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3.Key Techniques Of HDTV
There are a lot of key technologies involved in the implement of the HDTV systems; some of them are listed below:
MPEG-2
ATSC employs MPEG-2, a data compression standard.MPEG-2 typically achieves a 50-to-1 reduction in data.It achieves this by not retransmitting areas of the screen that have not changed since the previous frame.
Interlacing Scanning
The term interlacing refers to the practice of drawing all of the odd numbered lines on the CRT, and then drawing all of the even numbered lines, which are drawn interspersed with the odd numbered lines.For 1080i, the 540 odd numbered lines are one field, and the 540 even numbered lines are the other field.When interlacing is employed, there are always two fields per frame.
Progressive Scanning
These systems light all the horizontal lines in the same instant, which can make the image seem “smoother” and more like film (or real life).
Motion Compensated Processing
In this process a computer in the receiver turns a 24 or 30 frames/s image into a true 60 or 120 frames/s image. This motion vector is used for creating the missing frames. (How called compensated processing). But it requires the networks and studios to make maximum use of motion vectors, which may or may not be the case.
The 3:2 Pull-down Issue
Theatre film is usually 24 frames/s while TV usually operates at 30 or 60 frames/s. 3:2 pull-down, also called telecining, is the process of converting a 24 frames per second image into a 60 frames or fields per second image.
120 Hertz technology
The best monitors show the image at the frequency of 120Hz. 120 is an exact multiple of 24, 30 and 60. Thus in theory the HDTV monitor can show any programme without introducing judder.
This article put the emphasis on the technologies used in HDTV broadcasting. Three different technologies used in HDTV signal modulation will be discussed in details as follows:
The main difference between analog and digital transmitter is that, the output of the former is a carrier wave which has been modulated by analog signal while the latter a series of discrete values (this process is so-called channel coding). Terrestrial Broadcasting allows lager power than satellite broadcasting, which makes it possible to send a higher-power signal to the receiver. Because of these reasons, we can use multilevel signal which send the signal in the form of a series of dc levels.
Figure 1.a illustrates the simplest system. There are only 2 possible statements which were denoted by one bit each for the power of the transmitter. The system in Figure 1.b has 4 possible values for the power of the transmitter and 1 symbol denotes 2 bits so there are 4 possible combinations. For this reason the bandwidth of this system is half that of the system in Figure 1.a. Figure 1.c illustrates a system of ATSC US which uses 8 levels for the power of the transmitter. The bandwidth of this system is 1/3 of that of the system in Figure 1.a.
Click on the following links to see more details:
3.1 Vestigial Sideband Modulation (VSB)
3.2 Quadrature Amplitude Modulation (QAM)
3.3 Coded Orthogonal Frequency Division Multiplexing (COFDM)
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