vijay kaushik
TRANSCRIPT
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CHAPTER 1
INTRODUCTION TO AIR
1.1INTRODUCTION
All India Radio (abbreviated as AIR), officially known as Akashvani is the radio
broadcaster ofIndia and a division of Prasar Bharati (Broadcasting Corporation of
India), an autonomous corporation of the Ministry of Information and Broadcasting,
Government of India. Established in 1936, today, it is the sister service of Prasar
Bharati's Doordarshan, the national television broadcaster. The word Akashvani was
coined by Professor Dr. M.V. Gopalaswamy for his radio station in Mysore during
1936.
All India Radio is one of the largest radio networks in the world. The headquarters is at
the Akashvani Bhavan, New Delhi. Akashvani Bhavan houses the drama section, the
FM section and the National service. The Doordarshan Kendra (Delhi) is also located
on the 6th floor of Akashvani Bhavan.
A famous thing that happened with the AIR was that during his regular broadcasts from
the Azad Hind Radio, Subhash Chandra Bose used to refer to the pre-independence
AIR as Anti Indian Radio.
The first radio program in India was broadcast by the Radio club of Bombay in June
1923. It was followed by the setting up of a broadcasting service that began
broadcasting in India in July 1927 on an experimental basis at Mumbai and Kolkata
simultaneously under an agreement between government of India and a private
company called the Indian Broadcasting company ltd. The operation of AIR began
formally in 1936, as a government organization, with clear objectives to inform,
educate and entertain the masses.
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http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Prasar_Bharatihttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/New_Delhihttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/Delhihttp://en.wikipedia.org/wiki/Azad_Hind_Radiohttp://en.wikipedia.org/wiki/Subhas_Chandra_Bosehttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Prasar_Bharatihttp://en.wikipedia.org/wiki/Government_of_Indiahttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/New_Delhihttp://en.wikipedia.org/wiki/Doordarshanhttp://en.wikipedia.org/wiki/Delhihttp://en.wikipedia.org/wiki/Azad_Hind_Radiohttp://en.wikipedia.org/wiki/Subhas_Chandra_Bose -
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Serve the rural, illiterate and underprivileged population, keeping in mind the
special needs and interests of the young, social and cultural minorities, the tribal
population, and of those residing in border regions, backward or remote areas.
Promote national integration.
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CHAPTER 2
STUDIO CHANNEL IN A TYPICAL AIR STATION
2.1 INTRODUCTION
The broadcast of a programme from source to listener involves use of studios,
microphones, announcer console, switching console, telephone lines / STL and
Transmitter. Normally the programmes originate from a studio centre located inside the
city/town for the convenience of artists. The programme could be either live or
recorded. In some cases, the programme can be from OB spot, such as commentary of
cricket match etc. Programmes that are to be relayed from other Radio Stations are
received in a receiving centre and then sent to the studio centre or directly received at
the studio centre through RN terminal/telephone line. All these programmes are then
selected and routed from studio to transmitting centre through broadcast quality
telephone lines or studio transmitter microwave/VHF links. A simplified block
schematic showing the different stages is given in Fig. 2.1.
Fig. 2 .1 Simplified block schmatic of broadcasting chain
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2.2 Studio Centre
The Studio Centre comprises of one or more studios, recording and dubbing room, a
control room and other ancilliary rooms like battery room, a.c. rooms, switch gear
room, DG room, R/C room, service room, waiting room, tape library, etc. The size of
such a centre and the number of studios provided depend on the programme activities
of the station. The studio centres in AIR are categorised as Type I, II, III and IV. The
number of studios and facilities provided in each type are different. For example a type
I studio has a transmission studio, music studio with announcer booth, a talks studio
with announcer booth, one recording/dubbing room and a Read Over Room. Type II
has one additional drama studio. The other types have more studios progressively.
2.3 Broadcast Studio
A broadcast studio is an acoustically treated room. It is necessary that the place where
a programme for broadcast purposes is being produced should be free of extraneous
noise. This is possible only if the area of room is insulated from outside sound.
Further, the microphone which is the first equipment that picks up the sound, is not able
to distinguish between wanted and unwanted signals and will pick up the sound not
only from the artists and the instruments but also reflections from the walls marring the
quality and clarity of the programme. So the studios are to be specially treated to give
an optimum reverberation time and minimum noise level. The entry to the studios is
generally through sound isolating lobby called sound lock. Outside of every studio
entrance, there is a warning lamp, which glows Red when the studio is ON-AIR.
The studios have separate announcers booths attached to them where first level fading,
mixing and cueing facilities are provided.
2.4 Studio Operational Requirements
Many technical requirements of studios like minimum noise level, optimum
reverberation time etc. are normally met at the time of installation of studio. However
for operational purposes, certain basic minimum technical facilities are required for
smooth transmission of programmes and for proper control. These are as follows:
Programme in a studio may originate from a microphone or a tape deck, or a
turntable or a compact disc or a R-DAT. So a facility for selection of output of any
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of these equipments at any moment is necessary. Announcer console does this
function.
Facility to fade in/fade out the programme smoothly and control the programme
level within prescribed limits.
Facility for aural monitoring to check the quality of sound production and sound
meters to indicate the intensity (VU meters).
For routing of programmes from various studios/OB spots to a central control room,
we require a facility to further mix/select the programmes. The Control Console in
the control room performs this function. It is also called switching console.
Before feeding the programmes to the transmitter, the response of the programmeshould be made flat by compensating HF and LF losses using equalised line
amplifiers.(This is applicable in case of telephone lines only)
Visual signalling facility between studio announcer booth and control room should
also be provided.
If the programmes from various studios are to be fed to more than one transmitter, a
master switching facility is also required.
2.5 Mixing
As already mentioned, various equipments are available in a studio to generate
programme as given below:
Microphone, which normally provides a level of 70 dBm.
Turntable which provides an output of 0 dBm.
Tape decks which may provide a level of 0 dBm.
CD and R-DAT will also provide a level of 0 dBm.
The first and foremost requirement is that we should be able to select the output of any
of these equipments at any moment and at the same time should be able to mix output
of two or more equipments. However, as we see, the level from microphone is quite
low and need to be amplified, so as to bring it to the levels of tape recorder/ tape decks.
Audio mixing is done in following two ways:
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i) Required equipments are selected and then outputs are mixed before feeding to
an amplifier. This is called low level mixing (Fig. 2.2). This is not commonly
used now days.
Fig. 2.2 Low level mixing
ii) Low-level output of each equipment is pre-amplified and then mixed. This is
called high level mixing.
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Fig. 2.3 High level mixing
Low level mixing system may look economical since it requires one single pre-
amplifier for all low level inputs, but quality of sound suffers in this system as far as
S/N ratio is concerned. Noise level at the input of best designed pre-amplifier is of the
order of 120 dBm and the output levels from low level equipment 70dBm. In low
level mixing, there is signal loss of about 10 to 15 dB in mixing circuits. Therefore, the
S/N ratio achieved in low level mixing is 35 to 40 dB only.
High level mixing system requires one pre-amplifier in each of the low level channels
but ensures a S/N of better than 50 dB. All India Radio employs High level mixing.
2.6 Announcer Console
Most of the studios have an attached booth, which is called transmission booth or
Announcer booth or play back studio. This is also acoustically treated and contains a
mixing console called Announcer Console. The Announcer Console is used for mixing
and controlling the programmes that are being produced in the studio using artist
microphones, tape playback decks and turn tables/CD players. This is also used for
transmission of programmes either live or recorded.
