Television and Artificial Satellite (With Block Diagram) | Electronics

Throughout the history of human civilization, man has contrived in different ways to extend his sense of sight. The microscope, the telescope and the camera were important landmarks along the way. These inventions were based upon an important characteristic of the human eye known as ‘persistence of vision’. Television is one of those miracles which enables us to view a scene occurring at a far distance. By the word ‘television’ we literally mean ‘seeing at a distance’ just as we have accepted long ago hearing at a distance.

Plan of Television System:

A television system comprises of a transmitter and a receiver. In the transmitter section a television camera breaks the image into tiny parts which after suitable processing fed to the transmitting antenna. The antenna then radiates the signal in the form of electromagnetic waves. These electromagnetic waves are received by the receiving antenna where they are fed to the television receiver to produce a visible image on the screen of the picture tube.

Fig. 11.1 is a simplified presentation of a television system that employs both AM picture transmitter and FM sound transmitter. By a single antenna the picture and sound signals are transmitted at different frequencies to avoid interference. These two signals are also received by a single antenna wherefrom they are separated by the AM picture section and FM sound section.

The picture signal originating from TV camera terminates in a picture tube while the sound signal originating from microphone terminates in the Speaker.

Picture Analysis of TV:

A picture is, in general, characterized by a distribution of light and shade over it. An area of an image whose size corresponds to the smallest detail and has a uniform total value, is called the picture element. A complete picture consists of a large number of these tiny picture elements.

So for transmitting a picture properly it is required to transmit the visual information of each picture element separately so that their respective identity is not lost at the receiving station. This is done suitably by using the phenomenon of photo-electricity which converts light energy into electrical signal.

In practice, all the electrical impulses of the different picture elements are picked up one after another and then transmitted rapidly through a single channel. This process is called TV scanning. At the receiving end these impulses are converted into picture elements in the same sequence. Because of the rapid scanning and owing to ‘persistence of vision’ all the picture elements appear simultaneously before our eyes.

The scanning of the image can be done by a number of ways, out of which the method of linear interlaced scanning is used widely now-a-days. In this method, the scanning beam goes over the series of slightly inclined lines downwards to the right. Schematic illustration of linear interlaced scanning is given in Fig. 11.2(a) and (b). Fig. 11.2(a) shows the downward movement of the scanning beam while the upward movement is shown by Fig. 11.2(b).

These movements may be explained by going through the following steps:

i. In Fig. 11.2(a), the scanning spot starts to move from P1 to the right-hand point Q and then reversing its motion it comes quickly to R3 and proceeds so on up to the point S.

ii. The upward movement of the spot now starts from S, as shown in Fig. 11.2(b), and comes at U, at the centre of the top line.

iii. The downward movement of the spot further starts from U, as shown in Fig. 11.2(a) and it traverses the even number line such as 2, 4, 6, 8, etc., and ends at V.

iv. At this stage, from the point V the scanning spot again traverses up and comes back to P.

The whole frame thus is accurately scanned using two sets of scanning lines. The interlacing of the scanning lines reduces considerably the flickering of the image.

In actual cases, the number of lines per picture frame varies from one system to another. A typical value is 525 lines per picture frame so that there are alternate pairs of 262.5 lines constituting a field. The entire picture is scanned about 25 to 30 times per second depending on the frequency of the power supply.

TV Camera (Pick-Up Device):

The function of the TV camera is to generate electrical signals suitable for each picture element and to scan the image in proper sequence. A typical TV camera known as iconoscope is shown in Fig. 11.3.

Its action can be explained as follows:

Light from an illuminated scene, as shown by arrow mark, is made to focus by optical lenses on a photosensitive mosaic screen. The mosaic is a coating of millions of light-sensitive globules each about 0.001 inch in size and is insulated well by using a mica shoot. At the oilier side of the mica sheet there is a conducting signal plate coated with a film of graphite.

The globules insulated by the mica from the metallic coating thus form tiny electric capacitors with a mica dielectric and the common metallic signal plate. Therefore, in accordance with the intensity of the light, electrons are emitted from each globule and thereby charge up its individual capacitor.

The stored electrical image on the mosaic cannot be transmitted as a whole. For this a sharply focussed electron beam formed by the electron gun in the narrow elbow of the tube is made to scan the mosaic through the attraction of the highly positive second anode, called the collector ring.

The beam neutralizes successfully the previous positive charge formed by photo-emission and as a consequence discharges the globule capacitor. At that instant a voltage generates across the load resistor R_L connected to the signal plate. The amplitude of this voltage varies with the light intensity of the corresponding picture element and it is known as picture signal.

It is to be noted in this connection that the TV camera of iconoscope type, has a low video output and so it has been replaced by ‘image orthicon’ or ‘vidicon’ camera tubes which are much sensitive and can televise anything visible with naked eye.

i. Image Orthicon:

The television camera of image orthicon type is mostly used now-a-days. It can be divided in four different sections.

