduce with correct gradation characteristics, it is necessary to electrically correct the gamma value γ of the picture tube either on the transmitting side or inside the receiver. Generally, television video signals are transmitted after the gamma value γ of the picture tube is corrected on the transmitting side, and the gamma correction formula is used to pre-correct the gamma characteristics of the receiver on the transmitting side.

EW　”’　-Eb　”” It is expressed as

Conventionally, when storing a television video signal in a frame memory or digitally transmitting it, the gamma-corrected television video signal is linearly quantized. Since the gamma value exceeds 1, if the video signal voltage E0 is linearly quantized.

The brightness reproduced under the above ideal situation is quantized so that brightness - 3 = small quantization step for low level parts and large quantization step for high brightness level parts. be done. Although the degree of change in brightness step at this time is somewhat gentler than in the logarithmic series, it has reasonable characteristics.

However, when considering general television viewing conditions, the influence of tube surface reflection due to background light etc. cannot be ignored. (2
) The input and output characteristics of the picture tube are shown by the equation.

In reality, it is affected by reflected light from the screen, etc., and even if the input voltage is -0, the screen brightness B does not become 0.

That is, by using the lowest luminance Bbn on the tube surface and the highest luminance B8 on the tube surface, it can be expressed as in equation (4).

B= (B,l Bb)V' +Bb -+4
)Furthermore, if the contrast ratio is expressed as C, then Bb -1°B,, = C, and if the tube surface brightness B is 1 and normalized to C, equation (4) can be expressed as the following equation (5). I can do it.

B= (C-1)V' +1... (5
) Therefore, due to the influence of +Bb, = 4 for the voltage
− Using the linear quantization method, low brightness levels (
In the part where ■ is small), the brightness quantization step is (2)
It is even smaller than when assuming Eq.

In other words, when quantization is performed using the conventional method, the steps in the low part of the 5 brightness levels are too small, and the brightness steps in the relatively middle part of the brightness level are made too large. Therefore, if the quantization level is set to about 55 bits (32 levels), which is lower than the normally used 8 bits, a brightness step that is compatible with the visual characteristics of two people is not realized, and the brightness level in the reproduced image will change. There was a drawback that quantization noise was noticeable in the middle part. ′〈
Purpose of the Invention The present invention takes advantage of the fact that human senses are logarithmic with respect to brightness, and uses electrical control of the video signal so that the brightness steps of the reproduced image are logarithmically evenly spaced. The quantization characteristics are determined, which has the advantage that quantization noise is less noticeable even if the number of quantization levels is reduced.

<Description of structure and operation of the invention> The electrical quantization characteristics of the video signal are determined so that the steps of the reproduced brightness are logarithmically evenly spaced, including the influence of tube surface reflection due to background light, etc. now.

Since we are thinking of making the steps of the screen brightness B with respect to the quantized input voltage ■ logarithmically evenly spaced, we take the logarithms of both sides of equation (5) to form the equation regarding the logarithm of the brightness. When converted to .

log B=log ((C-1) V' +1)
...(6). Furthermore, ■ is the video signal voltage E
0 and normalize the right side of equation (6) from O to 1 as the video signal compression voltage S.

It becomes log C. Therefore, the electrical quantization characteristics of the video signal such that the reproduced brightness steps are logarithmically evenly spaced are as follows:
It is obtained by compression-converting the video signal voltage E0 using equation (1) and linearly quantizing it on S. Also,(
l) Inversely transform the equation and obtain E. As the picture tube input voltage (7)
As shown in the formula.

Video signal compression voltage S quantized by this equation (7)
From this, the quantized picture tube input voltage ■ can be obtained. Here, the contrast ratio is C.

It is most desirable to use a gamma value γ that matches the characteristics of the picture tube actually used. Further, the optimal values of the contrast ratio C and the gamma value T may change due to the influence of the brightness distribution of the image to be reproduced. However, these values do not need to be precisely optimized, and from actual measurement values and experimental results, r = 2.5, C = 20.
It has been found that it is best to use degrees.

Next, embodiments of the present invention will be described using the drawings. FIG. 1 is a block diagram of an embodiment of the present invention, in which 1 is an A
2 is a variable conversion circuit, 3 is a frame memory, and 4 is a variable inverse conversion circuit.

