File - ce

MODULE 3 (18 hours)
Transducers: Basic principles of Strain guage, LVDT, Thermistor, Photodiode, Typical
moving coil microphones and Loud speaker. Block diagram of Digital Multimeter .[8hrs].
Basic principles of TV: Interlaced Scanning-Block Diagram of PAL TV receiver (color).
Basic principles of DTH, brief descriptions of MP3, multichannel audio 5.1,7.1.
Question Bank
1) Give the basic principle of a photo diode.
2) Give the principle of thermistor?
3) Explain strain guage with its guage factor and its uses.
4) Briefly describe the CRT scanning principle in a TV.
5) What is a thermistor? Why it is well suited to precision measurement, control and
6) Briefly explain interlaced scanning used in a TV receiver.
7) What are the advantages of a thermistor compared to other temperature sensors.
8) What is a transducer? Differentiate between active and passive transducers.
9) Briefly explain the basic principles of scanning?
1) What is a transducer? Explain different types and their operation.
2) With a block diagram, explain the basic principles of DTH.
3) Write short notes on (1) LVDT (2) Thermistor?
4) Describe the block schematic of PAL TV receiver.
5) With neat sketches describe the working principle and important properties of
(1) carbon microphone (2) moving coil microphone ?
6) Sketch a neat block diagram of monochrome TV receiver and explain each block in it.
7) With neat diagrams, explain the working principle of (1) Photodiode (2) Loudspeaker.
8) Explain with diagrams the principle of operation of LVDT. Discuss some of its
9) Briefly explain the principle, operation and application of thermistor.
10) Briefly explain the principle, operation and application of microphone.
A transducer is a device that is used to convert a physical quantity into its corresponding
electrical signal.
In most of the electrical systems, the input signal will not be an electrical signal, but a
non-electrical signal. This will have to be converted into its corresponding electrical
signal if its value is to be measured using electrical methods. Transducers are widely used
in measuring instruments.
The block diagram of a transducer is given below.
A transducer will have basically two main components. They are
1. Sensing Element
The physical quantity or its rate of change is sensed and responded to by this part of the
2. Transduction Element
The output of the sensing element is passed on to the transduction element. This element
is responsible for converting the non-electrical signal into its proportional electrical
There may be cases when the transduction element performs the action of both
transduction and sensing. The best example of such a transducer is a thermocouple. A
thermocouple is used to generate a voltage corresponding to the heat that is generated at
the junction of two dissimilar metals.
Selection of Transducer
Selection of a transducer is one of the most important factors which help in obtaining
accurate results. Some of the main parameters are given below.
Selection depends on the physical quantity to be measured.
Depends on the best transducer principle for the given physical input.
Depends on the order of accuracy to be obtained.
Transducer Classification
Some of the common methods of classifying transducers are given below.
Based on their application.
Based on the method of converting the non-electric signal into electric signal.
Based on the output electrical quantity to be produced.
Based on the electrical phenomenon or parameter that may be changed due to the
whole process. Some of the most commonly electrical quantities in a transducer are
resistance, capacitance, voltage, current or inductance. Thus, during transduction,
there may be changes in resistance, capacitance and induction, which in turn
change the output voltage or current.
Based on whether the transducer is active or passive.
Types of Transducers:
There are two types of transducers, they are:
Active transducers
Passive transducers
Active transducers:
Active transducer is a device which converts the given non-electrical energy into
electrical energy by itself. Self generating type - do not require an external power,
and produce an analog voltage or current when stimulated by some physical form of
energy. Thermocouple, Photovoltaic cell and more are the best examples of the
Passive transducers:
Passive transducer is a device which converts the given non-electrical energy into
electrical energy by external force. require an external power to operate, and the
output is a measure of some variation in passive components (e.g. resistance or
capacitance). Strain gauge, Differential Transformer are the examples for the Passive
Strain is the amount of deformation of a body due to an applied force. More specifically,
strain (ε) is defined as the fractional change in length. Strain can be positive (tensile) or
negative (compressive).
A strain gauge is a sensor whose resistance varies with applied force. It converts force, pressure,
tension, weight, etc., into a change in electrical resistance which can then be measured.. When
external forces are applied to a stationary object, stress and strain are the result. Stress is defined
as the object's internal resisting forces, and strain is defined as the displacement and deformation
that occur.
The strain of a body is always caused by an external influence or an internal effect. Strain might
be caused by forces, pressures, moments, heat, structural changes of the material and the like. If
certain conditions are fulfilled, the amount or the value of the influencing quantity can be
derived from the measured strain value. Special transducers can be designed for the
measurement of forces or other derived quantities, e.g., moments, pressures, accelerations,
displacements, vibrations and others. The transducer generally contains a pressure sensitive
diaphragm with strain gages bonded to it.