The technical facilities provided in a typical announcer booth, besides an Announcer
Console are one or two microphones for making announcements, two turn tables for
playing the gramophone records and two playback decks or tape recorders for recorded
programmes on tapes. Recently CD and Rotary Head Digital Audio Tape Recorder (R-
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DAT) are also included in the Transmission Studio. Audio block schematic of
transmission studio is shown in Fig. 2.4.
Fig. 2.4 Announcer Console
2.7 Control Room
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For two or more studios set up, there would be a provision for further mixing which is
provided by a control console manned by engineers. Such control console is known as
switching console. Broad functions of switching console in control room are as follows:
Switching of different sources for transmission like News, O.Bs. other
satellite based relays, live broadcast from recording studio.
Quality monitoring.
Signalling to the source location.
Communication link between control room and different studios.
Audio block schematic of control room is shown in Fig.2. 5.
Fig. 2.5 Block Schematic of Control Room
CHAPTER 3
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TRANSMITTER
The electronic equipment used to produce radio Frequency for radio transmission is
called transmitter.
The function of the transmitter is to generate the R.F carrier of proper frequency and of
sufficient power. The output of a transmitter is applied to the antenna, which radiates
the signal into space.
Basically, a transmitter consists of two sections.
1. RF section
2. AF section
3.1 RF Section
a) Master oscillator: - An oscillator is the heart of a transmitter. It generates radio
frequency voltage with high degree of frequency stability. Crystal oscillator hasmaximum degree of frequency stability and it is used as master oscillator. The
frequency generated depends upon the thickness of the crystal.
b) Buffer amplifier: - A buffer amplifier is used to isolate an oscillator so that the
frequency of the oscillator is not affected by the operation of succeeding stages.
c) Int. Power amplifier: - It is a class c amplifier used to increase the power level and
deliver to succeeding stages.
d) Power amplifier: - This is a plate modulated class c amplifier. Its O/P is fed to the
transmitting antenna through a feeder line.
3.2 AF Section
This section is designed to convert the information Into corresponding electrical
variations of sufficient magnitude. The electrical signals are amplified, modulated and
fed to the power amplifier.
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passed to the low pass filter fixed in baseband interconnection unit. The signal is fed to
the VCO unit where oscillator frequency of 415 MHz and at the same time music is
modulated. This frequency is multiplied by 4 times in RF amplifier. The exact
frequency 1440 MHz is adjusted in reference oscillator. The signal is further amplified;
the power of transmitter is connected to the antenna filter which is then given to
antenna port. When power of one transmitter is connected to antenna port, the other
transmitter is connected to dummy load (stand by).
3.4 RECEIVER PATH
The transmitted signal at the other station is received through antenna feeder cable .It is
then passed through antenna filter tuned for receiver frequency. The signal is then given
to RF front-end amplifier where it amplified by 12 db. The RF amplifier is consists of a
mixer, local oscillator and a multiplexer. The local oscillator frequency is generated in
VCO unit is fed to the RF amplifier (+10 dbm) and then it is multiplied by 4 times for
mixing with received frequency. The output I.F. 35 MHz with a gain of 20-25 db is then
given to the I.F. amp. Where it is amplified and demodulated. The detected music
signal is then amplified, filtered and then given to line transformer.
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Fig 3.2 Reciever
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3.5 SPECIFICATIONS
1.Frequency range -1427- 1660 MHz (1440 MHz working)
2. FM modulation.
3. 3 Watt power output.
4. Output impedance 50 unbalanced.
5. Total harmonic distortion
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CHAPTER 4
MICROPHONES
4.1 Introduction
Microphone plays a very important role in the art of sound broadcasting. It is a device
which converts accoustical energy into electrical energy. In the professional
broadcasting field microphones have primarily to be capable of giving the highest
fidality of reproduction over audio bandwidth.
4.2 Microphone Classification
Depending on the relationship between the output voltage from a microphone and the
sound pressure on it, the microphones can be divided into two basic groups.
4.2.1 Pressure Operated Type
In such microphones only one side of the diaphragm is exposed to the sound wave. The
output voltage is proportional to the sound pressure on the exposed face of the
diaphragm with respect to the constant pressure on the other face. Moving coil, carbon,
crystal and condenser microphones are mostly of this type. In their basic forms, the
pressure operated microphones are omni-directional.
4.2.2 Velocity or Pressure Gradiant Type
In these microphones both sides of the diaphragm are exposed to the sound wave. Thus
the output voltage is proportional to the instantaneous difference in pressure on the twosides of the diaphragm. Ribbon microphone belongs to this category and its polar
diagram is figure of eight.
4.3 Types of Microphones
There are many types of microphones. But only the most common types used in
broadcasting have been described here.
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4.3.1 Dynamic or Moving Coil MicrophoneThis is common broadcast quality
microphone which is rugged and can be carried to outside broadcast/recording etc. It
consists of a strong permanent magnet whose pole extensions form a radial field within
a round narrow gap. A moving coil is supported within this gap and a dome shaped
diaphragm usually of aluminium foil is attached to the coil. The coil is connected to a
microphone transformer whose secondary has sometimes tapings to select proper
impedance for matching.
With sound pressure changes, the diaphragm and coil move in the magnetic field,
therefore, emf is induced in the speech coil, which is proportional to the incoming
sound.
The primary impedance of the matching transformer is generally high (5 to 6 times of
thespeech coil impedance so that low frequencies are not lost and rising impedance
frequency characteristic is avoided as best as possible. The resonant frequency is
generally damped with special arrangements of absorption in acoustic cavity,
Bass/boost arrangements are provided by an equalising tube connecting the rear side of
diaphragm i.e. inside of microphone with the atmosphere. The diameter and length of
the tube is critically adjusted for achieving good frequency response. The output of the
microphone is 65 to 68 dBv and various shapes of the body make it OMNI UNI or
SEMI directional.
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Fig.4. 1 Dynamic Microphone (Moving coil)
4.3.2 Ribbon/ Velocity Microphone
Corrugated aluminium foil about 0.1 mm thick forms a ribbon which is suspended with
in two insulated supports. The ribbon is placed within the extended poles of a strong
horse shoe magnet. The ribbon moves due to the difference in pressure (at right angles
to its surface) i.e. from the front or rear of the mike. There exists the maximum
pressure difference between the front and rear of ribbon because of maximum path
difference.
The sound does not develop any pressure gradient when it comes from the sides of the
microphones because there is no path difference. It reaches the front and rear of ribbon
at the same time, hence no movement of ribbon. Thus, this microphone is bi-directional
and follows figure of eight directivity pattern with no pick up from sides.
Such a microphone has a clarity filter. This is a series resonant circuit at low
frequencies across the primary of microphone transformer. When switched to the
Talk or Voice position, the response is modified cutting down low frequencies by
about 8 dB at 50 Hz. This filter should therefore not be in circuit during music
performances.
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This microphone delivers 80 dBv with a very good frequency response. The output
impedance of this microphone is high. The popular method of providing d.c. voltage to
the condenser is known as Phantom Powering .
Variable directivity capacitor microphones are becoming popular these days.
