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REPORT
OnPractical Training At
Submitted To: Submitted By:
Mr.R.S.Tyagi Pratibha Chauhan
(Head of Office) (B.E 2nd Year)
M.B.M Engg. College
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PREFACE
Practical Training is an important constituent of any curriculum and B.E course
is no exception to this general rule. Practical Training helps a student in getting
acquainted with the manner in which his/her knowledge is being practically
used and this is different from what he/she has learnt from books.Hence,when
he/she switcher from the process of learning to that of implementing his/her
finds an abrupt change. This is exactly why a practical training session during
the curriculum becomes all the more important.
The duration of the practical training period prescribed for awarding the B.E
degree is 100 days in our College. This period has been divided in two parts viz.40 days practical training after 2
ndyear ,B.E session and a 60 days Training
after the 3rd
Year ,B.E session.
This report is describes my practical training after the 2nd
Year, B.E session that
I completed at the Doordarshan Kendra, Jaipur. This Training was in the formof, practical study of broadcast station. The report gives an introduction of the
Doordarshan Kendra and its different sections. The whole transmission of a TV
program from production to broadcast is explained in detail in this report. This
complete chain is divided in different segments for clear understanding. This
report consists of the working process of the production unit, Earth stationsatellite and transmitter.
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ACKNOWLEDGEMENT
It gives me immense pleasure to express my gratitude to the family of
Doordarshan Kendra, Jaipur for their prudent response in course of completingmy summer training .I am highly indebted to Mr.A.K.Sundrani, Station
Engineer DDK, Jaipur and Course Coordinator Mr. Harish Chandra (ASE) &
Mr.R.K.Jain (AE) for their guidance and whole hearted inspiration, it has been
of greatest help in bringing out the work in the present shape. The direction,
advice, discussion and constant encouragement given by them has been so
helpful in completing the work successfully.
I am extremely thankful to all the staff members of Doordarshan who have
spend their Valuable time in guiding and providing valuable information and
data required to accomplish this project.
I am extremely grateful to Mr. Arvind Rai(Dean, M.B.M Engg College) and
Head of ECE Department & all the faculty members of ECE Department for
guiding me.
Pratibha Chauhan
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CONTENTS
About DoordarshanDigitizationBasic TV SystemsEarth StationVideo Cassette RecorderVideo TapeMicrophoneLoudspeakerStudio CameraVision MixerConcept Of Satellite communication
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ABOUT DOORDARSHAN
The first television broadcast was viewed by the people of Rajasthan on 1st
August 1975 under the satellite Instructional Television Experiment targeting
the district of Kota, Sawai Madhopur and Jaipur. The footprint of the American
satellite ATS-6 fell on 388 villages in these district which were provided with
direct receiving sets. Special educational programmers were then produced atDelhi.
On 1st
March 1977, Upgrah Doordarshan Kendra (UDK) was set up at Delhi.
The programmes produced at UDK for Jaipur were relayed via high power
transmitters. On 1st
June 1987, Jaipur Doordarshan Kendra was set up. Initially
the Kendra produced only 30 minutes of programming and this was gradually
increased. Presently the Kendra originates about four hours of programming
daily.
The terrestrial channel covers 78%by population and &by area of Rajasthan.
The total numbers of transmitters are:
1) High Power: 8 Nos.2) Low Power: 80 Nos.3) Very low Power: 18 Nos.4) Transposers: 2 Nos
Transmitters in Rajasthan for DD2: 3 Nos (Jaipur, Jodhpur & Kota)
With its vision of Satyam, Shivam and Sunderam, Doordarshan has a mission to
reach every person with reliable news, balanced views, innovative educational
programmes and wholesome entertainment. The Jaipur Kendra Seeks to project
the life and literature, art and culture of Rajasthan in its variegated forms. It
strives not just to portray the resplendent past and the momentous present but to
redesign a human future.
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Doordarshan promotes discussion and dialogue to foster understanding and
goodwill among all sections of society. It endeavors to inculcate scientific
temper among the people. It attempts social reconstruction by waging a war
against casteism and superstition. It aims to wipe out illiteracy and ignorance
and promote health and well-being.
As a public service broadcaster Doordarshan never-even in the midst of the
vicious competition abandon its values and standards of decency. It does not
merely inform but also educates, not merely entertain but also enrich the lives of
all its viewers.