If a strip of conductive metal is stretched, it will become skinnier and longer, both
changes resulting in an increase of electrical resistance end-to-end. Conversely, if a strip
of conductive metal is placed under compressive force, it will broaden and shorten. If
that the strip does not permanently deform), the strip can be used as a measuring element
for physical force, the amount of applied force inferred from measuring its resistance.
Guage Factor:
The gauge factor
is defined as:
is the change in resistance caused by strain,
is the resistance of the undeformed gauge, and
is strain.
Types of Strain Guages:
Unbonded strain gauges consist of a wire stretched between two points as shown in
Figure. Force acting on the wire (area = A, length = L, resistivity = r) will cause the wire
to elongate or shorten, which will cause the resistance to increase or decrease
proportionally according to:
R = ρL/A
and ∆R/R = GF·∆L/L,
where GF = Gauge factor
Bonded strain gauges consist of a thin wire or conducting film arranged in a coplanar
pattern and cemented to a base or carrier. The gauge is normally mounted so that as much
as possible of the length of the conductor is aligned in the direction of the stress that is
being measured. Lead wires are attached to the base and brought out for interconnection.
Bonded devices are considerably more practical and are in much wider use than
unbonded devices
Semiconductor strain Guage. The use of semiconductor material, notably silicon, for
strain gauge (SG) application has increased over the past few years. As in the case of the
metal SGs, the basic effect is a change of resistance with strain. In the case of a
semiconductor, the resistivity also changes with strain, along with the physical
dimensions. This is due to changes in electron and hole mobility with changes in crystal
structure as strain is applied. The net result is a much larger gauge factor than is possible
with metal gauges.
Semiconductor strain gages make use of the piezoresistive effect in certain
semiconductor materials such as silicon and germanium in order to obtain greater
sensitivity and higher-level output. Semiconductor gauges can be produced to have either
positive or negative changes when strained. They can be made physically small while still
maintaining a high nominal resistance. Semiconductor strain gauge bridges may have 30
times the sensitivity of bridges employing metal films, but are temperature sensitive and
difficult to compensate. Their change in resistance with strain is also nonlinear. They are
not in as widespread use as the more stable metal film devices for precision work;
however, where sensitivity is important and temperature variations are small, they may
have some advantage.
The semiconductor strain gauge physically appears as a band or strip of material with
electrical connection, as shown in Figure. The gauge is either bonded directly onto the
test element or, if encapsulated, is attached by the encapsulation material. These SGs also
appear as 1C assemblies in configurations used for other measurements.
Strain Guage Measurements
To measure such small changes in resistance, strain gages are almost always used in a
bridge configuration with a voltage excitation source. The general Wheatstone bridge,
illustrated in Figure, consists of four resistive arms with an excitation voltage, VEX, that is
applied across the bridge.
The output voltage of the bridge, VO, is equal to:
From this equation, it is apparent that when R1/R2 = R4/R3, the voltage output VO is zero.
Under these conditions, the bridge is said to be balanced. Any change in resistance in any
arm of the bridge will result in a non zero voltage. Therefore, if you replace R 4 in the
above Figure with an active strain gauge, any changes in the strain gauge resistance will
unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of
the strain gage is designated as RG, then the strain-induced change in resistance, ∆R, can
be expressed as ∆R = RG·GF·Є, where GF is the Gauge Factor. Assuming that R1 =
R2 and R3 = RG, the bridge equation above can be rewritten to express VO/VEX as a
function of strain as in Figure below. Note the presence of the 1/(1+GF·e/2) term,
indicates the nonlinearity of the quarter-bridge output with respect to strain.
LVDT are an acronym for Linear Variable Differential Transformer, a common type of
electromechanical transducer that can convert the rectilinear motion of an object to which
it is coupled mechanically into a corresponding electrical signal.
Figure shows the components of a typical LVDT. The transformer's internal structure
consists of a primary winding centered between a pair of identically wound secondary
windings, symmetrically spaced about the primary. The coils are wound on a one-piece
hollow form of thermally stable glass reinforced polymer, encapsulated against moisture,
wrapped in a high permeability magnetic shield, and then secured in a cylindrical
Stainless steel housing. This coil assembly is usually the stationary element of the
Position sensor.
The moving element of an LVDT is a separate tubular armature of magnetically
Permeable material called the core, which is free to move axially within the coil's hollow
bore, and mechanically coupled to the object whose position is being measured. This bore
is typically large enough to provide substantial radial clearance between the core and
bore, with no physical contact between it and the coil.