Fig 4.3 Condenser microphone
4.3.3 Electret Microphone
It is a modified form of condenser microphone in which the polarising voltage is
avoided. In fact a plastic polymer containing metallic dust keeps the metal particles
permanently charged with in the plastic insulation and such a polymer within the
diaphragm foil or fixed plate delivers the electrical signal on the principle of the
condenser mike. The hissing noise gets avoided since there is no external polarising
resistor as a load. The microphone has high impedance and is generally having FET
pre-amplifier. The microphone costs very little but developes excellent quality designs
in many forms.
Perhaps this microphone is going to flourish most in comparison to all other.
4.4 Special Microphones
4.4.1Combined Microphones
More than one microphone is placed within the same unit to achieve a particular
purpose e.g. Western Electric 639 combines a dynamic (OMNI) microphone with a
Ribbon (Bi-directional) microphone to get a cardioid pattern. Thus we have a choice of
three patterns by switching in either or both units. AKG 202 consists of two dynamic
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microphones to obtain a flat frequency response. In fact the dual arrangement is
coaxially mounted with the smaller in the front and the bigger diaphragm in the rear in
a single housing. The two are connected electrically with a phase correcting network.
The response shows a cross-over at 500 Hz. One unit is adjusted for frequencies above
and the other unit for frequencies below to achieve good response. A switch is provided
to cut down low frequencies with 50 Hz dropping upto 20 dB with respect to 1 kHz.
The sensitivity is 53 dBv and mike is uni-directional resembling a cardioid pattern.
4.4.2 Lip Ribbon Microphone
It is also called noise-cancelled mike since the ribbon even if held close, does not pick
up breathing noises due to a guard in between. The stainless steel mesh acts as a wind
shield. The design and other features resemble the ribbon mike.
4.4.3 Lapel Microphone
Both carbon and ribbon types are available. The microphone is very small and light-
weight and is suspended around the neck keeping the mike just below the chin. It is
most suitable for running commentary or in a lecture.
4.4.4 Contact Microphone
It is generally a dynamic microphone of lower sensitivity. It is normally placed close to
the source of sound, when it is not supposed to pick up other stray noises.
4.4.5 Gun Mike
It has two forms, (short gun and long gun). A dynamic mike placed at the end of a
perforated tube extends its directivity in the front. The short gun about 18 long can
pick up a talk from about 10 feet distance and a long gun with a tube of about 3 feet
length can pick up sound from a distance of about 20 to 25 feet. The quality suffers but
is
intelligible. This microphone is useful when sound from a distant spot is to be picked
up. An example is the picking of the sound of bat hitting a cricket ball.
4.5 Important Characteristics
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Important characteristics of the microphones are as under :
4.5.1Frequency Response
This characteristic indicates the relative signal output voltage of a microphone at
different frequencies for a constant acoustic level input at all the frequencies. These
days it is possible to attain an almost flat frequency response over the audio range of 20
Hz to 20 kHz. Frequency response of a microphone depends on :-
Direction of arrival of sound, and
Distance between the source and the microphone
Frequency response specified by manufacturers is generally that obtained by using a
calibrated sound source at a specified distance in an anechoic test room or duct. The full
20 Hz to 20 kHz spectrum may not be necessary or required in some applications. In
some microphones a roll off at low frequency end is provided to cut off low frequency
noise. If a microphone covers the essential audio range 100 Hz to 7 kHz within + 1 dB
it is considered to be a broadcast quality microphone.
4.5.2 Directivity
Microphones can be designed either to respond equally to sounds from an angle or to
discriminate those arriving from specific directions. Microphones which respond
equally at all angles are called omni-directional. The microphones which pick up
equally from front and rear and have very little pick up equally from sides are called Bi-
directional and have a polar diagram as figure of eight. The microphones which pick up
maximum from the front with slight reduction in the sides and very less pick up from
the rear are called Cardioid (means heart shape). Microphones directivity is often a
principal reason for choosing between different models for particular applications.
4.5. 3Sensitivity
The ability to pick up weak sound and to deliver more electrical signal determines the
sensitivity. It is measured in dBs below 1 volt as the electrical output from a
microphone when a standard sound pressure of one microbar i.e. 1 dyne per sqr. cm. is
applied at the diaphragm of the microphone. The velocity or Ribbon Mike gives an
output of about 70 dBv and dynamic Mike 65 to 68 dBv.
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4.5.4 Distortion
The microphone should not add or subtract the amplitude or frequency of the sound
during conversion. The maximum change in complex wave form cannot be measured,
as such the tests are conducted under sine wave conditions and within the broadcasting
range offrequencies, the distortion is not allowed to exceed beyond a specified value,
typically less than 0.5% at 1000 Hz.
4.5.5Termination Impedance
The microphone must have a proper impedance and a balanced or unbalanced output
suited to the pre-amplifier. In the broadcast chain the microphone lines cover long
distances, therefore, the impedance is chosen in the range of 50 ohms to 60 ohms at the
microphone terminals. The commercial microphones in public address system do not
require lengthy mike cables and prefer high electrical output across high impedance
which is generally above 5 k ohms.
Moreover broadcasting microphones use balanced output with Mike cable containing
two live conductors and a earth shield commercial microphones have unbalanced output
connected to single core of mike cable which is shielded.
The above arrangement used in commercial practice is not suitable for broadcasting
set-up mainly because, the noise pick up on unbalanced lines and high impedance of
circuit become objectionable and prone to loss of high frequencies when the cable is
long. Therefore, the termination of broadcast type of microphone will have balanced
output with impedance in the range of 50, 70, 100, 200, 300 or 600 ohms to suitably
match the input impedance of the pre-amplifier.
In some modern microphones, the pre-amplifier is an integral part of microphone and
high level output is brought out. In another modern variety the cable is not used at all.
The sound picked by microphone is modulated on miniature FM transmitter and a
power of 100 mW or so is radiated. Such microphones do not have any termination but
an antenna and are called cordless or RF microphones or Radio Microphones, Sound
signals are available on demodulation at the receiver for mixing with other
microphones.
4.6 Guidelines for choice and placement of Microphones
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4.6.1 Choice of Microphone
a) Frequency response : A flat frequency response (50-10000 Hz) is preferred for music
and drama programmes whereas a gentle roll on low frequency side below 200 Hz is
preferred for talks and announcement.
For OBS, the above requirements can be somewhat relaxed further.
b) Directivity pattern: For announcements, in modern practice cardioid microphones
without proximity effect (like AKG-2-way cardioids are preferred though at one time
we were using omni-directional microphones.
For talks/discussions and music cardioid or bi-directional microphones are preferred.
An omni directional microphone can sometimes be used for discussions. For OBs
having PA system cardioid with very good front to back ratio only should be used. For
open air discussions/interview etc. omni directional microphones can be used. Special
type of microphones such as lavalier microphones should be used where situation
demands use of such a microphone
4.6.2 Placement of Microphones
Placement of microphone has important bearing on the quality of its output. A few
general guidelines given in the following paragraphs should help in improvement of
programme production.
a) As far as possible, microphones should be placed with its 0o axis facing the source of
sound to avoid off axis colouration.
b) Phasing of Microphone
Whenever two or more microphones are used with their outputs mixed together, it
should be ensured that their outputs are in phase. A simple test for above is as follows:
Fade in one microphone and monitor the sound level output for any programme. With
all other controls undisturbed, fade in the second microphone. If the outputs of the two
microphones are in phase, an increase in the overall sound level output should result. If
there is loss of sound level and deterioration in quality (attenuation of low frequencies)
leads of one of the microphones should be reversed in the microphone socket and the
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test repeated. In the case of ribbon bi-directional microphone the same result can be
achieved by rotating the microphone through 180o.
