Friday, April 27, 2012

How Television screen works?

The Basic idea of "video" is a special property of the human eyes, the Persistence of vision.
Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina.

Hence, the Cinema we are watching in the Theater is created by moving 24 still images successively before a projector in one second.

However, they found that it is creating some flickers in the video (ie, jump from one still image to another).
They resolved the issue by duplicating the images; ie, 48 images per second. Thus we have a smooth video motion.

(Image: This animated cartoon of a galloping horse is displayed at 12 drawings per second )

Thus, we have to create 48 images per second on the TV screen.
However, in PAL system, we are creating 50 frames per second. This is to make the system compatible with the electric power system india. (230V/50Hz)
This is done by moving an electron beam very fast over the TV screen.
When the beam hits different phosphor spots on the TV screen, it is illuminated.
The phosphor spots are arranged in the form of a matrix.
There are 625 phosphor spot lines arranged horizontally on the PAL Television System(India).
Thus, the electron beam should move from the first line to the 625th line within 1/48th of a second.

In Black and White TV, the TV signal contains only the brightness information at a particular spot.
However, in the colour TV, the TV signal contains both the colour and brightness information.

The TV screen has the RGB (Red, Green, Blue) spots arranged horizontally.
The combination of the three colours at various intensities can reproduce any colour on the TV screen.

(Image: How 3 Colour Electron Gun  Works)

In order to create 50 frame per second, the electron beam should move in a very high speed and it will make the circuitry very complex.
Hence we can use interlaced scanning method, ie scanning alternate lines in one frame.
Successive scan TVs are also available in the market, which will provide a better image quality.

(Image: Interlaced Scanning in the TV)

Thus the TV signal received in the TV contains a luminescence signal and a chrominescence signal.

Different Television broadcasting system differ on the basis of the number of lines, number of frames per second etc.
The TV System in US is NTSC and it is SECAM in Europe.

The modern High defenition LCD TVs differ slightly from the above explained operation.
It usually contains 1080 lines, successive scanning and 16:9 Aspect ratio.

the NTSC system was developed as the first color TV standard. This basic color system works well, and after 40 years is still in use in North America and Japan. NTSC stands for: National Television System Committee, which was the organization that defined this color TV standard. NTSC is based on 525 picture lines and 60 Hz (60 scans across the screen in one second). The major problem with NTSC is that hue errors might occur. In order to correct this, all NTSC receivers are equipped with a special hue control.

PAL stands for: Phase Alternating Line and was introduced 15 years after NTSC. The system is based on 625 lines and 50 Hz. It has more picture detail than NTSC, because it uses 100 more lines and is able to write colors with greater accuracy. If there is a color distortion on one line, it will correct this on the next line by reversing the error. If, for instance, the intended green shifts to a yellow, on the next line it will correct this with a reverse error. The result is that the next line will be cyan (blue-green), which results in an optical green; the yellow and cyan will blend to green.
The major disadvantage of the PAL system compared with NTSC, is that it makes use of 50 scans per second rather than 60. This results in flicker and a more unstable picture. However, the problem of flicker is solved when the raster frequency is adjusted to 100 Hz (100 scans per second).

SECAM or Sequence Couleur à Mémoire, (which translates as color sequence in memory) was developed around the same time as PAL. Whereas with both NTSC and PAL color errors still occur, with SECAM there are no color errors
The systems are not compatible because of the difference in the number of lines and the raster frequency, and the way they handle color reproduction. For those who want to watch movies recorded in a different standard, TVs and VCRs have been developed which are able to handle two or three systems. Conversion of TV, satellite or cassette programs to other systems results in lower quality.

Note: TVs, DVDs, VCRs etc purchased in different countries may not work in another country due to the various reasons discussed above.

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