In operation, the LVDT's primary winding is energized by alternating current of
appropriate amplitude and frequency, known as the primary excitation. The LVDT's
electrical output signal is the differential AC voltage between the two secondary
windings, which varies with the axial position of the core within the LVDT coil. Usually
this AC output voltage is converted by suitable electronic circuitry to high level DC
voltage or current that is more convenient to use.
How Does An LVDT Work?
Figure illustrates what happens when the LVDT's core is in different axial positions. The
LVDT's primary winding, P, is energized by a constant amplitude AC source. The
magnetic flux thus developed is coupled by the core to the adjacent secondary windings,
S1 and S2. If the core is located midway between S1 and S2, equal flux is coupled to
each secondary so the voltages, E1 and E2, induced in windings S1 and S2 respectively,
are equal. At this reference midway core position, known as the null point, the
differential voltage output, (E1 - E2), is essentially zero.
As shown in Figure, if the core is moved closer to S1 than to S2, more flux is coupled to
S1 and less to S2, so the induced voltage E1 is increased while E2 is decreased, resulting
in the differential voltage (E1 - E2). Conversely, if the core is moved closer to S2, more
flux is coupled to S2 and less to S1, so E2 is increased as E1 is decreased, resulting in the
differential voltage (E2 - E1).
Figure shows how the magnitude of the differential output voltage, EOUT, varies with
core position. The value of EOUT at maximum core displacement from null depends
upon the amplitude of the primary excitation voltage and the sensitivity factor of the
particular LVDT.
Why Use An LVDT?
Friction free operation
Infinite resolution
Unlimited mechanical life
Environmentally robust
Absolute output
Sensitive to stray magnetic field
AC to DC conversion required at output
Affected by temperature change
The vibration of the transducer may affect the movement of the core
A thermistor is a type of resistor used to measure temperature changes, relying on the
change in its resistance with changing temperature. It is the combination of the two words
thermal and resistor.
Thermistor is a temperature sensitive resistor. Thermistors are generally composed of
semiconductor materials (metallic compounds including oxides such as manganese,
copper, cobalt, and nickel, as well as single-crystal semiconductors silicon and
Although positive temperature coefficient units are available, most thermistors have a
negative temperature coefficient (TC); that is, their resistance decreases with increasing
temperature. The negative T.C. can be as large as several percent per degree Celsius,
allowing the thermistor circuit to detect minute changes in temperature.
High value of temperature sensitivity, a reasonably wide temperature range that can be
covered and small size are the important advantages of thermistors in the field of
tempearature measurement.
The resistors are in the form of beads, rods and discs of different sizes. The beads are
available with glass envelope encapsulation. The beads may be as small as 0.15 to 1.25
mm in diameter and may have resistance as high as 10 Kilo ohm.
(1) Because they are semiconductors, thermistors are more susceptible to permanent
decalibration at high temperatures. The use of thermistors is generally limited to a few
hundred degrees Celsius and manufacturers warn that extended exposures even well
below maximum operating limits will cause the thermistor to drift out of its specified
(2) Thermistors can be made very small which means they will respond quickly to
temperature changes. It also means that their small thermal mass makes them especially
susceptible to self-heating errors.
(3) Thermistors are a good deal more fragile and they must be carefully mounted to avoid
crushing or bond separation.
The resistance temperature characteristics shows that the thermistor has a very high
negative temperature co efficient of resistance making it an ideal temperature transducer.
Thermistor finds important application in surface temperature studies like that of power
transistor or heat sinks.
A photodiode is a type of photodetector capable of converting light into either current or
voltage, depending upon the mode of operation. The common, traditional solar cell used
to generate electric solar power is a large area photodiode.
Photodiodes are similar to regular semiconductor diodes except that they may be either
exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber
connection to allow light to reach the sensitive part of the device.
Silicon photodiodes are semiconductor devices responsive to high energy particles and
photons. Photodiodes operate by absorption of photons or charged particles and generate
a flow of current in an external circuit, proportional to the incident power. Photodiodes
can be used to detect the presence or absence of minute quantities of light and can be
calibrated for extremely accurate measurements from intensities below 1 pW/cm2 to
intensities above 100 mW/cm2.
Silicon photodiodes are utilized in such diverse applications as spectroscopy,
photography, analytical instrumentation, optical position sensors, beam alignment,
surface characterization, laser range finders, optical communications, and medical
imaging instruments.
