The hottest 3D comb video decoder meets the challe

2022-07-29
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3D comb video decoder meets the challenge of standard definition video optimization

with the popularity of large screen HD LCD TV and plasma display, digital TV broadcasting and HD signal interface are becoming more and more popular

however, it is still necessary to support traditional signal sources with the highest quality. 3D comb video decoder is a key processing module, which has a decisive impact on the overall performance of the system

at the earliest development stage, TV only supports the playback and display of monochrome (i.e. black and white) images. With the development of technology, color TV broadcasting is also supported, but the backward compatibility of black-and-white TV display devices still needs to be maintained

TVs need to accommodate color information within the available bandwidth and continue to display undistorted black-and-white pictures in the same format as early TVs

in the composite video signal, the color information and the available brightness information share the same bandwidth. Sine waves of different amplitudes and phases represent the chrominance content of any transmitted image (Fig. 1). Therefore, the chromaticity and brightness must be separated to correctly display the picture

Figure 1: luminance information and chrominance information share the same spectrum of the composite video signal

chrominance information is placed at the high end of the spectrum, which is a multiple of the line length. The difficulty in display lies in how to correctly extract luminance and chrominance information, and how to maintain full bandwidth without causing display artifacts

if there is no luminance chrominance separation, the color information will make the picture brighter or darker when the carrier cycles forward or backward. At the same time, the color information will also appear incorrectly in the black-and-white part of the image (Figure 2)

figure 2 Images without luminance and chrominance separation contain a large number of artifacts

using a simple notch filter or band-pass filter to separate luminance and chrominance will cause chrominance residue in the luminance signal path and luminance residue in the chrominance signal path (Fig. 3). Residual information can cause serious image artifacts, such as "point creep" (Figure 4). The residual chrominance information in the chrominance path may also cause artifacts such as cross color (Fig. 5)

figure 3 Using notch filter or band-pass filter to separate brightness and chroma

figure 4 The TV mistakenly interprets the residual color in the brightness path as brightness information, resulting in poor point creep effect

figure 5 The residual luminance signal in the chrominance path causes cross color artifacts

the signal delay function of the comb filter itself causes phase length interference and phase cancellation interference. The frequency response of the comb filter is composed of a series of uniform spikes in the shape of a comb. Compared with notch filter and band-pass filter, 2D comb filter can provide higher video decoder performance (Fig. 6)

figure 6 The three line 2D comb filter will adopt the working principle of input-output one line delay

2d comb filter: if there are similar lines above and below the image target line, the chromaticity and brightness can be more completely separated

ntsc (National Television System Committee) system, the chromaticity sine wave signal changes 180 ° line by line. Any two consecutive lines are added, the luminance content is doubled, and the chrominance content is offset

on the contrary, if two lines are subtracted, the luminance content is offset and the chrominance content is doubled. For example, for a full screen color bar, each active line looks the same. At a given signal level, the luminance content of each line is the same as that of samples with different thicknesses. In addition to the phase change, the chrominance content of each line is the same

video decoder (such as ADI's decoder) adopts five line 2D comb filter, which can provide better performance for NTSC and PAL signal sources. Depending on the complexity of the image, the comb processor must determine whether to combine the current line with the next or previous line

the comb processor cannot perform any line combination on some images. At this time, it can cut into the current line. The adaptive 2D comb video decoder can provide an acceptable performance level. However, when the continuous lines are different, the 2D comb filter does not work properly, and turns to the notch filter to separate the brightness and chromaticity of the line area

although it is very important to successfully realize the separation of brightness and chroma without image artifacts or bandwidth constraints (which will be converted into low contrast images), there are two identification methods for many coaxiality errors of video signals: extended measurement method and geometric measurement method. Other aspects, such as poor time base or non-standard weak RF signals, will also bring many challenges

acceptable artifacts or image defects of small CRT displays are unacceptable for new generation plasma displays and liquid crystal displays. Because with the increase of resolution, size and display contrast, even other organic matter and moisture are small image defects will be obvious

adaptive 3D comb filter technology

high definition (hd) signal source, digital interface and high-resolution display can bring excellent visual experience. However, through channel switching or input, users may see beautiful HD images or traditional composite video broadcasting (CVBS)

with the help of high-quality adaptive 3D comb filter technology, the quality of standard definition (SD) composite video images has been significantly improved (Fig. 7)

figure 7 The typical architecture of the decoder with a built-in 3D comb filter complements the space-time function of the previous generation of 2D comb filter decoder, and adds timing recognition.

