Streaming media 4--Digital TV Fundamentals

1. Analog Color TV System

2. TV Scan and Sync

Television technology uses the principle of photoelectric conversion to realize the transformation of optical image to TV signal , which is usually done in the camera. When the subject is imaged by the camera lens in the optical conductivity layer of the camera tube, the different points of the photoelectric target will excite the number of photoelectron with different illumination, which causes different potential fluctuation of the additional light conductance, and the electric image corresponding to the photo is formed. Using the visual inertia of the human eye, the physical quantity of the pixels in the representation image can be transmitted one by one in a certain order at the transmitting end, and the original image is re-marked on the receiving side by the same law. As long as this sequence is fast enough, the human eye will feel the image shining at the same time. In television technology, the established law of this transmission image is called scanning. The electric images formed in the optical conductivity layer of the camera tube are sequentially connected to each point under the scanning of electron beam, and their brightness changes are continuously converted into electrical signals; the scanned signal is transmitted through a single channel, and then an electron beam is used to scan a fluorescent screen with electro-optic conversion characteristics, which is converted from an electrical signal into a light image.

The principle of TV system scanning

Under normal circumstances, the current television system is widely used in electronic vacuum camera and imaging devices are electron beam scanning for photoelectric and electro-optical conversion, and with the input of CCD camera and flat panel display, the use of a variety of pulse digital circuit can achieve the above conversion. Cathode ray Tube (CRT) scanning processing, wherein the video signal is composed of luminance (Luma) and chroma (chroma) signal components, the component video sends the luminance and chroma signal respectively.

Cathode ray Tube (CRT) Scan processing

Scanning method has two kinds of progressive scan and interlaced scanning .

<1>progressive scan

The way the electron beam is scanned sequentially from left to right, from top to bottom, is called progressive scanning. The line-shaped highlight trajectory formed by the electron beam sequential scan screen is called a grating .

(1) line scan : The scanning of the electron beam along the horizontal direction is called the line scan. The scan from left to right is referred to as the line scan positive path, the short line is being swept, and (a) the solid line in the figure is shown. A right-to-left scan is called a row-scan inverse, or a line-sweep. (a) shown in the dotted line. The scanning process is long and the inverse time is short. Obviously, for each image, the more the number of scans, the higher the decomposition power of the image, the finer the image, but the wider the bandwidth of the TV signal, the higher the demand for the channel.

(2) Frame Scan: The electron beam is scanned in a vertical direction called a frame scan. The scan from top to bottom is called frame scan positive path, short frame scanning, and bottom-up scan is called frame scan inverse, short frame back sweep. Figure (a) shows the scanning trajectory of the frame scan, and (b) is the sweep trajectory of the frame scan inverse. Similarly, the frame scan forward time is much larger than the frame scan inverse time.

In fact, both the line scan and the frame scan are carried out simultaneously, that is, when the electron beam is scanned horizontally and in the vertical direction , the motion trajectory of the electron beam is a horizontal and vertical movement of two directions. As the horizontal direction of the electron beam scanning speed is much larger than the vertical direction of the speed, so that on the screen formed a slightly oblique downward horizontal light line, hundreds of lines of dense scanning light line to form a uniform gate-like luminous surface, is called grating . A row-by-line scan is a single frame. The disadvantage of progressive scanning: to make the image continuous without flicker, you need to change the frame 50 times per second, that is, the frame rate of 50Hz, but the bandwidth of the image signal is too wide, so that the TV equipment complications. In order to compress the bandwidth of the image signal, and to overcome the flicker phenomenon, it is suggested that the interlaced scanning method can be used for reference of film technology. Current radio and television use interlaced scanning .

Progressive Sweep Grating

<2>. Interlaced scanning

Interlaced scanning is a frame of the image is divided into two fields to scan, the first sweep odd lines, called odd number field, the second sweep even the number of lines called even field. Odd field and even-number field images are inlaid together to form a complete picture

Interlaced image reproduction

Interlaced grating, the electron beam sweeps through the 1th line and goes back to the beginning of line 3rd, (A is shown, and then on the 5th, 7 、......, the line sweeps until the last row. The odd line sweeps through the even rows (Figure B), thus completing a frame scan (Figure C). As a result, an interlaced image is made up of two parts: one is a caused by parity number line, the other is an odd number of fields, and the other part is made up of even lines, called an even number field, and two fields are combined to form a frame. So in an interlaced scan, either the camera or the display, it takes two times to get or display an image to get a complete picture.

Interlaced grating

In an interlaced scan, the number of rows scanned must be odd. As mentioned above, a frame screen is divided into two fields, the first scan is half of the total number of rows, and the second one scans the other half of total rows. Interlaced scanning requires that the first field end in half of the last line, no matter how the electron beam is retraced, it must return to the center of the top of the display so that the next second scan is embedded in the middle of the first scan line. It is for this reason that the total number of rows must be odd. Interlaced scanning for the transmission of the active image has a continuous sense without flicker, the need to scan 50 games per second, that is, the field frequency of 50Hz. and two field for a frame, then scan 25 frames per second, that is, frame rate of 25Hz, thereby reducing the frame rate, compressing the image signal frequency bandwidth, and overcome the flicker phenomenon.

　　The number of rows scanned per second is called the line FH, and the number of fields scanned per second is called the field frequency FC, and the number of frames per second is referred to as frame frequency ff. FC and FF are two different concepts.

black and white and color TVs are interlaced, while computers typically use non-interlaced scanning when displaying images. 　　

3. Color TV Signal Type

4. Digital TV Image

5. Image sub-sampling

6. Digital TV

7. Digital TV image format

Streaming media 4--Digital TV Fundamentals