Planar diffused silicon photodiodes are simply P-N junction diodes. A P-N junction can
be formed by diffusing either a P-type impurity (anode), such as Boron, into a N-type
bulk silicon wafer, or a N-type impurity, such as Phosphorous, into a P-type bulk silicon
wafer. The diffused area defines the photodiode active area.
To form an ohmic contact another impurity diffusion into the backside of the wafer is
necessary. The impurity is an N-type for P-type active area and P-type for an N-type
active area. The contact pads are deposited on the front active area on defined areas, and
on the backside, completely covering the device. The active area is then deposited on
with an anti-reflection coating to reduce the reflection of the light for a specific
predefined wavelength. The non-active area on the top is covered with a thick layer of
silicon oxide. By controlling the thickness of bulk substrate, the speed and responsivity of
the photodiode can be controlled.
Note that the photodiodes, when biased, must be operated in the reverse bias mode, i.e. a
negative voltage applied to anode and positive voltage to cathode.
Silicon is a semiconductor with a band gap energy of 1.12 eV at room temperature. This
is the gap between the valence band and the conduction band. At absolute zero
temperature the valence band is completely filled and the conduction band is vacant. As
the temperature increases, the electrons become excited and escalate from the valence
band to the conduction band by thermal energy. The electrons can also be escalated to the
conduction band by particles or photons with energies greater than 1.12eV, which
corresponds to wavelengths shorter than 1100 nm. The resulting electrons in the
conduction band are free to conduct current.
Due to concentration gradient, the diffusion of electrons from the N type region to the Ptype region and the diffusion of holes from the P type region to the N-type region,
develops a built-in voltage across the junction. The inter-diffusion of electrons and holes
between the N and P regions across the junction results in a region with no free carriers.
This is the depletion region. The built-in voltage across the depletion region results in an
electric field with maximum at the junction and no field outside of the depletion region.
Any applied reverse bias adds to the built in voltage and results in a wider depletion
The electron-hole pairs generated by light are swept away by drift in the depletion region
and are collected by diffusion from the undepleted region. The current generated is
proportional to the incident light or radiation power. The light is absorbed exponentially
with distance and is proportional to the absorption coefficient. The absorption coefficient
is very high for shorter wavelengths in the UV region and is small for longer wavelengths
(Figure). Hence, short wavelength photons such as UV, are absorbed in a thin top surface
layer while silicon becomes transparent to light wavelengths longer than 1200 nm.
Moreover, photons with energies smaller than the band gap are not absorbed at all.
Microphone is a sound transducer that produces an electrical output signal proportional
to the sound wave acting upon its flexible diaphragm.. It is widely used in audio
recording, communication systems and also in instruments that measure sound and noise.
Types Of Microphone
Carbon Microphone
Moving Coil Microphone
Carbon Microphone
The simplest type is carbon microphone, which is used in telephones. The microphone
consists of a metallic cup filled with carbon granules. A movable metallic diaphragm
mounted in contact with the granules covers the open end of the cup. Wires attached to
the cup and the diaphragm are connected to an electrical circuit so that a current flows
through the carbon granules. Sound waves vibrate the diaphragm, varying the pressure on
the carbon granules. The electrical resistance of the carbon granules, changes with
varying pressure, causing the current in the circuit to change according to the vibrations
of the diaphragm. The varying current may either actuate a telephone
receiver or may be amplified and transmitted to a distant receiver. If
the current variation is suitably amplified, it may also be used to
modulate a radio transmitter.
Graham Bell built the first microphone in 1876 when
Moving Coil Microphone
A moving coil microphone functions on the basic principle of Electromagnetic induction.
It has a copper wire coil, suspended within the magnetic field of a permanent magnet and
a diaphragm, which is exposed to sound waves.
An incoming sound hits the flexible diaphragm , it moves back and forth in response to
the sound pressure acting upon it. The attached coil of wire also moves within the
magnetic field of the magnet. As a copper wire coil moves in the magnetic field a voltage
is generated as given by…
where V is resulting voltage from B is magnetic field, l is the length of the copper wire
and u is the velocity at which it passes thought the field. Hence V is proportional to the
pressure of the sound wave acting upon the diaphragm. ie. Louder the sound, larger will
be the output signal.
Moving coil microphones are cheap and robust making them good for the rigors of live
performance and touring. They are especially suited for the close miking of Bass and
Guitar speaker cabinets and Drum kits. They are also good for live vocals as their
resonance peak of around 5kHz provides an inbuilt presence boost that improves
speech/singing intelligibility.