DOORDARSHAN - TODAY
Channels 24
Studio Centers 64
Transmitters 1400
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DIGITIZATION
Prasar Bharati, the umbrella organization under which Air and Doordarshanfunction crosses another milestone as its these two media units, enter the digital
era.
The inauguration of a new Broadcasting House and Tower B of Doordarshan in
New Delhi recently is an important milestone in the history of public
broadcasting in India as it marks its entry into the digital era.
Prasar bharati sought to reorient its growth strategies and succeeded in making
its presence felt as a vibrant and socially relevant public broadcasting
organization; Going digital is the latest manifestation of this trend.
Doordarshan has acquired state-of-the-art facilities for production and
transmission of programs. With a network of over 1400 terrestrial transmitters,
Doordarshan covers 90 % of the population and is way ahead of the reach of all
the satellite put together. Moreover, as the Review Committee envisionised,
Doordarshans channels telecast a healthy mix of entertainment and socially
relevant programmes reflecting the varied cultures and languages of the nation.
The Tenth Five-Year Plans proposals for Doordarshan focus on digitization
,Currently 20 out of doordarshans 25 channels are digital. Digital earth stations
have already been set up in Mumbai, Hyderabad, Bangalore, Patna, Jallandhar
Chennai, Thiruvananthpuram, Ahmadabad, Kolkata, Bhubaneswar, Shilling and
Aizawl. Engineers of Doordarshan put up, in record time, the Ku band
transmission facility at Todapur, near New Delhi, for broadcasting DD Direct
Plus, And the Direct TO Home (DTH)broadcast service. Ku band transmission,
which ensures near total coverage, is a cost effective alternative to terrestrial
transmission. Doordarshan has distributed 10,000 DTH receiver systems and
200 cable head ends in select states of north, central and north eastern India,
where television coverage is below the national average. One of the compelling
reasons for introducing DTH is to ensure that programmes of AIR and
Doordarshan reach every household in the country. The DTH service would
help Doordarshan and AIR expand their reach with investment much lower than
that require for increasing the number of transmitters.
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VIDEO CASSETTE RECORDER
Device that records audio and video electrical signals onto magnetic tape. Machine that records on and plays a video tape The video cassette recorder (or VCR, less popularly video tape recorder) is
a type of video tape recorder that uses removable cassettes containing
magnetic tape to record audio and video from TV broadcast so it can be
played back later. Many VCRs has their own tuner and can be
programmed to record the signal on a particular channel during a
particular time interval.
Introduction
The main purpose of the video recorder is recording and replaying video and
audio signals. Although built-in tuners and timers have become integral parts of
the average video recorder, they are not prerequisites for reaching the main goal
audio and video registration and playback.
How the VCR works??
The Helical Scan System in an audio cassette deck, which only registers audio
signals, the tape passes over a static recording/playback head at constant speed.
The higher the speed of the tape , more tape particles pass the head opening and
the higher the frequencies that can be registered. With the extremely narrow
head opening, it is possible to record and play back the entire tone range, up to
18000 or 20000 Hz despite a slow tape speed of no more than 4.75 centimeters
per second.
However, to register video signals, a range of 3.2 MHz is required and so a tape
speed of approximately 5 meters per second is prerequisite. This is over 100
times as fast as the tape speed for an audio cassette deck. The required high
recording speed for video recorders are realized by the helical scan system
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without such a high tape speeds. The system basically consists of a revolving
head drum that has a minimum of two video heads
The head drum has a diameter of approximately 5 cm and rotates at a speed of
1500 revolutions per minute. The (12.65 mm) wide videotape is guided
around half the surface of this drum, in a slightly oblique manner. This is
achieved by positioning the head drum at a slight angle. The tape guidance
mechanism then ensures that the tape is guided through the device at a speed of
approximately 2 cm per second (half of the low speed that is used in audio
cassette decks).
In the meantime, the rapidly revolving video heads write narrow tape tracks of
no more than 0.020 to 0.050 mm wide on the tape, next to each other, diagonaltrack which equals half an images The first head writes one track, i.e., the first
field (the odd numbered scanning lines). The second head writes a second track,
i.e., the other half of the image (the second field; the even numbered scanning
lines), which precisely fits in the first image. This corresponds to the interlacing
principle, as applied in television. One full revolution of both heads results in
two diagonal tracks right next to each other, together forming one entire image
scan (a frame)
. This means that two apparently contradictory requirements can be realized
simultaneously; low tape speed of only 2 cm per second and at the same time a
high registration speed (relative tape speed)
Of no less than 5 meters per second. These two requirements make it possible to
record the high video frequencies up to 3.2 MHz at the same time; the low tape
speed gives a time capacity up to three hours.
