What Is Cued Articulation?

Sharpness refers to the sharpness of detailed shadow lines and their boundaries on the image. Clarity, generally from the perspective of a video recorder, compares the image quality by looking at the clarity of the playback image, so the term definition is often used. Cameras generally use the term resolution to measure their ability to "decompose the details of the subject." Units are "TVLine" (also called lines), 4K resolution, 8K resolution, etc. ^[1] .

Paraphrase

It means that when viewed from the horizontal direction, it is equivalent to erecting each line of scanning lines and multiplying by 4/3 (

Introduction

1.Overview

TV'S clarity

Resolution and clarity

Human eye resolution and TV clarity

The resolving power of the human eye refers to the ability of the human eye to discern the details of the observed object or the details of the image. Specifically, it is the ability to distinguish two points on the plane. The human eye has limited resolution. Under certain conditions of a certain distance, a certain contrast, and a certain brightness, the human eye can only distinguish points that are as small as a certain degree. If the points are smaller, they cannot be seen clearly. According to the resolution of the human eye, it determines the index of image sharpness that film and television workers strive to achieve, and also determines the reasonable value of image pixels.

The ability of the human eye to distinguish image details is also called "visual sharpness". The magnitude of visual acuity can be expressed by the angle of view at which two points can be clearly observed. This minimum resolution angle of view is called "visual acuity angle". The larger the visual acuity angle, the coarser the details of the image that can be identified; the smaller the visual acuity angle, the finer the image details that can be identified. Under the conditions of medium brightness and medium contrast, when viewing still images, the visual acuity angle is between 1 and 1.5 minutes for people with normal vision. When viewing moving images, the visual acuity angle is larger.

In order to expand the research object from two points to one surface, the visual acuity angle is extended from the angle between the human eye to the two points to an adjacent black and white line from the observation point (human eye) to a certain distance. ". If you observe a series of continuous horizontal black and white lines arranged in the vertical direction, you can show the vertical sharpness of the image; if you observe a series of continuous vertical black and white lines arranged in the horizontal direction, It can show the horizontal sharpness of the image.

TV uses this principle to determine how high the vertical definition and how high the horizontal definition of the TV should be. Then, from the sharpness, it is calculated how many horizontal scanning lines and how many vertical scanning lines are needed. The scan line deduces how many horizontal and vertical pixels are needed, that is, the resolution of the corresponding image and the scanning format of a single TV image are established, and it is then combined with the number of images per second and other indicators , And finally established the corresponding TV system. Below, we take PAL TV as an example to illustrate how its clarity and resolution evolved.

2.Vertical sharpness

As mentioned above, according to the principle of visual acuity angle, the fineness with which the human eye can distinguish the adjacent black and white horizontal lines arranged in the vertical direction is called vertical sharpness, but how to identify and measure this fineness? This will be described with reference to FIG. 4. Assume that the picture height is H, and there are M horizontal lines with a certain width in black and white in the vertical direction, and the width of each horizontal line in the vertical direction is h. If the human eye can clearly distinguish these horizontal lines at a distance of L, the visual acuity angle can be expressed as:

= h / L (radian)

Because the width of each line pair is

h = H / M

Then

= H / (LM) (radian)

After turning radians into angles,

= 3438H / (LM) (minutes)

That is

M = 3438 (H / L) (1 / )

Experiments show that the optimal distance for viewing an image should be 4 to 5 times the height of the picture, and the total viewing angle at this time is about 15 degrees. In this case, it can be guaranteed that the human eye can see the complete picture without rotating. This distance can not only prevent eye fatigue caused by the eyeballs constantly turning when viewed too close, but also avoid the reduction of the ability to discern the image when viewed too far away, and prevent scenes outside the screen from entering the field of vision. If the viewing distance L is selected to be 5 times the screen height H, that is, L = 5H, and it is substituted into the above formula together with the visual acuity = 1.5 points, then

M = 3438 (1/5) (1 / 1.5)

= 458 (line)

This 458 line is what we call 458 TV lines, referred to as "line" for short. From the calculation above, you can see that when viewing the image at a distance of 5 times the screen height, the vertical resolution of the human eye is about 458 lines. At this time, the vertical sharpness of the image is exactly 458 lines. In this way, when developing a scanning format for a television system, its vertical pixels should be considered based on 458 line sharpness.