However the inertia of the coil reduces high frequency response. Hence they are NOT
best suited to studio applications where quality and subtlety are important such as high
quality vocal recording or acoustic instrument micking.
Loudspeakers are sound transducers that convert complex electrical analogue signal into
sound waves .They are available in all shapes, sizes and frequency ranges with moving
coil, piezo electric, electrostatic etc as the common types.
Loudspeaker produces sound by converting electrical signals from an audio amplifier into
mechanical motion. Sound is created from the forward and backward motion of the
loudspeaker cone, which is a concave plastic or paper disc. The cone is mounted and
centered on a concave metal frame by a ring of flexible rubber. Glued to the centre of the
cone is a hollow cylinder of thin, lightweight aluminum. A length of thin, insulated wire
is wound upon the bobbin to form the voice coil; both ends of the voice coil wire are
connected to the voice coil terminals on the frame. The voice coil is positioned inside a
narrow cylindrical groove or air gap in the centre of a magnet. The coil is suspended in
the air gap by a flexible fabric disc. An audio amplifier is connected to the voice coil
terminals. The coil emits a magnetic field as audio signals from the amplifier travel
through the voice coil wire. The voice coil field alternately pulls and pushes the coil,
bobbin and cone assembly towards and against the magnetic field from the magnet,
which causes the forward and backward cone motion.
Moving Coil Loudspeaker
A coil of fine wire, called the "speech or voice coil", is suspended within a very strong
magnetic field, and is attached to a paper or Mylar cone, called a "diaphragm" which
itself is suspended at its edges to a metal frame or chassis. Then unlike the microphone
which is pressure sensitive, this type of sound transducer is a pressure generating device
When an analogue signal passes through the voice coil of the speaker, an electromagnetic field is produced whose strength is determined by the current flowing through
the "voice" coil, which inturn is determined by the volume control setting of the driving
amplifier. The electro-magnetic force produced by this field opposes the main permanent
magnetic field around it and tries to push the coil in one direction or the other depending
upon the interaction between the north and south poles. As the voice coil is permanently
attached to the cone/diaphragm this also moves in tandem and its movement causes a
disturbance in the air around it thus producing a sound or note. If the input signal is a
continuous sine wave then the cone will move in and out acting like a piston pushing and
pulling the air as it moves and a continuous single tone will be heard representing the
frequency of the signal. The strength and therefore its velocity, by which the cone moves
and pushes the surrounding air produces the loudness of the sound.
The human ear can generally hear sounds from between 20Hz to 20kHz, and the
frequency response of modern loudspeakers called general purpose speakers are tailored
to operate within this frequency range as well as headphones, earphones and other types
of commercially available headsets used as sound transducers
Television means viewing at a distance. TV broadcasting or telecasting involves the
transmission of both sound and picture at the same time.In TV, light signals from the
object being televised are converted into electrical signals by a TV camera and
transmitted to distant points by radio carrier waves. The TV receiver separates the signals
from the carrier waves and converts them into light signals which form a picture of the
televised object on the screen of the picture tube.
Separate carrier waves are used for the transmission of picture and sound signals but
they are radiated by the same transmitting antenna. At the receiving end the same
receiving antenna receives both carrier waves, but the TV receiver converts these signals
separately into sound waves which drive a loudspeaker and light waves which produce a
picture on the screen of the picture tube. For proper display of the picture and the
reproduction of the accompanying sound, several control signals have also to be
TV Broadcasting System
A block diagram of a complete TV system for transmission and reception of picture and
sound signals is given in figure below.
Block Diagram of Monochrome TV Transmitter
Block Diagram of Monochrome TV Receiver
At the TV studio, the TV camera focuses an optical image of the screen on a
photosensitive plate in the camera and picture elements of varying light intensity are
converted into correspondingly varying electrical signals by the process of electronic
scanning. The electrical signals so formed by the scanning of the picture image by an
electric beam are called video signals. At this stage, certain synchronizing signals meant
to keep the reassembly of the picture at the receiver in step with the scanning at the
studios are also added to the video information. The composite video signal so formed is
amplified by the video amplifiers and is made sufficiently strong to amplitude modulate a
picture carrier wave which is transmitted by the transmitting antenna.
The sound picked up by the microphone is converted to electrical currents at audio
frequencies (AF) and is strengthened by the audio amplifier which frequently modulates a
separate RF carrier whose frequency is 5.5 MHz above the frequency of the video carrier.
The FM sound carrier is radiated by the same transmitting antenna as used for the
transmission of video or picture carrier. Thus at TV transmitting station , two RF carriers,
one for transmission of picture signals and the other for sound signals are radiated by a
common transmitting antenna. The picture / video signal is amplitude modulated and the
sound is frequency modulated.