If more than two microphones are used simultaneously second and third microphones
and so on should also be tested similarly.
c) Working Distance
Whenever a directional microphone is kept fairly close to the source of sound low
frequencies in the output of the microphone may get disproportionately boosted thereby
giving rise to boomy sound. This effect known as proximity effect is most pronounced
in bi-directional ribbon microphones such as RCA 44 Bx. This effect should be
normally avoided by placing the microphones fairly away (30-45 cm) from the source
of sound. However, proximity effect can be used to advantage for special sound effects
(warmth, intimacy, etc.) by placing the microphones closer to the sound source.
It may however, be noted that some of the directional microphones such as AKG type
D200, D202, D222 and D224 do not exhibit the above proximity effect and hence these
can be used closer to the source of sound
. In outdoor locations because of the higher ambient noise level, the working distancemust be kept less than the corresponding distance when working indoors.
d) Balancing
There are two aspects of balancing in microphones :
- When single microphone is used for more than one source of sound, distance between
the microphone and different sources of sound be adjusted suitably so that the desired
sound levels are achieved from all sources.
- When a microphone is used in an enclosed space (as in studios) reverberant energy or
indirect sound plays an important role in the quality of the output of the microphone.
When the microphone is placed close to the sound source, direct sound is predominant
and the output appears to be dry . On the other hand
when the microphones is far away from the sound source the reverberant (indirect)
sound is predominant and the output lacks in clarity. Hence for proper balancing ofdirect to indirect sound pick up the microphone should be kept at an optimum distance.
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In an acoustically treated room like a studio, a normal distance of 30-45 cms, is found
to be satisfactory. An important Note : Balancing should always be judged by
monitoring the microphone output at a location outside the studio (say from the
attached announcers both or recording room etc.) Recording of a programme should not
be started till a proper balance is achieved.
e) Microphone should not be placed very close to a reflecting surface such as announcer
table top or bare walls of a studio.
f) A talker should not hold the script between his face and the microphone otherwise
shadowing effect will occur at high frequencies.
g) For placement of microphone for pick up of musical instruments following
guidelines may be kept in view :
- For stringed instruments (violin, sitar, sarangi, etc.) 0o axis of the microphone should
be preferably placed normal to the front face of the instrument.
- For instruments with large sound output (like drums and other percussion and bass
instruments) the microphone should be placed well away from the source of sound.
- For wood wind instruments where the instrument is not particularly directional (such
as flute) the microphone may be placed about 60 cms. away so that instrument does not
speak straight at it.
h) If a source of sound is placed on the dead axis of a microphone it sounds as if this
source of sound is placed at a considerable distance from the microphone. This effect
could be made use of in a drama production.
i) Talking very close to a microphone may cause blasting on explosive consonants such
as P . Hence it should be avoided.
4.7 Care of Microphones
Microphone needs a very careful handling as it is a delicate equipment. Some important
precautions in handling microphones are listed below :
- Do not allow it to fall or get any type of knock.
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- If microphones are to be carried, pack each one of them separately in a cushioned box
and keep them apart. Do not pack any tools, magnetic substances or types in the same
box.
- Do not open a microphone nor test the continuity of the microphone or its transformer.
If the microphone is connected to the mike cable, test the mike cable continuity only
after isolating the mike. Multimeter current is enough to disturb the ribbon or
diaphragm resulting in major damage.
- While testing the mike in a studio do not speak very loud nor blow into it. Speak
gently, rubbing the mike on its side in a gentle manner and announce its type, the stand
on which it is resting and the channel to which it is connected, so that the control
engineer may know the complete details of the test being given. He may verify that the
sound picked up and the rubbing noise are both from the same microphone under test.
- If there are folk singers or loud instruments like Nagaswaram/drums etc. present, keep
sensitive mikes at a distance from the source of sound.
- Do not allow the microphones to get wet during rain. Use a wind shield or PVC
coverings depending upon the situation.
A list of different types of microphones used in All India Radio & Doordarshan is given
in table 1. Table 1 Different Types of Microphones
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CHAPTER 5
THE DOORDARSHAN KENDRA DELHI
5.1 AN OVERVIEW
Doordarshan Kendra DELHI is part of the DD India, the largest television network inthe world. Doordarshan with over 5 high power TerrestrialTransmitters,62 low
power,5 very low power transmitter and 3 production centers serve DELHI Inaugurated
on 28th may 2000 by the then broadcast minister mr. ARUN JATELY. Doordarshan
Kendra delhi currently produces and telecasts 168 hrs of local programmes per week.
Now more than 85 per cent of the 60,385,118 populations of M.P., With the
introduction of DTH almost cent percent of the population can now receive DDK delhi
programmes without cable connection. Doordarshan studios have been established at
Gwalior, Bhopal and Indore to foster regional diversity. People all over India are
watching Doordarshans programmes. It is also received in 64 countries spread over
the continents of Asia, Africa, Europe, Australia and America.
5.2 TV Scenario in Delhi.
As per the 2001 census there are 60,385,118(5.5 million) house holds in M.P., 74.9 per
cent of them are in the rural sector (44,42550) the remaining 25.1 per cent (13,52656)
are in the urban sector. In 2001, 38.8 per cent of the households owned TV sets . Of
these 62.3 per cent were in rural areas and the remaining 37.7 per cent in urban areas.
Even if we estimate 10 15 per cent growth per annum. Of these estimated 3 million
TV households 40 45 per cent is estimated to have cable connection i.e., 1.3 million
and the remaining 1.7 million are without cable connection, and totally depend on DDK
Bhopal for their TV viewing. The introduction of DTH, DD Direct Plus has
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considerably increased DD viewership in MP. From the available sales estimates of set
top boxes and receivers it is estimated that MP has 3 to 4 lakh DTH households.
5.3 TECHNICAL INFORMATION OF TRANSMITTING
FACILITIES AT DDK, DELHI:
Doordarshan Kendra, delhi is equipped with studio, two terrestrial transmitters and one
digital up-link station. The two terrestrial transmitters are of 10 KW power each.
One is for DD-National and the other is for DD-News telecasting.
5.4 TERRESTRIAL TRANSMITTER PARAMETERS:
DD-NEWS :CH #31 (VHF-Band-III) Pictures IF: 551.25 MHz, Sound IF: 556.75
MHz
DOWNLINK PARAMETERS OF DD-NEWS SATELLITE
PROGRAMMES
Latitude Co-ordinates 23 1425 ( North )
Longitude Co-ordinates 77 2320 ( East)
Main Sea Level 300 Mtrs.
Antenna Hight 100 Mtrs.
Effective height of the antenna above
sea level400 Mtrs.
Peak power (Both DD-I & DD-II) 10 KW each
Black power 06 KW each
Antenna gainArt Direction8 Db wide band,
Jampro Antenna.
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FREQUENCY OF OPERATION
Band DD-II
Band III
Channel 31(.)
Video carrier 551.25 MHz
Audio carrier 556.75 MHz
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CHAPTER 6
SYSTEM FUNDAMENTAL OF MONOCHROME
AND COLOUR TV
6.1Picture formation
A picture can be considered to contain a number of small elementary areas of light or
shade which are called PICTURE ELEMENTS. The elements thus contain the visual
image of the scene.
