What Is a Sweep Account?

The scanning methods of the display are divided into "progressive scanning" and "interlaced scanning".

Interlaced scanning is that each frame is divided into two fields, and each field contains all the odd or even scanning lines in a frame. Usually, the odd lines are scanned first to obtain the first field, and then the even lines are scanned to obtain the second field.

Introduction to scanning methods

Whether it is progressive scanning or interlaced scanning, there are three concepts of video file, transmission, and development. These three concepts are the same but not the same. The earliest appeared was interlaced scanning imaging, and interlaced transmission was produced at the same time. Interlaced video files only appeared in the digital video era. The purpose is to be compatible with the original interlaced scanning system (interlaced scanning is still widely used). application).

Generally, the display is divided into two scanning modes: progressive scanning and progressive scanning. Compared to interlaced scanning, progressive scanning is an advanced scanning method. It means that when the image displayed on the display screen is scanned, the scanning starts from the first line in the upper left corner of the screen, and the entire image scanning is completed at one time. Therefore, the image display screen flickers small, and the display effect is good. Most advanced displays use progressive scanning.

Due to the visual persistence effect, the human eye will see smooth motion instead of flickering field-by-field images. However, this method caused a large time interval between the two images, which caused the image screen to flicker. Therefore, this scanning method is relatively backward and is usually used in early display products.

Let's start with interlaced display.

As we all know, as long as the frame rate reaches 24fps to achieve smooth, the movie is executed according to this standard. However, considering the AC frequency of 50 or 60 Hz, television standard setters have set a frame rate of 25 or 30 fps. What if the TV uses progressive scan and scans 25 or 30 frames per second? Yes in theory, but not in practice. The reason is that under the irradiation of the electron beam, the phosphor of the CRT will immediately emit light, but as soon as the electron beam leaves, it will dim almost instantly. Although the human eye can't see such a noticeable change, it still feels that the brightness is attenuated. It can be imagined that if the 30p scanning mode is used to display, when the electron beam moves from the upper half to the lower half of the screen, the brightness of the upper half of the screen will have an observable attenuation, so the lower half of the screen will appear brighter. This is just a moment, in fact the brightest part (of course, the area just passed by the electron beam) will constantly move, resulting in flicker. You have also seen this phenomenon, that is, the feeling of seeing a CRT computer monitor on a TV.

In order to solve this problem, the best solution is to double the refresh rate to 60p scanning and scan each frame twice, because within one-sixth of a second, the brightness attenuation that the human eye can feel is very small. However, if you do this, the total number of lines scanned per unit time will double, and the speed of horizontal scanning will increase. In this way, the technical requirements will be too high, and cannot be done under the conditions at that time. So the standard-setters came up with a compromise, first scanning the odd lines (upfield) for one-sixth of a second, and then scanning the even lines (downfield) for the next 60th of a second. The two complement each other to form a complete picture. Although the brightness of the upper field is attenuated when scanning the lower field, it is not easy to detect because the bright and dark parts are intertwined.

(Note: It has been realized since then. A few years ago, the refresh rate of CRT monitors generally reached 85Hz, and some even reached 120Hz. Some TVs can also display progressive display.)

Developed to fixed pixel devices (such as LCD, plasma, etc.), because all the points are illuminated at the same time, and the brightness is always fixed, there are no related problems encountered by CRT, so some flat-screen TVs can support 24p display, while CRT was Yes.

Scan mode scan difference

Each frame of image is scanned sequentially by the electron beam one line after another. This scanning method is called progressive scanning. After each frame of image is scanned through two fields, interlaced scanning is performed. In the two fields, the first field (odd field) scans only the odd lines, and then scans 1, 3, 5 ..., and the second field (even field). Scan only the even rows, scan 2, 4, 6 ... rows in that order. Interlaced scanning technology plays a big role in the case of insufficient transmission signal bandwidth. The main difference between the display effect of progressive scanning and interlaced scanning is the stability. The interline flicker is more obvious. Progressive scanning overcomes the problem of interlaced scanning. Disadvantage, the picture is smooth and natural without flicker. In the standard display mode of a television, i means interlaced scanning and p means progressive scanning.