3.Horizontal clarity

The determination of horizontal sharpness is the same as the idea of determining vertical sharpness. However, due to the aspect ratio of the TV screen and the relationship between the vertical and horizontal sharpness on the overall image quality, the number of horizontal sharpness lines can be easily calculated without the complicated derivation mentioned above ^[2] .

The aspect ratio of a traditional TV screen is 4: 3, which is determined in advance based on the aspect ratio of the original movie screen. Experiments show that when the image is displayed, the horizontal sharpness and vertical sharpness should be close to or the same to obtain the best image quality. Using these two points, and according to the principle of vertical sharpness calculation, multiply the number of vertical sharpness lines by the screen aspect ratio 4/3, and immediately calculate the number of horizontal sharpness lines N of the image as

N = 4/3 M

= 4/3 × 458

= 610 (line)

This means that when viewing a 4: 3 picture at a distance of 5 times the picture height, the horizontal resolution of the human eye is about 610 lines. At this time, the horizontal sharpness of the image is exactly 610 lines.

These are the sources of TV vertical and horizontal clarity. It is not difficult to see here that after clarifying the vertical and horizontal "resolution" of the human eye, the concept of "definition" of television is also clarified: the definition of television refers to the black and white straight lines that television has displayed When the screen is full in the vertical or horizontal direction, the smallest number of lines that can be recognized by the human eye, or the maximum number of lines that can be recognized. The maximum number of such horizontal lines arranged in the vertical direction is the vertical definition of the television; the maximum number of such vertical lines arranged in the horizontal direction is the horizontal definition of the television.

It can be seen that sharpness is an important concept and physical quantity that was put forward before the number of scanning lines and pixels of a television image was determined, and the resolution concept and physical quantity expressed by "horizontal image system × vertical pixels" are not a thing at all. . The resolution is good for the image signal and for the screen pixels of the display equipment. Both are fixed and the sharpness is variable. Although the resolution of the image signal has an impact on the sharpness of the TV image, the signal resolution is not the sharpness of the image that people see; the pixels of the display device also affect the sharpness of the image, but it is not what people see Image sharpness. The resolution of the image signal is the source, and the sharpness of the final displayed image is the result; in terms of quantity, the sharpness is always smaller than the resolution. The image resolution of the same resolution through different transmission channels and different display devices is different. Therefore, there is no direct conversion relationship between resolution and sharpness. If there is a conversion relationship, it can only be "self-to-self" conversion, and cannot be converted between source and end, source and middle, and middle and end. We will talk about this issue later.

Concept introduction

It is known from the introduction above that connecting all the horizontal scanning points (horizontal pixels) on the same vertical line from the vertical direction can form many vertical lines; connecting all the scanning points (horizontal pixels) of each horizontal scanning line ) Connected from the horizontal direction, you can form many horizontal lines. So, if there are already 458 horizontal scanning lines and 610 vertical lines filling the screen, can these lines reproduce the 458 lines of vertical resolution and 610 lines of horizontal resolution calculated above? The answer is no, because it involves the "aperture effect" and the validity of the scan line.

Diaphragm effect

The so-called aperture effect refers to the phenomenon that when the size of the scanning electron beam spot is as small as the physical spot or image detail spot spot, the image details of the corresponding size will be blurred, that is, the image clarity is affected by the electron beam aperture. (Diameter) size limitation. This phenomenon exists in both the imaging process and the development process. Taking the camera as an example, this is specifically: Because the electrical signal of the scanning electron beam spot is the average value of the area of the captured physical detail spot, when the scanning electrical signal spot and the physical detail are in size At about the same time, if the scanning electron beam is scanned over two black and white adjacent physical details, the electron beam is scanned on the physical details light spot, then the abrupt boundary of the black and white light points can be reproduced, and the high Horizontal sharpness; on the contrary, if the scanning electron beam sweeps not the two black and white adjacent light spots themselves, but its edges, then the boundary of the black and white abrupt changes of the light spots cannot be reproduced, but one The boundary of the light and dark transition, so that the image details become blurred, and the horizontal sharpness of the image is reduced by a degree of 50%. Statistically speaking, there is a 50% probability that it will sweep on the spot and just on the boundary of the spot. In this way, for the entire image, 50% of the area maintains the original sharpness; 50% of the area has only half the original sharpness, that is, 25% of the total sharpness of the original image is retained. Therefore, the final 50% and 25% of the sharpness of the entire image are added up to obtain a resolution equivalent to 75% of the original.

