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. 

Some Important Topics In Control System

     What are the topics?

•        Open loop & Close loop control system.
• Transfer  Function.
• Elements or Components of Close loop system.
• Characteristics equation of a transfer function.
• Time response for 1st order system.
• Time response for 2nd order system.
• Transient response specification of 2nd order system.

Introduction & Background:

   Control system is the behaviour of the system is described by the differential equations. The differential equation may be ordinary differential equation or difference equation. The first significant control device was James Watt’s flyball governer. It was invented in 1767. the working principle was to keep the speed of the engine constant by regulating the supply of the steam to the engine.

       open loop control system:

      The control system  without feedback is called open loop control system or non feedback control system. In open loop systems the control action is independent on the desired output.

    Examples: Automatic washing machine, Immersion    rod, a field control D.C motor.

 Advantages of open loop control system:

1.         Open loop control system are simple
2.   Open loop control system are economical.
3.    Less maintenance is required and not difficult.
4.    Proper calibration is not a problem.

Disadvantages of open loop control system:

1.       Open loop control system are inaccurate.
2.        These are not reliable.
3.       These are slow.
4.       Optimization is not possible.

Close loop control system:

       Close loop control systems are known as feedback control system.In close

       loop control system the control action is dependent on the desired output. 

         

        Example: Air conditioners provided with thermostate.

Advantages of close loop control system:

1.          These systems are more reliable.
2.          Close loop systems are faster.
3.         A number of variable can be handled simultaneously.
4.         Optimization is possible.

Disadvantages of close loop control system:

1.          Close loop systems are expensive.
2.           Maintenance is difficult.
3.           Complicated installation.

Transfer function:

      Transfer function is defined as the ratio of Laplas transfrom of the output to the Laplas transform of input with all initial conditions are zero.

       Consider a open loop control system                                

       R(s) = Laplas transform of input.

       C(s)= Laplas transform of output.

       G(s)= Transfer function.

       Transfer function G(s)=C(s)/R(s)

Characteristics equation of a transfer function:

             

      POLES:The poles of G(s) are Those values of ‘s’ which make G(s) tend to infinity. Above transfer function having simple poles at s=0,s=-2,double poles at s= -4

      ZEROS:The zeros of G(s) are Those values of ‘s’ which make G(s) tend to Zero. Here the simple Zero at s= -3.

Graphical representation of POLES & ZEROS:

        Poles are represented by ‘X’ & Zeros are represented by ‘O’.

         Now the Pole-Zero plot of the previous transfer function is                                          

Time Response of Control system:

       Time domain representation of a control system is known as time        response. Time response of a control system is divided into two parts. 

1.         Transient response
2.       Steady state response

                           

Time response of the 1st order system with unit step input:
DELAY TIME:
It is the time required for the response to reach 50% of  the final value in first time.
RISE TIME: It is the time required for the response to rise 10% to 90% of its final value for overdamped system and 0 to 100% for underdamped system.
       Rise time=

PEAK TIME: It is the time required for the response to reach the first peak of the time response or first peak overshoot.

      peak time =

MAXIMUM OVERSHOOT: It is the normalized difference between the peak of the time response and steady output.

      Maximum overshoot=  

SETTLING TIME: It is the time required for the response to reach and stay within the specified range   (2% to 5%) of the final value.

      Settling time= 

STEADY STATE ERROR: It is the difference between actual output and desired output as time ‘t’ tends to infinity.

      Steady state error= 

                                

The Basic Structure Of Private Automatic Branch Exchange

What is a PABX exchange

In generally the full meaning of PABX is Private Automatic Branch Exchange and in practically it stands for a telephone line exchange that is used for business or the office application  as opposed to one that is also used as a common carrier or Telephone Company that is operates for different kinds of  businesses or for the general public services. PABX system  are used to make connections amongst the internal telephones of a private organization or in the different institutes, Those are generally used for business oriented appliction. The PABX system are also connected with a public switched telephone network through trunk lines. In practicaly it is called PSTN line. As they inter connected telephones,  fax machines, modems, and many other parts, the usual term "extensions" that is given is referred to the ending point on the branch.

(i)Basic Blocks Of The PABX exchange:

The PABX exchange uses normal wire connection for total telecommunication system. But  it also uses optical fiber line for its telecommunication system. The basic block of the PABX exchange is given below

(ii)TNT Input lines

For the PABX exchange  8 input lines are taken from the TNT. These lines are taken on a rent. For each line,  has to pay about 100 taka per line. These lines are the input of the AX controller. For the another 8 lines are used as input lines to the AX controller but those lines are carried to the building through optical fiber.

(iii)AX Controller

AX controller is the major part of the PABX exchange. An AX controller mainly converts the input TNT lines for using in the telecommunication system. There are 12 ONSP card inside the AX controller. This AX controller is made by China. The ONSP cards of the AX controller are divided into 2 parts. 1^st part consists of 3 ONSP cards which has yellow colored input and output lines. 2^nd part consists of 9 ONSP cards which has gray colored output lines.

(iv)ONSP Card

An ONSP card is the major part of the AX controller. An ONSP card can take maximum 4 inputs from outside and give maximum 12/24 outputs from it. 1^st 3 ONSP cards can give 12 output lines each and last 12 ONSP cards can give 24 output lines each. These lines are connected to the telephone sets of different rooms of the varsity through wiring.

(v)ASU

ASU is a supporting device of AX controller. There are 2 ONSP cards in the ASU. These 2 ONSP card can give maximum 24 output lines from it.

(vi)Calculation of Total Number of Lines

Another 300 lines are managed about the same way for the internal communication.

(vii)Maintenance

The total PABX exchange system is maintained by navigator software which is mainly IP based. If any problem occurs then it is solved by this software. If anyone wants to call outside of the system area, then he first calls the member who is sitting in the exchange room. He then makes connection for the desired call. If anyone doesn’t know the called subscribers number, he also calls the exchange room and find the number.