Azimuth Settings
Compared with early video recorders, modern day video recorders have their
video tracks lying right next to each other. To avoid interference, the two video
heads are angled slightly away from each other. To avoid interference, the two
video heads are angled slightly away from each other. As a result, the video
head openings that transmit the magnetic tracks to the tape, create an angle
between them. The heads are 15 degrees angled in opposite direction, making atotal angle of 30 degrees. This diverted registration angle ensures no problems
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are caused if the heads slightly lose track when playing back and touch the next
track. The heads only register tape information at an angle that precisely
corresponds to the position of the head opening. This system is called the
azimuth recording system. If the video heads stray too far from the track, which
could lead to distorted images, tracking control can correct this.
SYNCHRONIZATION TRACK
The revolutionary speed of the head drum and the video heads needs to
maintain constancy within strict parameters. Moreover, the tracks must be
scanned during playback in precisely the same way as they were recorded. Each
tape track is synchronized at the recording stage by means of field
synchronization pulses. These pulses are generated in the video recorder by a
separate head which are recorded on a separate narrow track at the side of the
video tape. This is called the synchronization, servo or control track.
Video Systems
There are three major video systems in use today:
Video Home System(VHS) Betamax Video Hi8
When the video recorders were first introduced, Philips also developed a system
called V2000. Despite the fact that is a high quality system, it was not
successful in the market. Although Betamax was reasonably successful at first,
its popularity waned and VHS was adopted as the world standard.
Betamax
The Spony Betamax System, launched in 1975, was based on the pre-existing
professional Sony U-matic-system. In the Betamax system, the video tape is
guided along the head drum in aU shape for all tape guidance functions, such as
recording, playback and fast forward/backward. When the cassette is inserted,the tape is guided around the head drum (called threading). Threading the tape
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takes a few seconds, but once the tape is threaded, shifting from one tape to
another can be achieved rapidly and smoothly.
VHS
In VHS, the tape is guided through in an M-shape; so-called M-tape guidance
system. It considered simpler and more compact than the U-shaped system.
Threading is faster and is done every time the tape guidance function is
changed. It is therefore somewhat slower and noisier than the U-system. This
problem is being solved by Quick-start VHS video recorders, which allow
fast and silent changes in tape guidance functions. To avoid excessive wear, M-
tape guidance system recorders are provided with an automatic switch-offfeature, activated some minutes after the recorder is put on hold, which
automatically unthreads the tape. An improvement of the basic VHS system is
HQ(High Quality) VHS.
In the VHS system different starting points were used than in Betamax, such as
track size and relative speed. VHS has rather wide video tracks, but a slightly
lower relative tape speed, and that also counts for the audio tracks. In the end
the result is that there is not too much difference between the sound and image
qualities of both the systems.
SOUND RECORDING
Mono
In case of a mono video recorder, the audio signals which corresponds with the
image is transferred to a separate, fixed audio head.
As in an audio cassette deck, this head writes an audio track in longitudinal
direction of the tape. This is called linear or longitudinal track recording.
The video recorder has two erase heads. One is a wide erase head covering the
whole tape width which automatically erases all existing images,
synchronization and sound information when a new recording is made. The
other erase head is smaller and positioned at the position of the audio track With
this erase head, the sound track can be erased separately, without affecting thevideo information. In this way, separate audio can be added to a video
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recording. This is called audio dubbing, and can be particularly useful when
making your own camera recordings.
The linear audio track does have some restrictions. Due to its low tape speed, it
is not suitable for Hi-Fi recordings. Moreover, the audio tracks are so narrow(0.7 mm for VHS and 1.04 mm for Betamax) that not even stereo sound can be
properly recorded. The frequency range is limited as is the dynamic range
(which relates to the amount of decibels), and the signal-to-noise ratio is not
very high( the signal-to-noise ratio relates to the amount of noise compared to
the total signal. The higher this ratio, less the noise and better the signal will
be).
The sound quality of the mono track can be improved by a noise reduction
system. There is a way to get superior hi-fi stereo sound quality on a video tape
(used in hi-fi video recorders).