These are:

(i) The image section,

(ii) The electron gun section,

(iii) The scanning and deflecting section, and

(iv) The amplifier section.

A diagram of the camera is shown in Fig. 11.4.

The image section is the most vital part of the tube where the image of the scene to be televised is first focussed by an optical lens on a thin glass surface E coated with photosensitive film. The photosensitive surface emits photoelectrons from different points according to the light intensity of the image. The electrons emitted from the right-hand side of E are attracted towards the screen S kept at a positive potential and are made to pass through its meshes.

The number of meshes of the screen is about 1000 per inch and is fixed at the left-hand side of the glass target T of low resistivity at a distance of a few thousandths of an inch. The electrons are focussed on the target T by the help of the electrostatic field produced by the grid G₆ and the axial magnetic field of the focussing coil. Secondary electrons are now emitted from the different points of T which are attracted to S leaving positive charges at different points in proportion to the light intensity of the image.

The electron gun consists of a cathode K to emit electrons in the form of a narrow beam which is focussed on the target by using a number of cylindrical grids. In the figure, G₁ is the control grid, G₂ is the accelerating grid and G₄ is the focussing grid. The control grid G₁ is utilized for controlling the number of electrons in the beam which can be aligned by the help of the alignment coil.

The electron beam during its movement through the scanning section is deflected by horizontal and vertical deflecting coils and is kept narrow by the help of the focussing coil placed outside the scanning section. A decelerator grid G₅ is kept close to the target so that the electrons in the beam strike the target with a slow velocity.

A part of the electrons when strike the target neutralizes positive charges developed at different portions on it while the remaining electrons are turned back in the return beam. As a result electrons in the return beam vary in accordance with the intensity of the image and it is, therefore, current modulated.

The return beam strikes the grid G₂ which helps to emit secondary electrons. The emitted electrons are then deflected into the multiplier section by the deflecting grid G_s. The multiplier has a number of surfaces for emitting higher and higher secondary electrons.

The amplified stream of secondary electrons so produced is finally passed through the load resistor R_L across which the output is taken through a capacitor C. The maximum output current thus obtained represents the black portion of the scene while the less current corresponds to lighter shades.

ii. Vidicon:

The television camera of vidicon type is simpler than the other cameras and its cost is also lower. But the camera provides less fineness of resolution and so is used mainly for televising movie film. A diagram of the camera is shown in Fig. 11.5.

The image section of this camera consists of a thin conducting metallic signal plate which is transparent to light. As before, the image of the scene to be televised is focussed by an optical lens on one side of the signal plate while the scanning is done by a narrow electron beam on the other side of the plate coated with a film of photo-conducting substance. When the image of the scene is focussed on the photosensitive surface, its different parts get different resistance values according to the light intensity of the image.

The electron gun consists of a cathode K to emit electrons, a control grid G₁, an accelerating grid G₂ and an anode grid G₃. The anode grid is placed in front of the photo-conducting surface so that the electrons of the beam can easily pass through its meshes on the surface. Here also the alignment coils are used for aligning the electron beam, focussing coils make the beam narrow, and the horizontal and vertical deflecting coils serve the purpose of deflection.

One major disadvantage of this camera is that the resistance of the photoconductive film does not change instantaneously with the variation of the intensity of light.

Transmission of Signals in TV:

In TV, the picture signal is transmitted by modulating an RF carrier wave in amplitude. The carrier has a frequency range of 54-216 MHz or 470-890 MHz, in general. In practice, a few other signals called synchronizing signals are properly added with the picture signal for obtaining the so-called video signal of bandwidth 4.5 kHz. The function of the synchronizing signal is to maintain correct timing of the horizontal and vertical sweep motion, and also to keep the transmitter and receiver locked in step.

For transmitting the audio portion, frequency modulation is applied instead of the amplitude modulation. The modulated sound carrier has a maximum frequency deviation of 25 kHz.

Conversion to Original Image in TV:

The video and sound RF signals picked up by the receiving antenna are in turn amplified, super-heterodyned and then applied to a video detector where the video and sound IF signals are separated. The receiving arrangement of the television is shown in Fig. 11.6.

The sound IF signal is applied to the separate portion of a receiver which after audio ampli­fication goes to a loud-speaker. The video signal, on the other hand, is amplified by a video amplifier and then reassembled by an electronic beam on the picture tube or kinescope which is basically a specially equipped cathode-ray tube.

The kinescope has a fluorescent screen instead of the mosaic plate so that when electrons strike it with high energy, luminous radiation comes out. On the fluorescent screen, therefore, the brightness of the electron beam is varied with the variation of the amplitude of the picture signal and so with the brightness of the transmitted image.