At the receiving end both picture and the sound are intercepted by the same receiving
antenna and passed into wideband circuit called the tuner. In the tuner two separate IF’s
for picture and sound are formed by heterodyning (mixing) with a local oscillator. The
picture and sound IF are amplified in a common IF amplifier and then detected by the
video detector. At this stage sound IF of 5.5 MHz is separated and is fed into the sound
channel where it is detected by the method of FM detection and IF is amplified and fed
into the speaker to produce the sound.
The video signal from the video detector stage is amplified by a video amplifier and is
used to modulate the electron beam in the picture tube to produce a picture of the TV
screen. A portion of the composite signal is also fed to a synchronizing separator where
the synchronizing signals are separated from the video signals and is applied to the
deflection circuits to keep the electronic scanning beam in the picture tube in step with
the electronic scanning beam at the transmitter.
Scanning is the process by which the optical image of the televised object formed on the
photosensitive plate of the TV camera is broken into a series of horizontal lines by an
electron beam. There are two types of scanning approaches: progressive (also called
sequential) and interlaced.
Progressive Scanning
In progressive scanning, the television scene is first sampled in time to create frames, and
within each frame all the raster lines are scanned from top to bottom. Therefore, all the
vertically adjacent scan lines are also temporally adjacent and are highly correlated even
in the presence of rapid motion in the scene. The electron beam sweeps across each line
at a uniform rate, then flies back to scan another line directly below the earlier one and so
on till all the horizontal lines have been scanned in the desired sequence .After this the
electron beam flies back to the original position and starts the scanning sequence again.
As the electron beam scans across a line, it falls over portions of different intensities and
is accordingly converted into electrical currents of different amplitudes. The higher the
illumination of a particular spot on a picture, the greater is the amplitude of the
corresponding electrical currents produced by the TV camera. This electrical signal
which corresponds to variations of illuminations in the TV scene is the video signal
which is used to modulate the picture carrier for transmission to distant places.
At the receiving end also, a similar electronic beam traces out horizontal lines on the
fluorescent screen of the picture tube and horizontal and vertical scanning produces a
uniformly lit rectangular area called the raster. When the scanning electron beam is
modulated by video signals from the transmitter, the raster is converted into picture.
In order that the picture formed at the picture tube, corresponds to the televised scene, it
is necessary that the scanning at the transmitter is completely in step or in
synchronization with the scanning at the TV receiver.
Persistence of vision
It is the property of the retina of the human eye that any impression produced in he retina
by the light ray will persist for a fraction of a second even after the light source is
removed. If within this short interval of persistence of vision, which is generally one by
sixteenth of a second, a series of images are presented to the eye, the eye will see all the
images without any break and will get the impression of continuity.
When the electron beam strikes the face of the picture tube at a particular point, this point
continues to glow for a short period, even after the beam has moved to the next point and
persistence of vision makes it possible to televise the picture element by element and
when these elements are scanned rapidly enough, they appear to the eye as one complete
picture.The picture repletion rate used in TV in India is 25 per second.
When one picture does not completely blend into another, ficker effect is produced. This
can be avoided by interlaced scanning.
Interlaced Scanning
In interlaced scanning, all the odd-numbered lines in the entire frame are scanned first,
and then the even numbered lines. This process produces two distinct images per frame,
representing two distinct samples of the image sequence at different points in time. The
set of odd-numbered lines constitute the odd field, and the even-numbered lines make up
the even field. All current television systems use interlaced scanning. One principal
benefit of interlaced scanning is to reduce the scan rate (or the bandwidth). This is done
with a relatively high field rate (a lower field rate would cause flicker), while maintaining
a high total number of scan lines in a frame (lower number of lines per frame would
reduce resolution on static images). Interlace cleverly preserves the high-detail visual
information and, at the same time, avoids visible large-area flicker at the display due to
temporal post filtering by the human eye.
Here odd lines are coloured green and even ones yellow. When the colour is removed the
two images merge to form the picture.
In interlaced scanning followed in India, the total number of lines per picture is 625 and
that scanned per second is 625*25=115625 lines. These lines are not scanned at a stretch,
but the process is divided into two stages called fields. Each field hence will contain only
312 ½ lines in one frame. The scanning beam will scan alternate odd numbered lines at
double the rate and starting at “A” and ending at “B”. It then flyback to “C” to scan the
even numbered lines to finish scanning at “D”. Thus flicker effect is overcome by making
the picture repletion rate double the frame repetition rate.
