Introduction TO TV Signals PDF

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UNIT 1 INTRODUCTION TO TV SIGNALS A television system involves equipment located at the source of production, equipment located in the home of the viewer, and equipment used to convey the television signal from the producer to the viewer. The purpose of all of this equipment, is to extend the human senses of vision and hearing beyond their natural limits of physical distance. The aspects of vision that must be considered include the ability of the human eye to distinguish the brightness, colours, details, sizes, shapes, and positions of objects in a scene before it. Aspects of hearing include the ability of the ear to distinguish the pitch, loudness, and distributionofsounds. Analog (or analogue) television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal, the one in which the information to be transmitted, the brightness and colors of the points in the image and the sound waves of the audio signal are represented by rapid variations of some aspect of the signal; like its amplitude, frequency or phase, the values of which, at a given point in time, is any one of an infinite number of possible values. That means even the smallest change in the original color or sound is reproduced by the TV receiver, which in practice with the present systems is a very tiny positive quality that digital systems lack. However, this capability of analog also means that even the smallest unwanted change in the signal, caused by natural noise in electronics when amplifying the weak signals received, and also by on-air interference, also becomes reproduced by the receiver. So with analog, a moderately weak signal becomes snowy and subject to interference. In contrast, a moderately weak digital signal and a very strong digital signal transmit equal picture quality. All broadcast television systems preceding digital transmission of digital television (DTV) were systems utilizing analog signals. Analog television may be wireless or can can be distributed over a cable network using cable converters. The Sound and Light Spectrum Video is a combination of light and sound, both of which are made up of vibrations or frequencies. We are surrounded by various forms of vibrations: visible, tangible, audible, and many other kinds that our senses are unable to perceive. We are in the midst of a wide spectrum which extends from zero to many millions of vibrations per second. The unit we use to measure vibrations per second is Hertz (Hz). Sound vibrations occur in the lower regions of the spectrum, where as light vibrations can be found in the higher frequency areas. The sound spectrum ranges from 20 to 20,000 Hertz (Hz) and sound waves travels at a speed of 340.29 meter per second. Light vibrations range from 370 trillion to 750 trillion Hz. When referring to light, we speak of wavelengths rather than vibrations. As a result of the very high frequencies and the speed at which light travels (300,000 km per second), the wavelength is extremely short, less than one thousandth of a millimeter. The higher the vibration, the shorter the wavelength. Not all light beams have the same wavelength. The spectrum of visible light ranges from wavelength of 780 nm to a wavelength of 380 nm. We perceive the various wavelengths as different colors. The longest

wavelength (which corresponds to the lowest frequency) is seen by us as the color RED followed by the known colors of the rainbow: orange, yellow, green, blue, indigo, and VIOLET which is the shortest wavelength (and highest frequency)remember VIBGYOR ???. White is not a color but the combination of the other colors. Wavelengths which we are unable to perceive (occurring just below the red and just above the violet area), are the infrared and ultraviolet rays, respectively. Nowadays, infrared is used for such applications as remote control devices. Visible light is only visible because we can see the source and the objects being illuminated. The light beam itself cannot be seen. The beams of headlights in the mist for instance, can only be seen because the small water drops making up the mist reflect the light.

Luminosity or Brightness Besides differing in color (frequency), light can also differ in luminosity, or brightness. A table lamp emits less light than a halogen lamp, but even a halogen source cannot be compared with bright sunlight, as far as luminosity is concerned. Luminosity depends on the amount of available light. It can be measured and recorded in a numeric value. In the past, it was expressed in Hefner Candlepower, but nowadays Lux is used to express the amount of luminosity. Brightness Values: Candle light at 20 cm 10-15 Lux Street light 10-20 Lux Normal living room lighting 100 Lux Halogen lamp 750 Lux Sunlight, 1 hour before sunset 1000 Lux Daylight, clear sky 10,000 Lux Bright sunlight > 20,000 Lux

Luminosity is the basic principle of the black-and-white television. All shades between black and white can be created by adjusting the luminosity to specific values. Color Mixing There are two kinds of color mixing: additive and subtractive color mixing. The mixing of colorants, like paint, is called subtractive mixing. The mixing of colored light is called additive mixing. Color TV is based on the principle of additive color mixing. Primary colors are used to create all the colors that can be found in the color spectrum. Additive Color Mixing In video, the color spectrum contains three primary colors, namely red, green and blue (RGB). By combining these three, all the other colors of the spectrum (including white) can be produced. red + blue = magenta (cylamen) red + green = yellow blue + green = cyan (turquoise) green + magenta = white red + cyan = white blue + yellow = white red + blue + green = white Making colors in this way is based on blending, or adding up colored light, which is why it is called additive color mixing. Combining the three primary colors in specific ratios and known amounts enables us to produce all possible colors.

Have a look at this figures for better Understanding.

By combining the three primary colors red, green and blue, other colors can be mixed, including white.