High Voltage Power Supply in TV:

To the anode of the picture tube of a television set, voltage of the order of 5000 to 10000 volts is applied from a high voltage power supply. A high voltage of this order is essential for producing an intense electron beam over the screen of the picture tube.

In addition to this high voltage power supply there is also a conventional rectifier power supply for applying voltages to the electrodes of the tubes and to the electron gun. These voltages are, of course, low and so named as low voltage power supply to distinguish it from the high voltage one.

Colour Television:

In general, the principles of colour television are similar to that of black and white but in actual cases the designs and constructions are too complicated. In colour TV, the illuminated image is televised by using three separate cameras, each provided with an optical filter for transmitting a particular colour. The three colours, red, blue and green, are used in practice as others can be obtained by their proper combination.

The tricolour picture tube has three electron guns, one for each of the colour. The screen of the tube is made by a series of three dots of phosphor sensitive for the three colours, instead of a continuous layer. Between the electron guns and the screen a shadow mask is placed whose number of holes is equal to the number of group of dots.

The guns are oriented in such a manner that the beams pass at slightly different angles through the same aperture of the mask at a time. These three are then incident simultaneously on red, green and blue dots on the screen accordingly as they are originated in the electron beams. This arrangement is shown in a simplified form in Fig. 11.7. If the distance of the picture screen is greater, the three dots cannot be differentiated individually rather it appears as a composition of three colours.

Frequency Bands for Satellite Communication:

General Structure of a Satellite Communication:

The general structure of a satellite communication is shown by a simplified block diagram in Fig. 11.8.

Block Diagram of an Earth Station for Satellite Communication:

The basic block diagram of an earth station for communication through satellite is shown in Fig. 11.9.

Modem and Codec Used in Satellite:

The equipment that carries out modulation (MOD) and demodulation (DEMOD) is called MODEM- Similarly the equipment responsible for carrying out coding and decoding is termed CODEC. These two devices are widely used in digital satellite communication. Modem is used as an interface between analog and digital systems.

These play an important role in computer communication networks and ISDN systems. Codecs are used in digital television systems and normally consists of a pair of A/D converter and D/A converter. It is a kind box digital device as shown in the Fig. 11.10.

Some Terms Related to Satellite Communication:

1. Process Gain:

Process gain, G_p = BW_rƒ/R_inf,

Where BW_rƒ = rƒ bandwidth of the transmitted spread spectrum signal

R_inf = information rate which is the data rate in the information base band channel

ii. Jamming Margin:

It expresses the capability of a system to perform in interfering (hostile) environments. Jamming margin takes into account the requirement for a useful system output signal to noise ratio [(minimum (S/N)_out of the system)] and allows for internal losses. Thus the jamming margin is evaluated by the formula-

M_j = Jamming margin = G_P – [L_sys + (S/N)_{min out}]

Where L_sys = system implementation losses.

Direct Sequence Spread Spectrum Techniques:

Basic block diagram of a typical DS-SS Receiver is shown in Fig. 11.11.

The incoming RF signal is first down converted to IF and then the pseudo-random frequency, g₁(t) is multiplied to the IF signal and thereby the product is compared with the received IF signal in the correlator. The function of the correlator are to compare the two signals and recover original data.

Elements of a Digital Communication System:

Fig. 11.12 reveals a simplified block diagram illustrating the elements of a digital communication system.

Time Division Multiplexing (TDM) Used in Digital Signal Transmission:

The principle of time division multiplexing is illustrated in Fig. 11.13.

The time division multiplexing of channels is used in digital signal transmission techniques. Here different channels are assigned different time slots in the specified time intervals for transmission. In this technique the intermodulation effect (cross talk) is not present though it requires perfect synchronism between the multiplexing and demultiplexing commutators.

Time division multiplexing may also be said to have information channels to be interleaved into a single transmission channel in time. This process is commutation. Upon arrival at the receiver the interleaving channels are decommutated or distributed. Fig. 11.13 indicates the principle of TDM. Here each channel is assigned a time slot of duration T = T_s/M, where-

Actually this time slot T for individual channel consists of the channel pulse time slot T_P and a guard time ∆t to prevent excessive intersymbol interference or overlap of pulses from adjacent channels. This is shown in Fig. 11.14. Here each channel signal is band limited to frequency ƒ_m Hz and the M channels are sampled at rate ƒ_s.

The sync signal is used to synchronise the commutation (sampling), pulse time reference and frame time as well as to provide a receiver sync reference. This sync reference is derived from a frequency standard. At the receiver the sync tone is extracted by a pilot tone filter or phase locked loop and is employed to synchronise the distributor.

The TDM wave is then demultiplexed in time (distributed) to M individual channels. It should be noted that each channel in TDM transmits digital ward in the form of a group of bits which are arranged in the prescribed time slot. These M time slots in time T_s constitute a frame where each channel is identified by its position in the frame, and in the digital transmission individual frames are identified by the presence of synchronization bits that repeat a known pattern.