Television is a system that converts the succession of visual images into corresponding electrical signals and transmits those signals through a suitable communication channel to distant receivers where these signals are used to reproduce the original images. At first, the television is large in size and is constructed from various mechanical, electrical, and electronic devices. Nowadays these televisions are available in different portable and large sizes and completely make use of electronic components and hence they are occupying less space.
In Greek works "TELE' means 'far' i.e. at distance and 'VISION came from the Latin word which has the meaning 'to see'. Thus the term television means "To see at a distance".
The first demonstration of actual TV was given by J.L. Baird in London and C.F. Jenkins in the USA in 1927.
There are three monochrome television system standards were developed by scientists they are
1. The 525-line American system
2. The 625-line European system and
3. The 819-line French system
In India 625 lines CCIR - B (consultative committee for Intermediate) system is followed:
There are three types of color systems namely:
1. NTSC (National Television System Committee)
2. PAL (Phase Alternation by Line)
3. SECAM (Sequential Colerus A Memory).
FORMATION OF A PICTURE
The picture is fundamentally an assembly of a very large number of small dark and light areas representing the picture elements. So these picture elements are divided into a number of small element areas of black and white. Each small area of light or shade is a picture element. It is also called a pixel or a panel. All these elements together consist of the visual information in the scene. If these elements are transmitted and reproduced in the same degree of light or shade and in the proper position then the picture is produced.
SCANNING PROCESS
Scanning is the process of exploring (careful search or examination) the picture elements in a scene in a sequential manner. This technique is similar to reading a book. When we read a book, we start at the left top corner, read letter by letter, word by word towards the right side complete the line, and then go to the next line thus eye moves from left to right and after completing the line go to the next line. Thus horizontal and vertical motion is made.
In a similar manner scanning will be done in a camera tube and a picture tube. The scanning is done line by line horizontally from left to right at a fast rate and vertically from top to bottom at a slower rate.
1. The scanning process starts at the top left of the raster and then continues in the horizontal line.
2. At the end of each horizontal line the beam to go left at a faster rate. This return is called flyback or retrace.
3. As the beam retraces horizontally, it is stepped down vertically by one line to start the scanning of the second line. This process is known as vertical scanning. Vertical scanning is necessary to avoid the scanning of the same line repeatedly.
CHARACTERISTICS OF HUMAN EYE WITH REGARD TO THE PERCEPTION OF COLOURS
According to 'Helmrotz's theory' light-sensitive organs are two types. They are:
(i) Rods
(ii) Cones rods
The rods provide a bright sensation. That means they perceive objects in various shades of gray from black to white cones. The cones are sensitive to the color content of the picture. There are three types of cones, each responding to red, green, and blue (primary colors red, green, and blue).
The eye is most sensitive to green light. The relative sensation of brightness by individual spectral colors radiated at constant levels
ADDITIVE AND SUBTRACTIVE MIXING OF COLOURS
Colors are mixed in two ways:
1. Subtractive mixing
2. Additive mixing
Subtractive Mixing: When light falls on the surface, only colors of certain wavelength gets reflected and other gets absorbed. This depends on pigments contained in the surface. In this mixing process, the required color removal is obtained by subtracting individual primary colors from the white color In this reflecting properties of pigments are used, which absorb all wavelengths except their characteristic color wavelength. Le. to provide a color that reflects the particular color while absorbing all other colors.
For example, an apple absorbs blue and green and reflects only the wavelength of red.
Red White-blue-green;
Green White-blue-red;
Blue White-green-red
This type of mixing can be used in painting and color printing.
Additive Mixing: When colors are produced by adding two or more primary colors. This process is called additive mixing. This mixing process is used in color TV.
Red + Green Yellow; Red + Blue = Magneta;
Green + Blue = Cyan; Red + Green + Blue = White
DIGITAL CAMERA
Three primary colors R, G, and B are generated by using dichroic mirrors and color filters which have not been shown in Fig.5.6 CCD sensors are charge-coupled CMOS sensors used to convert light into its equivalent electrical signals. Each CCD sensor has typically 1024 CCD elements. Neutral density filters allow the user to control selecting shutter speed and aperture combinations in a variety of conditions such as blurring water motion in waterfalls, motion blur to moving objects, etc.
Analog color signals are converted into digital signals satisfactorily by using 8-bit ADC. The digital signal is then numerically processed for gamma correction, black level adjustment, and white level peak. It has some options like shutter speed etc. The chroma signals R-Y, B-Y, and Luma signal Y are obtained by suitably matrixing the RGB signals. The R-Y and B-Y signals are filtered digitally to reduce their bandwidth half so that they can be multiplexed to form a chroma data stream.
The three digital signals are then individually stored in separate field store memories and read alternately to give interlaced signals. Aperture correction is provided to the Y signal using digital delay lines. Signals are finally converted into analog Y and RGB signals by using a digital-to-analog converter.