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MICROPHONE
Microphone is a transducer, in which sound waves are translated intomechanical vibrations in a thin, flexible DIAPHRAGM these sound vibrations
are then converted by various methods into an electrical signal which varies inamplitude, frequency and phase according to the sound waves.
Characteristics of microphone:
Sensitivity Signal to noise ratio Frequency response Distortion Directivity Output impedance
SENSITIVITY: It is defined as output in milli volts for the sound pressure of1 Microbar.
SIGNAL TO NOISE RATIO: It is define to be ratio in dB of the output tothe output in the absence of sound.
FREQUENCY RESPONSE: It is define by the bandwidth of audio
frequencies in the output of microphone within 1dB of the output at 1000Hz.
DISTORTION: Two types of distortions in microphones
1. Non linear distortion.
2. Phase distortion.
DIRECTIVITY: The angle of the half power points in a polar diagramrepresents Directivity of a microphone.
Three types of directivity:-
1. Omnidirectional.2. Bidirectional.3. Cardioids or heart shaped4. Varieties of microphone: Capacitor microphone Ribbon microphone
Electrets microphone Carbon microphone
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Crystal microphone Laser microphone
LOUD SPEAKER
Loud speaker performs an opposite function to a microphone i.e. it converts
electrical signal into sound wave.
Two types of loud speaker:
Moving coil type. Horn type.MOVING COIL TYPE :(direct radiating loud speaker)
Principle:Interaction between the magnetic field and the current ,resulting ina force working on the movable coil. This force is proportional to the audio
current and hence causes vibratory motion in the coil,which makes a
diaphragm to vibrate and produce pressure variations in air, resulting in
sound waves.
Advantages-
1. Low cost .2. Compact size.
Applications: Radio receiver,TV receiver and in cassette players.
HORN TYPE : ( Indirect Radiating loud speaker)
Principle: same as in moving coil type loud speakerbut instead ofradiating power directly in open space of listenersarea, the power is firstdelivered to the air flared horn and from there to the air in the listeners
area.
Advantages-
1. Better impedance matching.2. High efficiency.Applications: Used in PA system and music concerts.
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EARTH STATION
INTRODUCTION
Communication using conventional technique like coaxial cable and microwave
relay link involves a large number of repeaters and is a costly affair. A better
option for transmission was introduced in 1945 by Arthur C Clark that
worldwide coverage can be obtained by using 3 microwave repeaters placed in a
geostationary.
Orbit at the height of about 36000Km with the period of 24 hrs Satellites
communication.
A satellite communication system consists of a number of earth stations
interconnected through a satellite serving as a microwave relay repeater. At
transmitting earth station, incoming signal modulated an incoming carrier
frequency. The modulated signal is uncovered using a local oscillator to a
microwave radio frequency. RF signal is amplified in a HPA (High Power
Amplifier) and transmitted to the satellite repeater through a transmittingantenna. The signal is received through a satellite antenna and after
amplification is converted to the downlink frequency using a translation
frequency provided by a local oscillator. After amplification to a high power
stage, the signal is down linked through a transmit satellite antenna to the
ground coverage area. At the receive earth station, signal received through the
antenna, after amplification by a sensitive low noise amplifier (LNA) is down
converted and de modulated to recover the base band signal.
EARTH STATION
Pat loss in a satellite system is much higher as compared to terrestrial radio
relay systems and therefore, earth station should have high transmit and receive
antenna.
Gains, high power amplifier on transmit side and more sensitive low noise
amplifier on the receive side.
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A simplified block diagram is shown in fig. information signal, received
through a terrestrial end link, are multiplexed and amplified in a base band
amplifier. Base band amplifier. Base band signals modulate a carrier at an
intermediate frequency. Modulated output is uncovered to a radio frequency. RF
signal in amplified in a HPA.
EQUIPMENT USED IN EARTH STATION
The earth station is generally equipped with following equipments:-
1. High power amplifier.2. Low noise amplifier.3. Up and down converter4. Power system.5. Antenna.6. End links
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BASIC TV SYSTEMS
The most commonly used color TV systems are:
1. NTSC
2. PAL
3. SECAM
NTSCThe work of ntsc was built on the basis of earlier attempts. The ntsc laid
foundations that made television a reality in the united states. At the dawn of
color tv it seemed at first that it was not going to compatible with
monochrome tv& would need special recievers.
a.R, G, B signals are combined in the suitable proportion to obtain Ysignals.
b.Luminance signals & two color signals are combined to obtain two colorsignals.
c. These are quadrature modulated & only sidebands are transmitted withsync signal
d.Color burst is transmitted on a black porch of the lines sync pulse.