In the case of a TV camera the scene is focused on the photosensitive surface of pick
up device and a optical image is formed. The photoelectric properties of the pick up
device convert the optical image to a electric charge image depending on the light and
shade of the scene (picture elements). Now it is necessary to pick up this information
and transmit it. For this purpose scanning is employed. Electron beam scans the charge
image and produces optical image. The electron beam scans the image line by line and
field by field to provide signal variations in a successive order.
The scanning is both in horizontal and vertical direction simultaneously.
The horizontal scanning frequency is 15,625 Hertz.
The vertical scanning frequency is 50 Hz.
The frame is divided in two fields. Odd lines are scanned first and then the even lines.
The odd and even lines are interlaced. Since the frame is divided into 2 fields the
flicker reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the
same as power supply frequency)
6.2 Number of TV Lines per Frame
If the number of TV lines is high larger bandwidth of video and hence larger R.F.
channel width is required. If we go for larger RF channel width the number of channelsin the R.F. spectrum will be reduced. However, with more no. of TV lines on the
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screen the clarity of the picture i.e. resolution improves. With lesser number of TV lines
per frame the clarity (quality) is poor.
The capability of the system to resolve maximum number of picture elements along
scanning lines determines the horizontal resolution. It means how many alternate black
and white elements can be there in a line. Let us also take another factor. It is realistic
to aim at equal vertical and horizontal resolution. Therefore, the number of alternate
black and white dots on line can be 575 x 0.69 x 4/3 which is equal to 528.
It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264 cycles
are there during 52 micro seconds. Hence the highest frequency is 5 MHz.
MHz552
10264f6
highest =
=
Therefore the horizontal resolution of the system is 5 MHz.A similar calculation for
525 lines system limits the highest frequency to 4 MHz and hence the horizontal
resolution of same value.
In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz.
6.3The PAL Colour Television System
5.3.1 The Colour Television
It is possible to obtain any desired colour by mixing three primary colours i.e. Red,
Blue and green in a suitable proportion.
6.3.1 Additive Colour Mixing
The figure 10 shows the effect of projecting red, green, blue beams of light so that they
overlap on screen.
Y= 0.3 Red + 0.59 Green + 0.11 Blue
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Fig. 6.1 Additive Colour Mixing
6.4 The Colour Television
It is possible to obtain any desired colour by mixing three primary colours i.e., red, blue
and green in suitable proportion. Thus it is only required to convert optical information
of these three colours to electrical signals and transmit it on different carriers to be
decoded by the receiver. This can then be converted back to the optical image at the
picture tube. The phosphors for all the three colours i.e. R, G and B are easily available
to the manufacturers of the picture tube. So the pick up from the cameras and output
for the picture tube should consists of three signals i.e. R, G and B. It is only in
between the camera and the picture tube of the receiver we need a system to transmit
this information.
Fig 6.2 Colour TV
Colour television has the constraint of compatibility and reverse compatibility with the
monochrome television system which makes it slightly complicated. Compatibility
means that when colour TV signal is radiated the monochrome TV sets should also
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display Black & White pictures. This is achieved by sending Y as monochrome
information along with the chroma signal. Y is obtained by mixing R,G & B as per the
well known equation :
Y = 0.3 R + 0.59 G + 0.11 B
Reverse compatibility means that when Black & White TV signal is radiated the colour
TV sets should display the Black & White pictures.
If we transmit R, G, B, the reverse compatibility cannot be achieved. Let us see how
If we transmit Y, R & B and derive G then :
Since Y = 0.3R + 0.59G + 0.11 B
G = 1.7Y - 0.51 R - 0.19 B
In such a case what happens with a colour TV set when we transmit black and white
signal. R and B are zero, but G gun gets 1.7 Y. The net result is black & white pictures
on a colour TV screen appear as Green pictures. So reverse compatibility is not
achieved.
6.5 Colour Difference Signals
To achieve reverse compatibility, when we transmit Y, R-Y and B-Y instead of Y, R &
B, we do not take G-Y as this will always be much lower than R-Y and B-Y and hence
will needs more amplification and will cause more noise into the system. G-Y can be
derived electronically in the TV receiver.
In the previous paragraph we have seen
G = 1.7 Y - 0.51 R - 0.19 B
So G-Y = -0.51 (R-Y) - 0.19 (B-Y)
Thus, colour difference signals fulfill the compatibility and reverse compatibility.
Because in this case the colour difference signals are zero if the original signal is
monochrome (i.e. R = B = G)
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So if we take R - Y
R - Y = R - (0.3 R + 0.59 R + 0.11 R) = 0
Similarly B - Y = 0
As such colour difference signals are zero for white or any shade of gray whereas, Y
carries the entire Luminance information.
It is to be noted while R, G, B signals always have positive value R-Y, B-Y and G-Y
signals can either be positive or negative or even zero.
The R-Y and B - Y chrominance signals may be recovered at the television receiver by
suitable synchronous demodulation. But sub-carrier is to be generated by a local
oscillator. This generated sub-carrier in the receiver must have same frequency as that
of transmitted sub-carrier and also the same phase. This is achieved by transmitting 10
cycles of sub-carrier frequency on the back porch of H synchronizing pulse. This 10
cycles sub-carrier signal is known as BURST or colour BURST.
Fig 6.3 Colour Difference Signal
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Fig 6..4 Block Digram of PAL Encoder
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CHAPTER 7
VIDEO CHAIN IN A TYPICAL DOORDARSHAN STUDIO
7.1 STUDIO CENTRE
A Studio centre of Doordarshan has the following objectives:
1) To originate programmes from studios either for live telecast or for recording on
a video tape.
2) To knit various other sources of programs available at the production desk i.e.,
camera output from studios, feed from other kendras, outdoor, playback from pre
recorded tape, film based programs slides, video graphics and characters generator
etc. This knitting or live editing includes generation of special effects and desired
transitions between various sources.
3) Processing/distribution of different sources to various destinations in technical
areas.
4) Routing of mixed programme for recording/transmission via master switching
room and Micro Wave to the transmitter or any other desired destinations.
Activities in a television studio can be divided into three major areas such as :
1) Action area,
2) Production control room, and
3) Central apparatus room,
7.1.1 Action area
This place requires large space and ceiling as compared to any other technical area.
Action in this area includes staging, lighting, performance by artists, and arrangement
to pick up picture and sound. Hardware required for these activities in a studio (typical
size 20 x20x8.5 cubic meters) are:
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2. Timing a production/telecast.
3. Editing of different sources available at the production desk.
4. Monitoring of output/off air signal.
Hardware provided in this area include:
1. Monitoring facilities for all the input and output sources(audio/video).
2. Remote control for video mixer, telecine and library store and special effect
(ADO) etc.
3. Communication facilities with technical areas and studio floor.
7.1.3 Vision mixing and switching
Unlike films, television media allows switching between different sources
simultaneously at the video switcher in Production control room operated by the Vision
Mixer on the direction of the program producer. The producer directs the cameramen
for proper shots on various cameras through intercom and the vision mixer (also called
VM engineer) switches shots from the selected camera/cameras with split second
accuracy, in close cooperation with the producer. The shots can be switched from one
video source to another video source, superimposed, cross faded, faded in or faded out
electronically with actual switching being done during the vertical intervals between the
picture frames. Electronics special effects are also used now days as a transition
between the two sources.