The process to make retrace invisible is called blanking. (Retrace is the pah followed by
the returning electron beam).
Colour Receiver
A colour receiver is similar to the black and white receiver as shown in Fig. The main
difference between the two is the need of a colour or chroma subsystem. It accepts only
the colour signal and processes it to recover (B-Y) and (R-Y) signals. These are
combined with the Y signal to obtain VR, VG and VB signals as developed by the
camera at the transmitting end. VG becomes available as it is contained in the Y signal.
The three colour signals are fed after sufficient amplification to the colour picture tube to
produce a colour picture on its screen.
As shown in Fig. the colour picture tube has three guns corresponding to the three pickup tubes in the colour camera. The screen of this tube has red, green and blue phosphors
arranged in alternate stripes. Each gun produces an electron beam to illuminate
corresponding colour phosphor separately on the fluorescent screen. The eye then
integrates the red, green and blue colour informations and their luminance to perceive
actual colour and brightness of the picture being televised. The sound signal is decoded in
the same way as in a monochrome receiver
Direct to home (DTH) television is a wireless system for delivering television programs
directly to the viewer's house. In DTH television, the broadcast signals are transmitted
from satellites orbiting the Earth to the viewer's house. Each satellite is located
approximately 35,700 km above the Earth in geosynchronous orbit. These satellites
receive the signals from the broadcast stations located on Earth and rebroadcast them to
Digital broadcast satellite transmits programming in the Ku frequency range (10 GHz to
14 GHz ). There are five major components involved in a direct to home (DTH) satellite
system: the programming source, the broadcast center, the satellite, the satellite dish and
the receiver.
Programming sources are simply the channels that provide programming for broadcast.
The provider (the DTH platform) doesn’t create original programming itself; it pays other
companies (HBO, for example, or ESPN or STAR TV or Sahara etc.) for the right to
broadcast their content via satellite. In this way, the provider is kind of like a broker
between the viewer and the actual programming sources. (Cable television networks also
work on the same principle.) The broadcast center is the central hub of the system. At the
broadcast center or the Playout & Uplink location, the television provider receives signals
from various programming sources, compreses I using digital compression, if necessary
scrambles it and beams a broadcast signal to the satellite being used by it. The satellites
receive the signals from the broadcast station and rebroadcast them to the ground. The
viewer’s dish picks up the signal from the satellite (or multiple satellites in the same part
of the sky) and passes it on to the receiver in the viewer’s house. The receiver processes
the signal and passes it on to a standard television. Lets look at each step in the process in
greater detail.
The Programming
Satellite TV providers get programming from two major sources: International
turnaround channels (such as HBO, ESPN and CNN, STAR TV, SET, B4U etc) and
various local channels (SaBe TV, Sahara TV, Doordarshan, etc). Most of the turnaround
channels also provide programming for cable television, so sometimes some of the DTH
platforms will ad in some special channels exclusive to itself to attract more
Turnaround channels usually have a distribution center that beams their programming to
a geostationary satellite. The broadcast center uses large satellite dishes to pick up these
analog and digital signals from several sources.
The Broadcast Center
The broadcast center converts all of this programming into a high-quality, uncompressed
digital stream. At this point, the stream contains a vast quantity of data — about 270
2megabits per second (Mbps) for each channel. In order to transmit the signal from there,
the broadcast center has to compress it. Otherwise, it would be too big for the satellite to
handle. The providers use the MPEG-2 compressed video format — the same format
used to store movies on DVDs. With MPEG-2 compression, the provider can reduce the
270-Mbps stream to about 3 or 10 Mbps (depending on the type of programming). This is
the crucial step that has made DTH service a success. With digital compression, a typical
satellite can transmit about 200 channels. Without digital compression, it can transmit
about 30 channels. At the broadcast center, the high-quality digital stream of video goes
through an MPEG-2 encoder, which converts the programming to MPEG-2 video of the
correct size and format for the satellite receiver in your house.
Encryption & Transmision
After the video is compressed, the provider needs to encrypt it in order to keep people
from accessing it for free. Encryption scrambles the digital data in such a way that it can
only be decrypted (converted back into usable data) if the receiver has the correct
decoding satellite receiver with decryption algorithm and security keys. Once the signal
is compressed and encrypted, the broadcast center beams it directly to one of its
satellites. The satellite picks up the signal, amplifies it and beams it back to Earth, where
viewers can pick it up.