White light is derived from a ratio of 30% red, 59% green, and 11% blue. This is also the ratio to which a color TV is set for black-and-white broadcasts. Shades of grey can be created by maintaining the ratio percentages and by varying the luminosity to specific values. *** 30% red + 59% green + 11% blue = white Light Refraction Light refraction is the reverse process of color mixing. It shows that white light is a combination of all the colors of the visible light spectrum. To demonstrate refraction a prism is used, which is a piece of glass that is polished in a triangular shape. A light beam travelling through a prism is broken twice in the same direction, causing the light beam to change its original course. Beams with a long wavelength (the red beams) are refracted less strongly than beams with a short wavelength (the violet beams), causing the colors to fan out. The first fan out is enlarged by the second fan out, resulting in a color band coming out, consisting of the spectrum colors red, orange, yellow, green, blue, indigo, and violet. There are no clear boundaries between the various colors, but thousands of transitional areas. A rainbow is a perfect example of the principle of light refraction in nature.

When white light, such as sunlight passes through a prism, it is refracted in the colors of the rainbow. Color Temperature Color temperature relates to the fact that when an object is heated, it will emit a color that is directly related to the temperature of that object. The higher the color temperature, the more 'blue' the light, and the lower the color temperature the more 'red' the light. Color temperature of light can be measured in degrees Kelvin (K). Daylight has a color temperature between 6000 and 7000 K. The color temperature of artificial light is much lower: approximately 3000 K. In reality, color temperatures range from 1900 K (candlelight) up to 25,000 K (clear blue sky). Television is set to 6500 K, simulating 'standard daylight'. Various light sources with different color temperatures.

Color temperature is expressed in degrees Kelvin. Persistence of Vision by Human Eye 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. The eye tends to retain an image for about 80 milliseconds after it has disappeared. This Advantage is taken of this in television and cinematography, where a series of still pictures (25 per second) create the illusion of a continuously moving picture. Other characteristics of the human eye are that it is less sensitive to color detail than to black-and-white detail, and that the human eye does not respond equally to all colors. The eye is most sensitive to the yellow/green region, and less in the areas of red and (particularly) blue.

INTRODUCTION TO TELE - VISION

Television means ‘to see from a distance’ (From the Greek tele (= far) and the Latin visionis (from videre = to see)). The desire in man to do so has been there for ages. In the early years of the twentieth century many scientists experimented with the idea of using selenium photosensitive cells for converting light from pictures into electrical signals and transmitting them through wires. The first demonstration of actual television was given by J.L. Baird in UK and C.F.Jenkins in USA around 1927 by using the technique of mechanical scanning employing rotating discs. However, the real breakthrough occurred with the invention of the cathode ray tube and the success of V.K. Zworykin of the USA in perfecting the first camera tube (the iconoscope) based on the storage principle. By 1930 electromagnetic scanning of both camera and picture tubes and other ancillary circuits such as for beam deflection, video amplification, etc. were developed. Though television broadcast started in 1935, world political developments and the second world war slowed down the progress of television. With the end of the war, television rapidly grew into a popular medium for dispersion of news and mass entertainment. The fundamental aim of a television system is to extend the sense of sight beyond its natural limits, along with the sound associated with the scene being televised. Essentially then, a TV system is an extension of the science of radio communication with the additional complexity that besides sound the picture details are also to be transmitted. In most television systems, as also in the C.C.I.R. 625 line monochrome system adopted by India, the picture signal is amplitude modulated and sound signal frequency modulated before transmission. The carrier

frequencies are suitably spaced and the modulated outputs radiated through a common antenna. Thus each broadcasting station can have its own carrier frequency and the receiver can then be tuned to select any desired station. Figure shows a simplified block representation of a TV transmitter and receiver.

PICTURE TRANSMISSION The picture information is optical in character and may be thought of as an assemblage of a large number of bright and dark areas representing picture details. These elementary areas into which the picture details may be broken up are known as ‘picture elements’, which when viewed together, represent the visual information of the scene. Thus the problem of picture transimission is fundamentally much more complex, because, at any instant there are almost an infinite number of pieces of information, existing simultaneously, each representing the level of brightness of the scene to the reproduced. In other words the information is a function of two variables, time and space. Ideally then, it would need an infinite number of channels to transmit optical information corresponding to all the picture elements simultaneously. Presently the practical difficulties of transmitting all the information simultaneously and decoding it at the receiving end seem insurmountable and so a method known as scanning is used instead. Here the conversion of optical information to electrical form and its transmission are carried out element by element, one at a time and in a sequential manner to cover the entire scene which is to be televised. Scanning of the elements is done at a very fast rate and this process is repeated a large number of times per second to create an illusion of simultaneous pick-up and transmission of picture details.