7.1.4 Vision Mixer (or Video Switcher)
Though the video switching is done by the VM at the remote panel, the electronics is
located in CAR. The vision mixer is typically a 10 x 6 or 20 x 10 cross bar switcher
selecting anyone of the 10 or 20 input sources to 6 or 10 different output lines. The
input sources include: Camera 1, camera 2, camera 3, VTR1, VTR2, Telecine 1,
Telecine 2, Test signal etc. The vision mixer provides for the following operational
facilities for editing of TV programs:-
(i) Take: Selection of any input source
or
Cut: switching clearly from one source to another.
(ii) DISSOLVE: Fading out of one source of video and fading in another
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source of video.
(iii) SUPERPOSITION OF TWO SOURCES: Keyed caption when selected inlay is
superimposed on the background picture.
(iv) SPECIAL EFFECTS: A choice of a number of wipe patterns for split screen or wipe
effects.
The selected output can be monitored in the corresponding pre-view monitor. All the
picture sources are available on the monitors. The preview monitors can be used for
previewing the telecine, VTR; test signals etc. with any desired special effect, prior to
its actual switching.
The switcher also provides cue facilities to switch camera tally lights as an indication to
the cameraman whether his camera is on output of the switcher.
7.2 Present day PCRs have
24 input video special effects switchers
(CD 680 or CD 682-SP)
Character generators
Telecine/DLS remote controls
Adequate monitoring equipment
7.2.1 Character Generator(CG)
Character Generator provides titles and credit captions during production in Roman
script. It provides high resolution characters, different colours for colorizing characters,
background, edges etc. At present bilingual and trilingual C.G are also being used by
Doordarshan.
Character Generator is a microcomputer with Texts along instructions when typed in at
the keyboard is stored on a floppy or a Hard disk. Many pages of scripts can be stored
on the disk and recalled when needed, by typing the addresses for the stored pages, to
appear as one of the video sources.
7.2.2 Sync Pulse-Generator(SPG)
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It is essential that all the video sources as input to the switcher are in synchronism i.e.,
start and end of each line or all the frames of video sources is concurrent. This
requirement is ensured by the sync pulse generator (SPG). SPG consists of highly
stable crystal oscillator. Various pulses of standard width and frequency are derived
from this crystal electronically which form clock for the generation of video signal.
These pulses are fed to all the video generating equipment to achieve this objective of
synchronism. Because of its importance, SPG is normally duplicated for change over in
case of failure.
It provide the following outputs:
Line drive
Field drive
Mixed blanking
Mixed sync
colour subcarrier
A burst insertion pulse
PAL phase Indent pulses
7.2.3Camera Control Unit (CCU)
The television cameras which include camera head with its optical focusing
lens, pan and tilt head, video signal pre-amplifier view finder and other associated
electronic circuitry are mounted on cameras trolleys and operate inside the studios. The
output of cameras is pre-amplified in the head and then connected to the camera control
unit (CCU) through long multi-core cable (35 to 40 cores), or triax cable.
All the camera control voltages are fed from the CCU to the camera head over the
multi-core camera cable. The view-finder signal is also sent over the camera cable to
the camera head view-finder for helping the cameraman in proper focusing, adjusting
and composing the shots.
The video signal so obtained is amplified, H.F. corrected, equalized for cable delays,
D.C. clamped, horizontal, and vertical blanking pulses are added to it. The peak white
level is also clipped to avoid overloading of the following stages and avoiding over
modulation in the transmitter. The composite sync signals are then added and these
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video signals are fed to a distribution amplifier, which normally gives multiple outputs
for monitoring etc.
7.2.3 Light Control
The scene to be televised must be well illuminated to produce a clear and noise free
picture. The lighting should also give the depth, the correct contrast and artistic display
of various shades without multiple shadows.
The lighting arrangements in a TV studio have to be very elaborate. A large number of
lights are used to meet the needs of key, fill, and back lights etc. Lights are
classified as spot and soft lights. These are suspended from motorized hoists and
telescopes. The up and down movement is remotely controlled. The switching on and
off the lights at the required time and their dimming is controlled from the light control
panel inside a lighting control room using SCR dimmer controls. These remotely
control various lights are inside the studios.
7.3 Sound mixing and control
As a rule, in television, sound accompanies the picture. Several microphones aregenerally required for production of complex television programs besides other audio
sources also called marred sound from telecine, VTR, and audio tape/disc replays. All
these audio sources are connected to the sound control console.
The sounds from different sources are controlled and mixed in accordance with the
requirement of the program. Split second accuracy is required for providing the correct
audio source in synchronisation with the picture thus requiring lot of skill from the
engineer. Even the level of sound sometimes is varied in accordance with the shot
composition called prospective.
7.3.1 Audio facilities
An audio mixing console, with a number of inputs, say about 32 inputs is provided in
major studio. This includes special facilities such as equalisation, PFL, phase reversal,
echo send/receive and digital reverberation units at some places Meltron console tape
recorders and EMI 938 disc reproducers are provided for playing back/creating audio
effects as independent sources (Unmarried) to the switcher.
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7.3.2Video Tape recorders
VTR room is provided at each studio center. It houses a few Broadcast standard
Videocassette recorders (VCRs). In these recorders, sound and video signals are
recorded simultaneously on the same tape.
Most of the TV centers have professional quality B-Format BCN-51 One inch VTRs.
For broadcast quality playback it is equipped with correction electronics i.e. a processor
which comprises velocity error compensation, drop-out compensation and time base
correction. It also comprises a digital variable motion unit enabling still reproduction,
slow motion and visible search operation.
New centers are being supplied with Sony U-matic high band VCRs along with
Sony Betacam SP VCRs, DVC Pro.
7.3.3Post Production Suites
Modern videotape editing has revolutionised the production of television programs over
the years. The latest trend all over the world is to have more of fully equipped post
production suites than number of studios. Most of the present day shootings are done
on locations using single camera. The actual production is done in these suites. The
job for a post production suites is:-
a. To knit program available on various sources.
b. While doing editing with multiple sources, it should be possible to have any
kind of transition.
c. Adding/Mixing sound tracks.
d. Voice over facilities.
e. Creating special effects.The concept of live editing on vision mixer is being replaced by to do it at leisure in
post production suites.
A well equipped post production suite will have:-
1. Five VTRs/VCRs, may be of different format remotely controlled by the editor.
2. Vision mixing with special effect and wipes etc. with control from a remote
editor panel.
3. Ampex Digital Optics (ADO) for special effects.
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4. Audio mixer with remote control from the editor remote panel.
5. Multi-track audio recorder with time code facilities and remote operation.
6. Character generator for titles.
7. Adequate monitoring facilities.
8. Supported by Offline editing systems to save time in post production suites.
9. One man operation.
7.4 Coverage of Outside events
Outside broadcasts(or OBs) provide an important part of the television programs.
Major events like sports, important functions and performances are covered with anO.B. van which contains all the essential production facilities.
7.4.1 Video Chain :
The block diagram on facing page connects all these sections and it can be observed
that the CAR is the nodal area. Now let us follow a CAM-I signal. CAM-I first goes to
a Camera electronics in CAR via a multi-core cable, the signal is then matched/adjusted
for quality in CCU and then like any other sources it goes to video switcher via PP(Patch Panel) and respective VDAs(Video Distribution Amplifiers) and optional Hum
compensator/Cable equilizers.