Scan mode transmission concept

Analog TV relies on a continuously changing current to transmit signals. As long as the signal is received, the CRT will start scanning with the signal current. Scanning the field first, the signal line must transmit the field signal first, and the transmission of the field signal will not start until the field transmission is completed. 60 fields are transmitted per second, the so-called 60i transmission mode. Later, there was a progressive scan imaging, which followed the progressive transmission. For example, a 60p display requires 60 full frames per second, which translates to 120 fields per second. Therefore, 60p transmission requires twice the bandwidth required by 60i, which requires a higher interface. Only the color difference (specifically YPbPr; YCbCr is interlaced signal) and VGA can be realized in the analog interface.

If you have a 30p source, you should use 1080p. So is 1080i definitely not working? That really isn't. In theory, as long as your TV is smart enough, for 30p film source, using 60i transmission can also fully reach the level of 60p, there is no difference. The theoretical basis is: transmitting 60 fields per second, which is equivalent to transmitting 30 frames per second, and transmitting 60 fields per second is no loss compared to transmitting 30 frames per second.

Flat-screen TVs are fixed-pixel devices, and all dots glow at the same time. Unlike the real-time scanning display of a CRT when a signal is received, a flat-screen TV must receive all the signals in that frame or field before it can be displayed. Under normal circumstances, the flat-screen TV will be delayed by 1/60 with respect to the time of signal transmission. We can imagine that since we can wait for all the field signals to be transmitted and display the odd lines immediately, why ca nt we wait and wait until the next field signals are all transmitted and then display the full frame together? Isn't it possible to achieve progressive scan quality? In fact, this is perfectly fine.

Scan mode video file

The camera does not actually scan from the beginning, because both the film and the electronic original receive light at the same time. However, to read the information on the photoreceptor (its core task is to sample the picture as YUV / RGB electrical signals), it still depends on scanning, so the order of reading each pixel must be considered. It can be read in order (row-by-row), or each row can be read in order, but all odd rows are read first and then even rows (interlaced). In order to be compatible with the TV's interlaced scanning system, the tape conveyor (a machine that samples pictures on film as YUV / RGB electrical signals and saves them on magnetic tape) and some cameras are interlaced. Of course, there are products of the progressive system.

Digital cameras became practical when people learned to use electronic originals for photosensitivity and digitized computer files to save video. In fact, the digital information is still stored in the YUV / RGB electrical signals, but the media is changed, which is essentially the same (here refers to uncompressed video, which is quite different after compression).

However, in the computer, regardless of whether it receives a progressive or interlaced signal, it will be written to the file one by one in sequence, without special distinction. As a result, there is definitely a difference between the video on the computer and the file that uses progressive and interlaced signals as the source. They are called progressive and interlaced video, respectively.

Scan mode de -interlacing

Where there is interlacing, there is anti-interlacing, which is mentioned here. Not all interlaced videos need to be deinterlaced, only those that have interlacing are required. So most PAL DVD's Rip work can theoretically skip the de-interlacing step directly (actually still need to correct unexpected interlaced frames).

Deinterlacing is mainly divided into Field Matching Inverse Interlacing (IVTC) and Deinterlace. Field matching is de-interlaced. If it is field-first, first specify the field of this frame. By comparing the coupling with the next field of the previous frame, this frame, and the next frame, try to find and combine the upper and lower fields that originally belong to the same frame. Regrouped together. Deinterlace is based on a single frame, and it is processed to the extent that you cannot see the interlacing. It is not a restoration, and it is still different from the original picture. For 3: 2pulldown videos, IVTC is generally adopted, and the specific process is as follows

H1 = tG1 + bG1 = tF1 + bF1

H2 = tG2 + bG1 = tF1 + bF1

H3 = tG3 + bG2 = tF2 + bF2

H4 = tG4 + bG3 = tF3 + bF3

H5 = tG5 + bG5 = tF4 + bF4 In this way, H1 and H2 are repeated, and one of them can be deleted by using the frame deletion procedure. This restores 3: 2pulldown to a 24fps progressive scan video that can be played on a computer. (25i / 30i videos are usually processed using Deinterlace.) ^[2]