A TV camera, the heart of which is a camera tube, is used to convert the optical information into a corresponding electrical signal, the amplitude of which varies in accordance with the variations of brightness. Fig. 1.2 (a) shows very elementary details of one type of camera tube (vidicon) to illustrate this principle. An optical image of the scene to be transmitted is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner side of the glass face-plate has a transparent conductive coating on which is laid a very thin layer of photoconductive material. The photolayer has a very high resistance when no light falls on it, but decreases depending on the intensity of light falling on it. Thus depending on the light intensity variations in the focused optical image, the conductivity of each element of the photolayer changes accordingly. An electron beam is used to pick-up the picture information now available on the target plate in terms of varying resistance at each point. The beam is formed by an electron gun in the TV camera tube. On its way to the inner side of the glass face - plate it is deflected by a pair of deflecting coils mounted on the glass envelope and kept mutually perpendicular to each other to achieve scanning of the entire target area. Scanning is done in the same way as one reads a written page to cover all the words in one line and all the lines on the page (see Fig. 1.2 (b)). To achieve this the deflecting coils are fed separately from two sweep oscillators which continuously generate saw-tooth waveforms, each operating at a different desired frequency. The magnetic deflection caused by the current in one coil gives horizontal motion to the beam from left to right at a uniform rate and then brings it quickly to the left side to commence the trace of next line. The other coil is used to deflect the beam from top to bottom at a uniform rate and for its quick retrace back to the top of the plate to start this process all over again. Two simultaneous motions are thus given to the beam, one from left to right across the target plate and the other from top to bottom thereby covering the entire area on which the electrical image of the picture is available. As

the beam moves from element to element, it encounters a different resistance across the target-plate, depending on the resistance of the photoconductive coating. The result is a flow of current which varies in magnitude as the elements are scanned. This current passes through a load resistance RL, connected to the conductive coating on one side and to a dc supply source on the other. Depending on the magnitude of the current a varying voltage appears across the resistance RL and this corresponds to the optical information of the picture. If the scanning beam moves at such a rate that any portion of the scene content does not have time to move perceptibly in the time required for one complete scan of the image, the resultant electrical signal contains the true information existing in the picture during the time of the scan. The desired information is now in the form of a signal varying with time and scanning may thus be identified as a particular process which permits the conversion of information existing in space and time coordinates into time variations only. The electrical information obtained from the TV camera tube is generally referred to as video signal (video is Latin for ‘see’). This signal is amplified and then amplitude modulated with the channel picture carrier frequency. The modulated output is fed to the transmitter antenna for radiation along with the sound signal. SOUND TRANSMISSION The microphone converts the sound associated with the picture being televised into proportionate electrical signal, which is normally a voltage. This electrical output, regardless of the complexity of its waveform, is a single valued function of time and so needs a single channel for its transmission. The audio signal from the microphone after amplification is frequency modulated, employing the assigned carrier frequency. In FM, the amplitude of the carrier signal is held constant, whereas its frequency is varied in accordance with amplitude variations of the modulating signal. As shown in Fig. 1.1 (a), output of the sound FM transmitter is finally combined with the AM picture transmitter output, through a combining network, and fed to a common antenna for radiation of energy in the form of electromagnetic waves. PICTURE RECEPTION

The receiving antenna intercepts the radiated picture and sound carrier signals and feeds them to the RF tuner (see Fig. 1.1 (b)). The receiver is of the heterodyne type and employs two or three stages of intermediate frequency (IF) amplification. The output from the last IF stage is demodulated to recover the video signal. This signal that carries the picture information is amplified and coupled to the picture tube which converts the electrical signal back into

picture elements of the same degree of black and white. The picture tube shown in Fig. 1.3 is very similar to the cathode-ray tube used in an oscilloscope. The glass envelope contains an electron-gun structure that produces a beam of electrons aimed at the fluorescent screen. When the electron beam strikes the screen, light is emitted. The beam is deflected by a pair of deflecting coils mounted on the neck of the picture tube in the same way and rate as the beam scans the target in the camera tube. The amplitudes of the currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast rate of scanning. The video signal is fed to the grid or cathode of the picture tube. When the varying signal voltage makes the control grid less negative, the beam current is increased, making the spot of light on the screen brighter. More negative grid voltage reduces the brightness. if the grid voltages is negative enough to cut-off the electron beam current at the picture tube there will be no light. This state corresponds to black. Thus the video signal illuminates the fluorescent screen from white to black through various shades of grey depending on its amplitude at any instant. This corresponds to the brightness changes encountered by the electron beam of the camera tube while scanning the picture details element by element. The rate at which the spot of light moves is so fast that the eye is unable to follow it and so a complete picture is seen because of the storage capability of the human eye. SOUND RECEPTION The path of the sound signal is common with the picture signal from antenna to the video detector section of the receiver. Here the two signals are separated and fed to their respective channels. The frequency modulated audio signal is demodulated after at least one stage of amplification. The audio output from the FM detector is given due amplification before feeding it to the loudspeaker. SYNCHRONIZATION It is essential that the same coordinates be scanned at any instant both at the camera tube target plate and at the raster of the picture tube, otherwise, the picture details would split and get distorted. To ensure perfect synchronization between the scene being televised and the picture produced on the raster, synchronizing pulses are transmitted during the retrace, i.e., fly-back intervals of horizontal and vertical motions of the camera scanning beam. Thus, in addition to carrying picture detail, the radiated signal at the transmitter also contains synchronizing pulses. These pulses which are distinct for horizontal and vertical motion control, are processed at the receiver and...