Advantages:a. Higher frame rateb. Atomic color editsc. Less inherent picture noise
Disadvantages:
a. Lower number of scan linesb. Smaller luminance signal bandwidth
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c. lower gamma rated. Undesirable automatic featuresPAL
This system is a refinement of NTSC system. It has adopted in our country.
This system was developed in the federal republic of Germany & was
introduced in year 1967.
The three color signals i.e. R , G , B signals are combined to obtain Cr & Y
signals.
Advantages:
Greater number of scan lines.Wider signal bandwidth.
Stable hues.
Higher gamma ratio.
Disadvantages:
More flicker.
Lower signal to noise ratio
Loss of color editing accuracy variable color saturation
SECAM SystemIt is the abbreviation of French term Sequential Colors AMemoroire.
This system is also refinement of NTSC system & entirely different fromPAL system.
Transmission of only one of two signals at a time.
A difference weightage factor is used.
The sub carrier is frequency modulated by color difference signals.
Advantages:
Stable hues & constant saturation.
Higher number of Sean lines.
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Concepts of satellite communication
1. GEOSTATIONARY SATELLITEA geostationary satellite is any satellite which is placed in a geostationary
orbit. Here satellites maintain a constant position relative to the surface of the
earth. Geostationary satellites do this by orbiting the earth approximately 22,300
miles above the equator. This orbital path is called the Clarke Belt. In contrastto geostationary satellites are commonly used for communications & weather
observation. The typical life expectancy of a geostationary satellite is ten to
fifteen years. Because satellites circle the earth at the equator, they are not able
to provide coverage at the Northernmost & Southernmost latitudes.
Geostationary orbits are often referred to as geosynchronous.
2. MEDIUM EARTH ORBITMedium Earth Orbit (MEO) refers to a satellite which orbits the earth at an
altitude below 22,300 miles & above the altitude of low earth orbit(LEO)
satellites. Medium earth orbit terrestrial terminals can be of lower power & use
smaller antennas than the terrestrial terminals of geostationary satellite systems.
However they cannot be as low power or have as small antennas as Low
Earth Orbit terrestrial terminals. Medium Earth systems better round trip time
than geosynchronous orbit systems, but not as low as LOW EARTH ORBIT
systems.
3. LOW EARTH ORBITLEO refers to satellite which orbits the earth at altitudes between 200 miles &
930 miles. low Earth Orbit satellites must travel very quickly to resist the pull
of gravity approximately 17,000 miles per hour because of this, low earth orbit
satellites can orbit the planet in as little as 90 minutes .low earth orbit satellitesystems require several to provide coverage of the entire planet .
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Low earth orbit satellites typically operate in polar orbits.
Low earth orbit satellites are used for applications where a short round trip time
(Rtt) is very important , such as mobile satellite services
Low earth orbit satellites have a typical service life expectancy of five to seven
years.
4. ROUND TRIP TIME
In a satellite network, round trip (RTT)is the time required for a single to travel
from a terrestrial system up to the satellite and back ,or for a signal to travel
from a satellite down to a terrestrial system and back up to the satellite again.
Round trip time is limited by the speed of light .Round Trip TIME will beshortest for a low earth orbit (LEO) satellite and longest for a geostationary
satellite.
An LNB- Low noise Block (also called an LNC-Low Noise Converter), is used
for communication (broadcast) satellite reception .the LNB is usually affixed
either in or on the satellite dish.
5. LNB
The purpose of the LNB is to utilize the super heterodyne effect ; and amplify
and convert a wide block (band) of frequencies. This helps compensate the
signal loss associated with typical coaxial cable at relativety high frequencies.
The term low noise relates to the quality of the 1st
stage input amplifier
transistor ,measured in either called noise temperature units or noise factor
units.Both noise factor and noise figure are easily converted into noise
temperature units .a lower Noise temperature rating is always better (i.e. an
LNB with a noise temperature of 100k is 2x as good as one rated 200k).