5 is a D/A converter, and 6 is a picture tube (television monitor). The operation of this circuit is to convert the analog video signal Ea into an A/D converter.
The converter 1 converts it into a digital signal (N bits), and inputs Eo to the variable conversion circuit 2. Variable conversion circuit 2
Then, let Eo be the value S from 0 to 1 obtained by equation (1).
Conversion table 7 that corresponds to the linearly quantized value Si
After Eo is converted into n (<N) bit data with reference to , it is stored in the frame memory 3. For example, A/D
If the conversion accuracy of converter 1 is 8 bits (N=8), Eo will be a numerical value in 256 steps from 0 to 255/255. If this is converted by setting the number of quantization bits to 5 (n=5), 0 to 31 (00000 to 11111
) is stored as data.

Next, in order to reproduce the stored data and display it on the picture tube, the values in the frame memory 3 are first input to the variable inverse conversion circuit 4. In this variable inverse conversion circuit 4.

Similar to the variable conversion circuit 2, the inverse conversion table 8 prepared in advance using equation (7) is referred to, and the data is converted into a picture tube input voltage normalized from 0 to 1.

FIG. 2 illustrates an example of conversion-inverse conversion tables 7 and 8 for converting an 8-bit signal into 5-bit signals. moreover,
■ is converted into an analog video signal Ea by the D/A converter 5.
' and the image is displayed on the picture tube 6. By passing these circuits.

The steps of the tube surface brightness B with respect to the quantized picture tube input voltage V can be made logarithmically evenly spaced.

Furthermore, if the steps of the tube surface brightness B are logarithmically evenly spaced from the input/output characteristics shown in FIG. 3, the quantization step width of the video signal voltage is 1 m-32.

1) The relationship Do > Dm-+ is established.

and 2) It can be seen that the condition that the relationship Di≦Dm-+ (if:m-1) is satisfied or one or more exists is satisfied.

In addition, in the above embodiment, variable conversion was performed after performing linear A/D conversion with higher accuracy than storing in memory, but equation (1) and ( 7
) It is also possible to add an analog nonlinear amplifier having the characteristics of the equation () or use a nonlinear A/D or D/A converter with the above characteristics.
The conversion characteristics shown in Equation 11 and Equation (7) are not required to be exact.

Reasonable approximations are also allowed.

<Effects of the Invention> As explained above, according to the present invention, there is an advantage that visual quantization noise is reduced. From a different perspective, when recording video with the same quality.

This has the advantage of requiring less frame memory capacity than the conventional case.

Further, although the above explanation has been given as an example of a system in which image data is simply stored in a frame memory and reproduced, it is also effective in the case where digitized image data is encoded and transmitted over a long distance using a communication line.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the present invention, FIG. 2 is an explanatory diagram for explaining a conversion-inverse conversion table, and FIG. 3 is an explanatory diagram for comparing the input/output characteristics of the present invention and a conventional method. In the figure, 1 is an A/D converter, 2 is a variable conversion circuit, 3 is a frame memory, and 4 is a variable inverse conversion circuit. 5 represents a D/A converter, and 7.8 represents a table.

Claims (2)

[Claims] (1) In a processing system that quantizes a video signal with gradations and performs digital processing including storage and/or transmission, i (0≦ The i<m)th quantization step width is D_
When i, 1) the relationship of D_0>D_m_-_1 is established, and 2) the quantization characteristic is such that there is one or more D_i satisfying the relationship of D_i≦D_m_-_1 (i≠m-1). A television luminance signal non-linear quantization processing method featuring: (2) Video signal voltage E_0 after gamma correction, normalized with the minimum value as 0 and maximum value as 1; Video signal compression voltage S, normalized with the minimum value as 0 and maximum value as 1; Gamma value of the picture tube When γ is the contrast ratio; A television brightness signal non-linear quantization processing method according to claim (1), characterized in that the brightness signal is quantized so that the brightness level steps of the grayscale image are logarithmically evenly spaced. . S=log{(C-1)E_0^r+1}/logC...
(1) (3) γ=2.5, C=20.0 and (1)
Claim No. (2) characterized in that the formula is calculated.
Television luminance signal non-linear quantization processing method described in .