Scanning lines in the vertical direction are similar to the aperture effect. When the width of the scanning line is equal to the width of the line, if it is scanned on the black and white lines, the original vertical sharpness can be maintained; if the scanning line is scanned between the black and white lines, the reproduced black and white lines are gray. The boundary of the black and white lines becomes blurred and the sharpness is reduced, so that the efficiency of this scanning line is reduced to 50%, and the vertical sharpness is also reduced to 50% accordingly. According to the statistics, the chance of scanning on black and white lines and between black and white lines each has a 50% probability, so the vertical sharpness of the last displayed image can also be obtained only the vertical sharpness of the scanning signal. 75% of the results.

Scanning situation

It should be noted that what we call "just swept on the lines" or "just swept between the lines" is a simplified statement. There are actually countless intermediate states, but the sharpness decreases in the intermediate state. The proportion is different, not as large as 50%, but the overall effect of various states on the sharpness is the same as the effect of simplifying it to "just scan on the line" or "just scan between the lines", that is, the end of the entire image It is only equivalent to 75% of the original definition, and the same is true for the aperture effect.

Therefore, in order to ensure the restoration of 100% sharpness, the vertical and horizontal scanning lines should be increased, that is, the vertical sharpness 458 line calculated above needs to be multiplied by a coefficient K. This coefficient K is called the effective coefficient. The coefficient is generally 1.3 to 1.4. After multiplying by the effective coefficient, the number of scanning lines obtained must be greater than 458 lines.

Scan calculation

If we do not adopt the method of multiplying the effective coefficient, we will calculate it based on the total effective rate of 75% calculated above, and we can also infer the number of lines m of the scan line that should be restored to 100% sharpness. :

m = 458 ÷ 0.75 = 611 (line)

Similarly, according to the 4: 3 ratio of the TV screen, the number of vertical scan lines n can also be easily calculated:

n = 611 × 4/3 = 815 (line)

It can be seen that in order to achieve ordinary people under normal viewing conditions to obtain 458 lines of vertical sharpness and 610 lines of horizontal sharpness images, in principle, 611 horizontal scanning lines and 815 vertical scanning lines are required. Because the vertical scanning line is not directly scanned from the vertical direction, but is a line formed by the pixels on the horizontal scanning line arranged in the vertical direction, so people may also call it a vertical scanning line or a horizontal scanning line. pixel.

The horizontal scanning lines mentioned above are the effective scanning lines that make up the image. If a reverse scanning line is added, the number of horizontal scanning lines will be larger. Due to the limitations of video bandwidth and other technical conditions in the formulation of the television system, the scan format of the PAL system was finally determined as:

Horizontal pixels × horizontal scanning lines in the vertical direction = 720 × 625

Among the 625 scanning lines, about 50 lines of backhaul lines are included, and the actual effective scanning lines are about 575 to 576.

Because when the TV system was formulated, the video bandwidth of the PAL TV was finally set to 6MHz. This bandwidth was not even 720 × 625. In actual use, the image format of the PAL TV had to be based on 720 × 625. The compressed and compressed pixels are horizontal pixels, and 625 horizontal scanning lines are retained. Therefore, the resolution of the PAL TV is reduced from 815 × 611 to 720 × 625, and then compressed to meet the 6MHz video bandwidth. The PAL TV cannot reach the vertical definition of 458 lines and the horizontal definition of 610 lines. It can only reach the vertical definition of 431 lines and the horizontal definition of 468 lines. These issues will be introduced in subsequent articles ^[3] .