Output from the switcher goes to stabilizing amplifier via PP and VDAs.
Output from the stab. Is further distributed to various destinations.
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Fig 7.1 video chain
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7.5 TV LIGHTING
7.5.1 GENERAL PRINCIPLES
Lighting for television is very exciting and needs creative talent. There is always a
tremendous scope for doing experiments to achieve the required effect. Light is a kind
of electromagnetic radiation with a visible spectrum from red to violet i.e. wave length
from 700 nm to 380 nm respectively. However to effectively use the hardware and
software connected with lighting it is important to know more about this energy.
7.5.2 Light Source
Any light source has a Luminance intensity (I) which is measured in Candelas. Candela
is equivalent to an intensity released by standard one candle source of light.
7.5.3 Basic Three Point Lighting
a) Key light : This is the principal light source of illumination. It gives shape and
modeling by casting shadows. It is treated like "sun" in the sky and it should cast only
one shadow. Normally it is a hard source.
b) Fill Light : Controls the lighting contrast by filling in shadows. It can also provide
catch lights in the eyes. Normally it is a soft source.
c) Back light : Separates the body from the background, gives roundness to the subject
and reveals texture. Normally it is hard source.
d) Background Light : Separates the person from the background, reveals background
interest and shape. Normally it is a hard source.In three point lighting the ratio of 3/2/1
(Back/Key/Fill) for mono and 3/2/2 for colour provides good portrait lighting.
7.6 TV CAMERA
7.6.1 Introduction
A TV Camera consists of three sections :
a) A Camera lens and optics : To form optical image on the face
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plate of a pick up device.
b) A transducer or pick up device : To convert optical image into an
electrical signal.
c) Electronics : To process output of a transducer to
get a CCVS signal.
7.6 .2 CCD CAMERAS
7.6.2.a Introduction
Any 7.6 TV CAMERA
convert the light information on it to a charge signal. All we need now is to have an
arrangement to collect this charge and convert it to voltage. This is the basic principle
on which CCD cameras are based.
7.6.3 Latest CCD Cameras
CCD were launched in 1983 for broadcasting with pixel count from a mere 2,50,000
which increased to 20,00,000 in 1994 for HDTV application. Noise and aliasing has
been reduced to negligible level. CCD cameras now offers fully modulated video
output at light level as low as 6.0 lumens. A typical specification for a studio camera
now available in market are some thing like 2/3 inch, FIT, lens on chip CCD with
6,00,000 pixel, 850 lines H resolution, S/N more than 60 dB, sensitivity F-8 (2000 lux)
etc.
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Fig 7.2Block Diagram of a typical Camera
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CHAPTER8
HIGH POWER TV TRANSMITTER
8.1 Design
All the TV transmitters have the same basic design. They consist of an exciter followed
by power amplifiers which boost the exciter power to the required level.
8.1.1 Exciter
The exciter stage determines the quality of a transmitter. It contains pre-corrector units
both at base band as well as at IF stage, so that after passing through all subsequent
transmitter stages, an acceptable signal is available. Since the number and type of
amplifier stages, may differ according to the required output power, the
characteristics of the pre-correction circuits can be varied over a wide range.
8.1.2 Vision and Sound Signal Amplification
In HPTs the vision and sound carriers can be generated, modulated and amplified
separately and then combined in the diplexer at the transmitter output.
In LPTs, on the other hand, sound and vision are modulated separately but amplified
jointly. This is common vision and aural amplification.A special group delay
equalization circuit is needed in the first case because of errors caused by TV
diplexer. In the second case the intermodulation products are more prominent and
special filters for suppressing them is required.As it is difficult to meet the
intermodulation requirements particularly at higher power ratings, separate
amplification is used in HPTs though combined amplification requires fewer
amplifier stages.
8.1.3 Power Amplifier Stages
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In BEL mark I & II transmitters three valve stages (BEL 450 CX, BEL 4500 CX and
BEL 15000 CX) are used in vision transmitter chain and two valves (BEL 450 CX
and BEL 4500 CX) in aural transmitter chain. In BEL mark III transmitter only
two valve stages (BEL 4500 CX and BEL 15000 CX) are used in vision transmitter
chain. Aural transmitter chain is fully solid state in Mark III transmitter.
BEL 10 kW TV TRANSMITTER
A block diagram of BEL 10 kW TV Transmitter is shown in Fig. 10. It consists of :
Monitoring Equipment Rack
Control Console Input Equipment Rack
Indoor Co-axial Equipment comprising of :
U-link Rack with U-link panel A and B, T-Transformer and 10 kW
Dummy Load.
Aural Harmonic Filter.
CIN Diplexer
Aural Notch Filter and Band Pass Filter.
Antenna system with junction box, feeder cables etc.
Fig 8.1 lock Diagram of 10kW TV Transmitter
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8.1.4 SOLID STATE POWER AMPLIFIERS
1) Has got two identical sections. Each capable of delivering 10 W.
2) Gets 28 V power supply through relay in 80 W AMP.
3) Sample of output is available at front panel for RF monitoring.4) Provides A DC output corresponding to sync peak out put for vision monitoring
unit.
5) Thermostat on heat sink is connected in series with thermostat or 80 W AMP
and provides thermal protection. (Operating temp. 70oC.)
Fig. 8.2 TX. Block Diagram
Fig 8.3 Vision Chain of Exciter
8.2TRANSMITTER CONTROL SYSTEM
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The transmitter control unit performs the task of transmitter interlocking and control.
Also it supports operation from control console. The XTR control unit (TCU) has two
independent system viz.
1. Main control system. (MCS)
2. Back-up Control System (BCS)
8.2.1 System Description of Exciter
Fig 8.4 Block Diagram of TV Exciter
8.2.2 Video Chain
The input video signal is fed to a video processor. In VHF transmitters LPF, Delay
equalizer and receiver pre-corrector precede the video processor.
8.2.2.a Low Pass Filter : Limits incoming video signal to 5 MHz.
8.2.2.b Delay Equalizer : Group delay introduced by LPF is corrected. It also pre-
distorts the video for compensating group delay errors introduced in the subsequent
stages and diplexer.
8.2.2.c Receiver pre-corrector : Pre-distorts the signal providing partial compensation
of GD which occurs in domestic receivers.Both the delay equaliser and receiver
precorrector are combined in the delay equaliser module in Mark III version.
8.3 DP/DG Corrector
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This is also used in the exciter preceding LPF (mark III) for pre-correcting the
differential gain and differential phase errors occurring in the transmitter.
8.3.1 Video Processor
The block diagram of video processor is given in fig. 3.
Functions
Amplification of Video signal
Clamping at back porch of video signal.
Clamping gives constant peak power. Zero volt reference line is steady irrespective of
video signal pattern when clamping takes place otherwise the base line starts an
excursion about the zero reference depending on the video signal.
Fig 8.5 Block Diagram of Video Processor
8.3.2 Vision Modulator
The block diagram of Vision modulator is given in fig. 4 and schematic diagram is
shown in fig.
Functions
Amplification of Vision IF at 38.9 MHz.
Linear amplitude modulation of Vision IF by video from the video processor in
a balanced modulator.
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8.4 IF Amplifier
IF is amplified to provide sufficient level to the modulator. It operates as an amplitude
limiter for maintaining constant output.