6. Linear- LNB
LNB stands for low noise blocker and it is essentially an amplifier on the end
of your small satellite tv dish or communications dish .the linear portion of
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linear LNB stands for the charecterstics of the radio waves that are transmitted
via the satellite in space to your dish (LNB).
7. Types OF Polariization
7.1. Circular polarization
Circular polarization is a characterisitic of radio waves that are transiitted
from a satellite in actual space ,in which the actual radio waves rotate in a
spiral .the spiral of radio waves either rotates clockwise or counters clockwise
.a great analogy of how circular radio waves approach your satellite dish
(LNB)is by picturing an airplane propeller.the airplane propeller follows a
specific path ,but as it approaches the radio wave is spinning vertically.
7.2.Linear polarization
Linear polarization is the characteristic of radio wave in which the radio wave
in which the radio wave rotates on a single horizontal plane.a great analogy is
instead of an airplane propeller the radio wave approaches your satellite dish in
the form of a helicopter .the radio waves spin horizontally .
Besides the direction of the wave being either horizontal or vertical ,satellite
receivers usually only are able to work with one type of polarization .however
,they are able to pick up both signals ,the difference is about 50% loss in signal
strength(-3dB).
8. Fixed Satellite Service(FSS)
Fixed Satellite services (FSS)refers to a satellite services which uses fixed
terrestrial terminals .In other words ,FSSis any satellite service where the
ground station does not change locations frequently .
Consumer satellite television is the most popular application of fixed Satellite
service.
The opposite of FSS is mobile satellite services (MSS).
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9. Mobile Satellite services
Mobile satellite service (MSS)IS A Satellite system which uses portable
terrestrial terminals. MSS terminals may be mounted on a ship , an airplane ,or
an automobile . MSS terminals may be carried by an individual .
The most promising application of Mobile satellite service is portable satellite
telephones which will enable phone services anywhere on the globe.
10. Uplink
Uplink is the signal path from an earth station to a satellite.
Uplink Frequencies
Satellite band Uplink frequencies
C Band 5.925 -6.425
Ku band 14-14.5ghz
Ka band 27.5-31 ghz
11. Downlink
Downlink is the signal path from a satellite towards the earth .
Downlink frequencies
Satellite band Downlink frequency
C Band 3.7-4.2 ghz
Ku band 11.2 -12.7 ghz
Ka band 18.3-20.2 ghz
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12. Elevation
Elevation is the angular measurement of a satellite above the horizion .it is
measured in degrees .A satellite which is higher in the sky will have a greater
elevation than one which is close to the horizon .
A satellite exactly level with the horizon would have an elevation of 0 degrees
.a satellite with an elevation of 90 degrees would be directly overheaded .
Knowing the elevation of a satellite from your location is critical to being able
to successfully point a satellite antenna to it.
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STUDIO CAMERA
It is fited on the tripod.it has a cable known as the triaxial cable ,both videoand audio signal pass through this cable.15 V supply is required .
Camera has three parts:
1. Lens2. Transducer3. ElectronicsLENS:- use to form optical image in shooting .
TRANSDUCER:-is used to convert the video signal into the electrical
signal.
ELECTRONICS:-is used to convert the electrical signal into the
complete composite video signal.
MAIN CONTROLS ON CAMERA:-
Focus Zoom Iris
General process of the shooting from studio camera:-
Filter AWB Focus and zoom Electronic shutter speed VTR
The studio camera is the backbone of the television industry. It is mounted on a
dolly pedestal so the camera operator may wheel it to different locations withrelatives ease during shot changes. Television techniques monitor and adjust the
video levels of the studio camera with the camera control unit.
Video Noise increases in proportion to the video gain previously described.Even the best cameras will deliver noisy pictures under low-light levels. A
noisy picture has a great amount of snow, or white vibrating spots, in the
picture. This occurs when the video signals produced by the pickup tube are not
strong enough to override the electronic interference the system usuallygenerates .At 12dB gain, the system is generating more electronic interference-
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and more videos camera noise. Having covered the basic operation and
electronic characteristics of a video camera, we now examine the types of video
cameras you will work with at NBS detachments.
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VIDEO TAPE
Tape Characteristics
There are three different characteristics which contribute to total tape quality:
mechanical, magnetic and electro-acoustical characteristics.
