A PRACTICAL OVERVIEW ON A TELEVISION SUBSTATION.

Every day changing the Technology & we live in an age of science. Science has linked up the distant parts of the world by discovered many wonders in the field of communications. Television is one of them. We can see the pictures of the whole world on the screen of television .To do these the length of special technique is used, so it is essential to realize this Transmission technique of TV.

OBJECTIVES

1. To acquire knowledge in television transmission system from a practical point of view. 

2. To know the signal receiving and transmitting information of television signal. 

3. To familiar with the audio and video signal reception technology.

4. To see the composite video signal and 625 lines of the composite signal individually. 

5. To gather knowledge about the other required used machines for receiving and transmitting and their functioning method.

FUNCTIONAL REQUIREMENTS TO POINT OUT THE SYSTEM

• Channel no:  11
• Frequency: 216 - 223 MHz
• Antenna gain: 9.6 db.
• Transmitting power:  10 KW.
• Effective radiated power:  96 KW.

SIGNAL RECEPTION:

Three basic process to every communication system namely transmitter, channel, and receiver. There are 2 types of system in communication. They are 

(a).Terrestrial Communication:

In this communication system, signal is conceded from earth to earth by microwave links. Usually we called it station by station signal pass. The TV substation uses this system generally. The mother station of the TV signal transmit the signal and the substation received this signal by their antenna. 

(b). Satellite Communication: 

Another communication system that use the TV substation is called satellite. Overall satellite system is divided into 3 zones. They are Indian Ocean Zone, Atlantic Ocean Zone, and Pacific Ocean Zone. The TV substation is in the Asian Ocean Zone. The name of this satellite is ASIA SAT 3S. It is placed at a distance of 47000 km from the centre of the earth and at a distance of 36000 km from the earth. It’s heading 105.5º east. Japan is situated at 0º position. This satellite is moving anticlockwise. The TV substation gets signal from 35^th transponder of this satellite. Transponder of a satellite is a part of the satellite that converts the Uplink signal to down link signal. Every transponder only converts the Uplink signal to down link signal for the earth station TV receiver. Every satellite has 3 states. They are
  i. Leo (LOW EARTH ORBIT)= From 500 to 1500 kilometers above the Earth's surface.

 ii. Meo (MEDIUM EARTH ORBIT)= From  5000 to 18000 kilometers above the Earth's surface.

 iii. Geo (GEOSTATIONARY EARTH ORBIT)= From 35863 kilometers above the Earth's surface.

The TV substation uses Geo which uses C band. Its’ frequency range is 3.7 to

4.2 GHz.

EXTRACTING THE DESIRED COMPOSITE VIDEO SIGNAL

 After receiving the signal, It passed through a Band pass filter to gets the desired signal. There are 3 bands.

i. Band 1: 41MHz to 68MHz. It has 4 channels. This band is used for navigation.

ii. Band 2: 88 MHz to 108MHz.  This band used for FM transmission.

iii.Band 3: 174 MHz to 230MHz. this band is divided of 7MHz. It has 8

channels.

The TV substation uses channel 11 that means it is in Band 3 and its’ frequency range is 216-230MHz. This time, the signal to noise ratio is also checked.

 MODULATION:

In modulation section, different techniques used for Audio and Video modulation. Audio modulation is accomplished by frequency modulation. Frequency modulation is that form of angle modulation in which the instantaneous frequency is varied linearly with an incoming message signal. Video modulation is accomplished by amplitude modulation. Amplitude modulation in which the amplitude of a sinusoidal carrier is varied in accordance with an incoming message signal. After amplitude modulation, They transmit this signal through vestigial sideband(VSB). This technique has several advantages.

 MIXING

After modulation, audio and video signal is mixed by a special type of coupling device called diplexer. In this diplexer unit, audio signal comes first. After mixing, this mixed signal is transmitted through the antenna.

