Introduction
To acquire
practical knowledge on TV signal transmitting, we were on a tour at Television
Substation of Khulna located at khalishpur (transmitting centre).We observe the
technique of telecasting and visited the entire transmitter and receiver
system.
Short description about the destination of our tour
The destination
of our tour was Television Substation of Khulna located at khalishpur 10 km
away from Khulna University. This substation has been operational since 1977.
The job of a substation is to retransmit the audio-video signal received from
the mother station, so that the receivers in that locality can get the low
video feed amplified from that substation. The Khalishpur TV Transmitting
station is mainly received the wave of 216 - 223MHz FM signal on channel no.11
from Jessore which consists of 4-5 link. That receives two types of signal, one
is am signal of about 222.75 MHz which is used for audio and other is 217.25
MHz which is used for video and at last both signals are transmits through the
air for receive to the viewers. It’s Transmitting power 10KW and efficient
radiated power 96KW. It’s centre area is 2000 miters. The
coverage area of this substation is 100 radius but can transmit 83 km radius.
The supported tower is BTCL, Khulna which height is 500 ft and the height of
the antenna is 15 ft with gain of 9.6db.The antenna gain The power is supplied from
PDB & NGP.
Objective
Our objective is to observe and understand the system of
operation of that substation. To see how the signal is received, amplified,
combined and retransmitted.
Microwave link
The Khulna TV
transmission station mainly receives the signal of 216-223MHz on channel no. 11
from Jessore which consists of 4-5 link programs. The audio frequency is
222.75MHz and video carrier frequency is 217.25MHz. For signal reception BTV
uses ASIA SAT 3S, location 105.50, east. It is a geostationary
satellite about 36000 km above the sea level. This satellite has three levels,
leo, meo and geo. The geo level contains the C band ranging from 3.7GHz to
4.2GHz which is used in this signal reception. Signals are divided in some
bands. 41 to 68 MHz is the first band used for navigation. 88 to 108 MHz is the
2nd band used for FM. 3rd band is 174 to 230 MHz. For
Khulna channel no. II there is 216 to 223 MHz.
Modulation used in the system
Frequency modulation for audio signals and amplitude
modulation for video signals are applied.
Audio and Video Mixing
There is a
combiner named diplexer which
combines the audio and video signals and sends them to the antenna (tower).
The program input equipment
The program
input equipment receivers the AM signal for video and FM Signal for sound and
lastly amplifies it and gets it to the transmission action. At the transmission
section modulation unit is made by “Thales” not only but also all the
equipment. The signal then comes to “power amplification section” where there
is many stage like rectify amplitude modulating at 557 MHz The IF stage for
video signal is 38.4 MHz and 34.4 MHz for audio.
Amplifying system
There is a space
for ten amplifiers in a machine made by Digital Axcera in the substations air
conditioned room. Amplifier no. 1 and 10 are for audio amplification and
amplifiers 2 to 9 are for video amplification. Although there were nine
amplifiers in that space and amplifier no. 4 was absent. There are two
balancing networks that balance the signal and make sure that absence of any
amplifier can not affect the signal power to be transmitted.
Transmitter
The model number
of transmitter is THALES TAV210K0ADDB3 which is made in France. The
specifications of the transmitter are as :
This transmitter
follows heterodyne action. It has 10 amplifiers. 1st and 10th
amplifier is used for audio transmission and 2nd to 9th
are used for visual transmission. Every amplifier needs 1.25KW for visual and
1KW for audio transmission. The exiting section of the transmitter is 2type.
Such as A-exciter and B-exciter. Every type exciter has 5 modules.
Function of the TV station
In TV center
there is a producer for each studio and a director to organize shows .the
studio are controlled by sub controlled receiver that helps the director to
organize the show in a order .there is also an o.b. Van for outdoor shooting
that contains a high gain Panasonics antenna for transmitting the signal to the
broadcasting station. In a studio there is also a studio for news. The switching control section makes the sequence of the show like drama, song
,poem, dance, etc. after
organizing the show the signal goes directly to the broadcasting antenna and
lastly to the viewers through T&T
microwave.
Function of antenna
There are wires
around the main transmission line which have 28000 volt for proper
transmission.. The special setup Austin transformer is needed to lighting the
neon light up above the aerial. The amplified video signal is passed through a
band pass filter giving a bandwidth of 7MHz (216MHz to 223MHz). Then it is
combined and sent to the tower. Then the tower transmits the signals. The tower
is Omni directional, has a height of 500ft and was set up in 1964. It transmits
the signal which can receives fairly from 90miles, so far, have also the
radiation effect around the aerial. To absorb all types of vibration all the
walls at studio, booth and the control room are covered by raw cooler and
partex type material.
Power system
Although the substation has own power system, it uses the
line supply. In case of power failure, backup power is provided shortly.
Discussion
The educational tour to Khulna TV
station was very educative. It is too much essential to increase our practical
knowledge. And this tour give us the
chance to observe practically the transmission system of a television station .It
is also help us to acquire theoretical knowledge easily. We want to thanks
specially our respected course teacher for providing this tour and the employees of Television Substation of
Khulna for their cordial co-operation.
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