The Dish
A satellite dish is just a special kind of antenna designed to focus on a specific broadcast
source. The standard dish consists of a parabolic (bowl-shaped) surface and a central feed
horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the
signal into a relatively narrow beam. The dish on the receiving end can’t transmit
information; it can only receive it. The receiving dish works in the exact opposite way of
the transmitter. When a beam hits the curved dish, the parabola shape reflects the radio
signal inward onto a particular point, just like a concave mirror focuses light onto a
particular point.
The curved dish focuses incoming radio waves onto the feed horn. In this case, the point
is the dish’s feed horn, which passes the signal onto the receiving equipment. In an ideal
setup, there aren’t any major obstacles between the satellite and the dish, so the dish
receives a clear signal. In some systems, the dish needs to pick up signals from two or
more satellites at the same time. The satellites may be close enough together that a
regular dish with a single horn can pick up signals from both. This compromises quality
somewhat, because the dish isn’t aimed directly at one or more of the satellites. A new
dish design uses two or more horns to pick up different satellite signals. As the beams
from different satellites hit the curved dish, they reflect at different angles so that one
beam hits one of the horns and another beam hits a different horn.The central element in
the feed horn is the low noise blockdown converter, or LNB. The LNB amplifies the
signal bouncing off the dish and filters out the noise (signals not carrying programming).
The LNB passes the amplified, filtered signal to the satellite receiver inside the viewer’s
The Receiver
The end component in the entire satellite TV system is the receiver. The receiver has four
essential jobs: It de-scrambles the encrypted signal. In order to unlock the signal, the
receiver needs the proper decoder chip for that programming package. The provider can
communicate with the chip, via the satellite signal, to make necessary adjustments to its
decoding programs. The provider may occasionally send signals that disrupt illegal
descramblers, as an electronic counter measure (ECM) against illegal users.
It takes the digital MPEG-2 signal and converts it into an analog format that a standard
television can recognize. Since the receiver spits out only one channel at a time, you can’t
tape one program and watch another. You also can’t watch two different programs on
two TVs hooked up to the same receiver. In order to do these things, which are standard
on conventional cable, you need to buy an additional receiver. Some receivers have a
number of other features as well. They pick up a programming schedule signal from the
provider and present this information in an onscreen programming guide. Many receivers
have parental lock-out options, and some have built-in Digital Video Recorders (DVRs),
which let you pause live television or record it on a hard drive. While digital broadcast
satellite service is still lacking some of the basic features of conventional cable (the
ability to easily split signals between different TVs and VCRs, for example), its high
quality picture, varied programming selection and extended service areas make it a good
alternative for some. With the rise of digital cable, which also has improved picture
quality and extended channel selection, the TV war is really heating up. Just about
anything could happen in the next 10 years as all of these television providers battle it
Common audio format for consumer audio recording or storage
Also a de-facto standard encoding for transfer and play back of music on digital
audio player
Designed by Motion Pictures Expert Group
MPEG audio compression
 MP3 is a digital audio codec
 Uses a lossy compression algorithm that greatly reduces the amount of data
required to represent the audio in recording
o Compress the source file by removing the portions of signal which are
o Algorithm take advantage of limitation of human hearing called audio
 Uses several complicated mathematical algorithms, that will lose
only that part of sound that are hard to be heard even in the orginal
o Perceptual coding
 Compression works by reducing accuracy of certain parts of sound
beyond the capacity of ear
Hence some frequencies are lost in the compression process & cant
be restored when converted to orginal format
 The loss is hardly noticed bcoz the compression method
tries to control it
MP3 file structure
 Standard format which is a frame consisting of 384, 5760r 1152 samples
 Has sequence of frames called elementary system
o All the frames have associated header (32 bits) & size info (9,17 or 32
 Help the decoder to decode the assopciated huffmann encoded
o Frames are not independent items & cannot be extracted on arbitrary
o Data block has audio info in terms of frequencies & amplitudes
o Header consist of a sync word, which identifies the beginning of a valid
 Followed by a bit 1 indicating MPEG standard and 2 that of Layer3
MP3 bit rates
 While doing audio encoding ie creating an MP3 file, there is a trade off between
amount of space and sound quality.
 Lower bit rates produce lower audio quality & produce smaller file size & vice
 CBR (Constant Bit Rate) encoding uses one rate for the entire file
 VBR (Variable bit Rate) uses bit rate that changes throughout the file
 128 kbps is the most common as it offer adequate audio quality and smaller file
Note: In addition to bit rates of an encoded audio, the quality of an MP3 file also depends
on the quality of encoder itself. As MP3 allows a bit of freedom for encoder algorithms,
quality of audio encoding is also dependent on choice of encoders and encoding
Related flashcards
Nuclear physics

42 Cards


15 Cards

Create flashcards