3d comb filter is similar to 2D comb filter, which separates brightness and chroma through the pixel combination of some rows. The main difference between the two is that the 2D comb filter combines the pixels of the continuous line of the image, while the 3D comb filter combines the pixels of the current line with the pixels in the same line in the image delay state (Fig. 8)

Fig. 8 shows that the result (a) of the decoder with adaptive built-in 3D comb filter is significantly better than that of the decoder with built-in 2D comb filter (b)

3d comb video decoding scheme can provide outstanding video quality. This method can effectively eliminate bad image artifacts, such as point creep, "hanging point" and cross color

in addition, due to the luminance and chrominance separation method of 3D comb video decoding, this method can maintain the full bandwidth of luminance and chrominance packets

all brightness bandwidth maintains high-frequency content, provides clear and distinct images, and users can distinguish small details. Full chromaticity bandwidth ensures brighter and clearer colors

2d comb video decoding mainly processes adjacent active video lines, or performs analysis, or both, while 3D comb processing performs frame to frame video pixel information comparison (Fig. 9). It compares the data of the current frame with the data of the previous frame in the memory

figure 9 The typical frame sequence of NTSC system shows the 3D comb filtering technology

if two frames are added at the same time, the chrominance information of each pixel is offset, and the luminance pixel data is doubled. Similarly, if the current frame is subtracted from the previous frame, the luminance pixel data is offset and the chrominance information is doubled

although 3D comb filtering has many advantages, designers still have to solve its performance limitations and some challenges. The 3D comb filter can perfectly separate the brightness and chroma of the image. On the contrary, the traditional 2D comb filter or notch filter can not achieve this effect

however, the perfect separation of luminance and chrominance can be achieved only when the pixels in the image are absolutely static. On the contrary, if the image is moving and the pixel data of two consecutive frames is also changing, the 3D comb filter cannot be used (FIG. 10). It is important that the video decoder checks each pixel and compares it with the previously stored pixel data to determine whether movement has occurred and which comb filter should be used

figure 10: comb filtering of moving images will produce obvious artifacts

due to the complexity of motion detection, the method used must be able to analyze each active pixel of the current and stored frames, so as to determine which method to use to separate information

3d comb filter technology combs static pixels, 2D comb filter technology processes no complex moving regions, while notch filter processes complex moving regions. The main challenge of 3D comb decoder is not the process itself, but the complex motion detection and adaptive switching among 3D comb filter, 2D comb filter and notch filter

if the comb filter is not competent,

the adaptive 3D comb filter relies on the decoder to correctly detect image movement. Otherwise, the comb filter cannot correctly process the pixel data, resulting in motion artifacts (FIG. 11). In Figure 11a, the bird's wings are downward. In Figure 11B, the wings have been dancing upward, while in Figure 11C, the wings are down again. This is the normal order in which a bird dances its wings

figure 11 The adaptive 3D comb filter relies on the decoder to correctly detect image movement. This is the normal order in which a bird dances its wings - down (a), up (b), and down (c)

many 3D comb decoders are used to check frame 1 and frame 3. It is found that frame 1 and frame 3 are the same, so they mistakenly believe that there is no image movement. Therefore, it is decided to use the 3D comb decoder to process the data (Fig. 12)

figure 12 Using the 3D comb decoder, the invalid detection causes obvious lattice artifacts (a). After the image movement correction, the lattice free artifact is generated (b)

the high-performance video decoder with 3D comb filter uses many frame memories to more accurately detect the motion between all frames. It is necessary to use a large number of frames, which is helpful for the decoder to accurately determine the time and position of applying the 3D comb filter

function maximization

3d comb filter should work normally. The memory buffer needs to store video pixel data frames for analysis and processing. ADI's 12 bit SD/HD TV video decoder, such as adv7802, is equipped with a 3D comb filter and a graphics digital converter. It maximizes memory space by handling other non 3D comb filter tasks, such as advanced timing noise reduction

due to the 3D comb filter, adv7802 uses a variety of technologies to compare the pixel data of the current frame with the previously stored data, so as to filter and eliminate image noise

external memory can also realize advanced time base correction. Frame based time base correction ensures that the decoder always outputs a fixed clock, a fixed number of samples per line, a constant number of lines per frame, and a correct field sequence

although TV applications usually do not require external memory, more and more manufacturers are transferring more receivers and electronic control equipment to a separate remote control box to minimize the thickness of the display panel. However, such designs also limit the number of cables directly connected to the TV, which may be difficult to route (Figure 13)

figure 13 The thin display panel transfers the receiver and electronic control equipment to a separate unit

the remote control box is connected to the display through a high-definition multimedia interface (HDMI) or similar link. When such components work, the TV needs stable pixel and clock data. Since time base correction allows the video decoder to be directly connected with the transmitter of the link, the decoder can even provide reliable timing data and pixel data for non-standard input devices

in addition to the separation of brightness and chroma, many other aspects of composite video processing also directly affect the picture quality. The performance of ADC input plays a decisive role in the overall picture quality received by the display

professional quality video decoder, such as adv7802, uses 12 bit ADC, and the signal-to-noise ratio (signal-to-noise ratio) is better than 62 dB. Noteworthy

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