8.4.1 Modulator
A balanced modulator using two IS-1993 diodes is used in the modulator.
8.4.2 Band pass amplifier
Modulated signal is amplified to 10 mW in double tuned amplifier which
provides a flat response within 0.5 dB in 7 MHz band.
Fig 8.6 Block Diagram of Vision Modulator
Fig 8.7Schematic Diagram of Vision Modulator
8.5 VSBF and Mixer
The block diagram of VSBF and Mixer is given in fig. 6. It consists of
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following stages :
VSB filter
ALC amplifier
Mixer
Helical Filter
Mixer Amplfier
Fig 8.8 Block Diagram of VSBF Mixer
8.5.1 VSB Filter
Surface Acoustic wave (SAW) filter provide a very steep side band response with high
attenuation outside designated channel. It has a linear phase characteristic with a low
amplitude and group delay ripple. (Fig. 7.)
Fig 8.9 Block Diagram of V.S.B.Filter
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8.5.2 Local Oscillator
The block diagram of Local Oscillator is given in fig. 8.It supplies three equal outputs
of + 8 dBm each at a frequency of fv + fvif. This unit has 3 sub units.
(1) fc/4 oscillator : Generates frequency which is 1/4 of desired channel frequency.
Fine freq. control is done by VC1.
(2) LO Mixer/Power divider : Here the above fc/4 frequency is multiplied by four
to obtain channel frequency of fc and then mixed with fvif. Power divider is
also incorporated to provide three isolated outputs of equal level.
Fig 8.10 Block Diagram of Local Oscillator
8.5.3 AUDIO CHAIN
8.5.3.a Aural Modulator
The aural modulator unit consists of audio amplifier, VCO, mixer and APC.
The block diagram of Aural modulator is given in fig. 9.
Fig 8.11 Block Diagram of Aural Modulator
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8.5.4 Audio Amplifier
A balanced audio signal at + 10 dBm from studio is converted to unbalanced signal by
audio transformer T4. The output of this is taken through potentiometer to the input of
Hybrid Audio Amp BMC 1003. A 50 micro second pre-emphasis is also provided.
8.5.5 VCO
This is a varactor tuned oscillator. Its frequency can be varied by coil L4. Transistor
TR-17 forms the oscillator. VCO output is frequency modulated by the audio signal.
Output level is 0 dBm
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CHAPTER 9
TV TRANSMITTER ANTENNA SYSTEM
TV Antenna System is that part of the Broadcasting Network which accepts RF Energy
from transmitter and launches electromagnetic waves in space. The polarization of the
radiation as adopted by Doordarshan is linear horizontal. The system is installed on a
supporting tower and consists of antenna panels, power dividers, baluns, branch feeder
cable, junction boxes and main feeder cables. Dipole antenna elements, in one or the
other form are common at VHF frequencies where as slot antennae are mostly used at
UHF frequencies. Omni directional radiation pattern is obtained by arranging the
dipoles in the form of turnstile and exciting the same in quadrature phase. Desired gainis obtained by stacking the dipoles in vertical plane. As a result of stacking, most of the
RF energy is directed in the horizontal plane. Radiation in vertical plane is minimized.
The installed antenna system should fulfil the following requirements :
a) It should have required gain and provide desired field strength at the point of
reception.
b) It should have desired horizontal radiation pattern and directivity for serving the
planned area of interest. The radiation pattern should be omni directional if the
location of the transmitting station is at the center of the service area and
directional one, if the location is otherwise.
c) It should offer proper impedance to the main feeder cable and thereby to the
transmitter so that optimum RF energy is transferred into space. Impedance
mismatch results into reflection of power and formation of standing waves. The
standard RF impedance at VHF/UHF is 50 ohms.
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Fig 9.1Turnstile Antenna and its Horizontal Pattern
9.1 Radiation Pattern and Gain
The horizontal and vertical radiation pattern are shown in fig. 9.1 and 9.2. The total
gain depends upon the type of the antenna panel and no. of stacks as given in table-1.
Fig. 9.2 Typical Horizontal radiation pattern
9.2 VESTIGIAL SIDE BAND TRANSMISSION
Another feature of present day TV Transmitters is vestigial side band transmission. If
normal amplitude modulation technique is used for picture transmission, the minimum
transmission channel bandwidth should be around 11 MHz taking into account the
space for sound carrier and a small guard band of around 0.25 MHz. Using such large
transmission BW will limit the number of channels in the spectrum allotted for TV
transmission. To accommodate large number of channels in the allotted spectrum,
reduction in transmission BW was considered necessary. The transmission BW could
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be reduced to around 5.75 MHz by using single side band (SSB) AM technique,
because in principle one side band of the double side band (DSB) AM could be
suppressed, since the two side bands have the same signal content.
It was not considered feasible to suppress one complete side band in the case of TV
signal as most of the energy is contained in lower frequencies and these frequencies
contain the most important information of the picture. If these frequencies are
removed, it causes objectionable phase distortion at these frequencies which will affect
picture quality. Thus as a compromise only a part of lower side band is suppressed
while taking full advantage of the fact that:
i) Visual disturbance due to phase errors are severe and unacceptable where
large picture areas are concerned (i.e. at LF) but
ii) Phase errors become difficult to see on small details (i.e. in HF region) in
the picture. Thus low modulating frequencies must minimize phase
distortion where as high frequencies are tolerant of phase distortions as they
are very difficult to see.
The radiated signal thus contains full upper side band together with carrier and
the vestige (remaining part) of the partially suppressed LSB. The lower side band
contains frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is
at 1.25 MHz.
9.3 Standards
The characteristics of the TV signal is sections 1 and 2 refer to CCIR B/G standards.
Various other standards are given in Table 1.
Table 1
Frequency Range Vision/sound carrier spacing channel
width
Vision sound carrier spacing 5.5 MHz
Channel width 7 MHz (B) in VHF OR 8 MHz (G) in UHF
Sound Modulation FM
FM deviation (maximum) + 50 kHz
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CONCLUSION
The practical training has proved to be knowledge buster for me and I have acquired a
good practical knowledge of the field which cant be gained nearly by reading books.
During my training atPARSHAR BHARTI A.I.R and DD, kingsways Delhi
11ooo9 was really very surprised and delighted to see the system configuration and
interconnections to such a large extent. I came to know and learn practical about
various stages and equipment involved right from the production of program to its
transmission; about which I heard or read only in text books. I also visited workshop
where, beside my training program, I also learnt various basic things about diodes,
capacitor, power supplies, multimeter, digital C.R.O. etc. which has a remarkable
experience. I really feel that my training at A.I.R and DD was very beneficial for
me.The training has proved me with a good knowledge of working of PARSHAR
BHARTI A.I.R and DD base for relating the theoretical knowledge with the practical
one. It was a very exciting, adventurous and exhaustive training which has raised my
practical skills to a great extent.
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BIBLIOGRAPHY
The documents which were a help to me in completion of my report are being obtained
from the following sources:
I. Sites :
1. Tcil-india.com
2. Google.com
3. Winkipedia.com
4. Emory.edu
5. Tycotelecom.com
6. Technologyforall.com
II. White Papers from different sites.
III. Books :
1. William Stallings, Wireless Communication & Networks, Pearson
Education, 2007.
2. Sanjay Sharma, Analog & Digital Communication System, Laxmi
Publication, 2009.