The Mechanical characteristics are related to the carrier, the thickness of the
tape, the slack resistance and especially the tape guidance system behavior. The
latter is a matter of cassette housing, the precision of which should meet the
highest requirements, just like the tape itself. The tape guidance should always
be tight but flexible, without friction, but also without getting tangled. The
vulnerable edges of the thin tape should not wear, even at the microscopic levelor after many hours of use. Damage at the edges will have a negative effect on
the sound production right next to the edges. Some tapes have a thin layer o
coating on the back, to reduce the friction to an absolute minimum.
Close-up of the Video Tape
As far as the magnetic characteristics are concerned, the tape should be resistant
to demagnetize i.e. should have a high coercive force. The reminisce, which is
the magnetism that remains on the tape after the magnetism generating field is
taken away, is another essential characteristic. For audio tapes a high reminisceis a pre requisite in order to produce a wide dynamic range, but in case ofvideotapes high reminisce is not required, because of the high registration
speed for five meters per second. The reminisce of an average normal
videotape has more or less the same value as that of a conventional Ferro-oxide
audio cassette.
The electro-magnetic characteristics largely determine the quality of the tape.
These Characteristics are:
Signal to noise ratio(S/N) Color reproduction Sound Quality
The Quality of the color reproduction (chrominance:C) is expressed by the color
signal to noise ratio(color S/N).The higher this value, the better the hues arereproduced and the deeper the colors are. Here too, dB is the standard, but the
latest tapes also achieve higher values, up to +5 dB. The video and color S/N
relate directly to how equally the particles are spread on the tape surface.
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Hf or high frequency output level, also called Sensitivity, express the capability
of the tape to register the high video frequencies. If this is not done properly, it
reference standard. The HF level relates to the size of the particles.
Tape and Purpose
Different applications require tapes of different quality. It makes sense to use
the right tape for the right purpose. There is no need to purchase a high quality
tape, if the application does not require it. For almost all normal application the
standard(or High Standard)tape will suffice.
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VISION MIXER
A Vision Mixer (also called video switcher, video mixer or production switcher)is a device used to select between several different video sources and in some
cases composite (mix) video sources together and add special effects. This is
similar to what a mixing console does for audio.
Explanation
Typically a vision mixer would be found in a professional television production
environment such as a television studio, cable broadcast facility, commercial
production facility, remote truck/OB van or linear video editing bay. The term
can also refer to the person operation the device.Vision mixer and video mixer are most exclusively European terms to describeboth the equipment and operators. In the United States, the common name for a
device of this kind is (video) production switcher and the common name for the
operator of the device is known as technical device.
Software vision mixers are also available.
Capabilities usage in TV Productions
Besides hard cuts (switching directly between two input signals), mixers canalso generate a variety of transmissions, from simple dissolves to pattern wipes.
Additionally, most vision mixers can perform keying operations and generate
color signals(called mattes in this context).Most vision mixers are targeted atthe professional market, with newer analog models having component video
connection and digital ones using SDI. They are used in live and videotaped
television productions and for linear video editing, even though the use of
mixers in video editing has been largely supplanted by computer based non-
linear editing.
Older professional mixer worked with composite video inputs. There is still a
number of consumer video switchers with composite video, S-Videos or evenFireWire available. These are often used for VJing, presentations, and small
multi-camera productions.
Operation
The main concept of a professional vision mixer is the bus,basically a row of
buttons with each button representing a video source.Pressing such a button will
set the video out of the bus.Older video mixers had two equivalentbusses(called the A and B bus,such a mixer is known as an A/B mixer).One of
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these busses could be selected as the main out(or program)bus. Most Modern
mixers, however, have one bus that is always the program bus,the second main
bus being the preview bus. These mixers are called flip-flop mixers, since the
selected source of the preview and the program buses can be exchanged. Both
preview and program bus usually has their own video monitor.
Another main feature of a vision mixer is the transition lever, also called a T-bar
or Fader Bar. This lever, similar to an audio fader, creates a transition betweentwo buses. Note that in a flip-flop mixer, the position of the main transition
lever does not indicate which bus is active, since the program bus is always the
active or hot bus. Instead of moving the lever by hand, a button (commonlylabeled mix auto or auto Trans) can be used, which performs the
transition over a user-defined period of time.
The Third bus on a vision mixer is the key bus. A mixer can actually have morethan one of these, but they usually share only one set of buttons. Here, a signal
can be selected for keying into the program. The image that will be seen in theprogram is called the fill, while the mask used to create the keys translucence is
called the source.