 AMPLIFIER SECTION

There are 10 amplifiers in the amplifier section of the TV substation.  No. 1 and No. 10 is used for audio and No. 2 to No. 9 is used for video. At present the no. 4 amplifier is not working in the TV Substation. There are 5 power supplies for these amplifiers and 8 transistors in every amplifier.

TRANSMISSION

The TV substation is recently using analog transmitter for transmission. This transmitter is made by a French company named Theles Company. There is another analog transmitter that is made by a Japanese company named NEC Company. 2^nd transmitter is used when 1^st transmitter fails. The TV substation has recently bought a digital transmitter to transmit parliamentary channel. This transmitter is made by an American Company named Axera. But the digital transmitter is not in used now .Because the government has not allowed the authority of the TV substation yet to transmit Parliamentary channel. The maximum power of the transmitter for video is 10KW and for audio is 1KW. Extraordinary cooling system is used for these transmitters. Room temperature is kept 17º-18º.

For transmission the TV substation uses an antenna. This antenna is OMNI directional. Type of this antenna is HKS-D-1214. Its’ height is 500 feet from the ground. It was made by Nippon Electronic Company LTD. Tokyo, Japan. This transmitter in progress its’ work from July, 1970.

Introduction of  THALES:

Thales is a French electronic device. It is a solid state device. It is installed in the Television Substation (transmitting centre). It is more efficient than previous TV transmitter NEC. It performs both analog system& Digital system. Here total control system of the machine in performed analogally.

THALES TV TRANSMITTER

1.     Exiter A

2.     Local control panel

3.     very low voltage power supply

4.     Exiter B

5.     CPU card

6.     Mains distribution panel

7.     Power amplifiers

Some Important Information’s About THALES to be pointed out:

1. Fresh air is blown to the transmitter by blower for cooling the transmitter. Dehydrometer is used to control humidity.

2. If voltage is got less then transmitter power is less. So regulated power supply will be maintained for transmission. Here 400 V & 38A supply is maintained.

3. Security board for current temp. Control sensors temperature and if exceeds goes terminated limit the transmitter automatically.

4.  It contains 100Km (radius of circle) capacity for transmitting in Khulna.

5. Both sound & picture is received well within 80 Km. Full range is not normally used.

6. There are 2 Excitor – Dual Band.

7.Some important parts for THALES are Audio IF, Sound converter,  Local Oscillator,  Vision converter,  Vision IF, video IF, Video converter , Local frequency device,  Sound converter ,  Audio IF, CPU etc.

8.CPU controls all the components digitally and takes all kind of automatic steps where necessary.

9. Visio converter converts IF to RF.

10. Amplified by Transistor, 4 terminal 2D MOSFET.

11. There Are 10 amplifiers. (8 for video (2^nd to 8^th), 2 for audio (1^st & 10^th))

12. There exists also Sync duplexer – superimpose video & audio signal.

13. BTV WORLD receives if it fails to Microwave.

14. There are two exciters (A & B). Usually one is used. If it is gone damaged then the other is used.

Report  About  NEC

NEC is the product of Nippon Electronics of Japan. It used in the Television Substation (transmitting centre). It is tube type transmitter. It also needs 400KW power supply.

THE TRANSMITTER INPUT AND OUTPUT ARE FOLLOWING:                           

Video   ( input )	Audio    ( input )	a.c.       ( input )	To antenna  ( visual sync   peak output)	To antenna (aural output )
01 v      (pp)	0-16   dbm	200-250  (3ø)	10  kw	500  kw
75 ohms	600 ohms	380~450 (3ø)	0.5  kw	2.5  kw

 

At last, it was a real pleasure to get an opportunity to observe such a project which is related to our theoretical knowledge and excellent experience of visiting  TV Centre. It is very necessary in our advanced learning. Generally bookish knowledge does not satisfy us unless we see things with our own eyes. This tour widens our knowledge, ideas and outlook.