Introduction:

The word television is derived from a combination of two words “Tele” – a Greek word denoting “far” and “vision” is taken from the Latin word “see”. The following section gives a simplified block diagram of TV transmitter and receiver

Overview of Television Transmitter and Receiver:

Figure below Shows the Basic monochrome television transmitter.
A TV camera is used to convert the optical information into a corresponding electrical signal.
In the BW TV, the camera contains optics designed to focus an image on a photocathode consisting of a large number of photosensitive elements.
The charge pattern so generated on the photo-sensitive surface is scanned by an electron beam, thereby producing an output current.
The resulting output current is called a video signal.
The type of scanning used in television is raster scanning.
The video signal is amplified and then amplitude modulated with the channel picture carrier frequency.
The modulated output is fed to the transmitter antenna for radiation along with the sound signal.
The microphone converts the sound associated with the picture being televised into proportionate electrical signal, which is normally a voltage.
The electrical output needs only a single channel for its transmission.
The audio signal from the microphone after amplification is frequency modulated employing the assigned carrier frequency.
The output of the sound FM transmitter is finally combined with the A.M. picture transmitter output, through a combining network and fed to a common antenna for radiation of energy in the form of F.M. waves.

Elements of a Television System:

The elements of a simple broadcast television system are:

An image source. This is the electrical signal that represents a visual image, and may be derived from a professional video camera in the case of live television, a video tape recorder for playback of recorded images.
A sound source. This is an electrical signal from a microphone or from the audio output of a video tape recorder.
A transmitter, which generates radio signals (radio waves) and encodes them with picture and sound information.
A television antenna coupled to the output of the transmitter for broadcasting the encoded signals.
A television antenna to receive the broadcast signals.
A receiver, which decodes the picture and sound information from the broadcast signals, and whose input is coupled to the antenna of the television set.
A display device, which turns the electrical signals into visual images.
An audio amplifier and loudspeaker, which turns electrical signals into sound waves (speech, music, and other sounds) to accompany the images.

Scanning and its need:

It is necessary to present the picture to the eye in such a way that an illusion of continuity is created and any motion in the scene appears on the picture tube screen as a smooth and continuous change. The scene is scanned rapidly both in the horizontal and vertical directions simultaneously to provide sufficient number of complete pictures or frames per second to give the illusion of continuous motion.

Horizontal scanning:

Fig. below shows the trace and retrace of several horizontal lines.
The linear rise of current in the horizontal deflection coils deflects the beam across the screen with a continuous, uniform motion for the trace from left to right.
At the peak of the rise, the sawtooth wave reverses direction and decreases rapidly to its initial value.
This fast reversal produces the retrace or flyback.
The start of the horizontal trace is at the left edge of raster.
The finish is at the right edge, where the flyback produces retrace back to the left edge.

Vertical scanning:

The saw tooth current in the vertical deflection coils moves the electron beam from top to bottom of the raster at a uniform speed while the electron beam is being deflected horizontally.
Thus the beam produces complete horizontal lines one below the other while moving from top to bottom.
Fig. (c) shows the trace part of the sawtooth wave for vertical scanning deflects the beam to the bottom of the raster.

Then the rapid vertical retrace returns the beam to the top.

Fig. (b) shows during vertical retrace the horizontal scanning continues and several lines get scanned during this period.
Because of motion in the scene being televised, the information or brightness at the top of the target plate or picture tube screen normally changes by the time the beam returns to the top to recommence the whole process.
This information is picked up during the next scanning cycle and the whole process is repeated 25 times to cause an illusion of continuity.

Need of Synchronizing & Blanking Pulses:

Vertical Sync Pulses:

A video picture is made up of video frames. To avoid picture flickering in Camera, this video frame is divided into 2 fields i.e. odd and even fields. These two fields are separated out at the camera point and then combined once again at the monitor end. This is also called interlacing of fields.

At the end of each frame or field, a vertical sync pulse is added. This sync pulse tells the electronic devices in the camera that the field has come to an end and gets them ready to receive the next frame or field. The duration of the pulse

depends upon the time the electronic devices take to receive the next field. The amplitude of this pulse is a 0.3 volts. This when added to the video signal, gives total amplitude of 1 volt peak to peak.

Horizontal Sync Pulse:

A video frame is made of lines. In NTSC (North American broadcast television standard) there are 525 lines per frame, while PAL (Phase Alternating Line) has 625 lines per frames. Each point in the line reflects the intensity of the video signal.

At the end of each line, a horizontal sync pulse is added. This sync pulse tells the electronic devices that a line has come to an end and to get ready for the start of the next line.

Blanking:

Retrace or flyback is the time required to move from the end of one line to the start of the next line or from the end of one field to the start of the next field.

No picture information is scanned during the retrace and therefore must be blanked out.

In television, blanking means "going to black level". The retrace must be very rapid, since it is wasted time in terms of picture information.

The blanking pulse puts the video signal at the black level; the synchronization pulse starts the actual retrace in scanning. Each horizontal sync pulse is inserted in the video signal within the time of the horizontal blanking pulse and each vertical sync pulse is inserted in the video signal within the time of the vertical blanking time.

Vestigial Side Band

Vestigial Sideband Modulation or VSB Modulation is the process where a part of the signal called as vestige is modulated, along with one sideband.

Along with the upper sideband, a part of the lower sideband is also being transmitted in this technique. A guard band of very small width is laid on either side of VSB in order to avoid the interferences. This VSB modulation is mostly used in television transmissions.

The transmission bandwidth of VSB modulated wave will be the total of message bandwidth and the width of vestigial sideband. Two guard bands are laid on both the sides of this VSB signal so as to avoid the interference of signals.

Advantages

• Highly efficient.

• Reduction in bandwidth.

• Filter design is easy as high accuracy is not needed.

• The transmission of low-frequency components is possible.

Disadvantages

• Bandwidth, when compared to SSB, is greater.

• Demodulation is complex.

Applications

The most prominent and standard application of VSB is for the transmission of television signals. Also, this is most convenient and efficient technique when bandwidth usage is considered.

Composite Video Signal:

Composite video is an analog video signal format that carries standard-definition video as a single channel. Video information is encoded on one channel and audio is carried on a separate connection. Composite video is also known by the initials CVBS for composite video baseband signal or color, video, blanking and sync, or is simply referred to as SD video for the standard-definition television signal it conveys. There are three dominant variants of composite video: NTSC, PAL, and SECAM.

TV broadcasting originally began with "monochrome" signals, those containing just sync and luminance (brightness) information.

The composite signal is also referred to as CVBS (Colour, Video, Blank, and Sync) or sometimes CCVS (Composite Colour Video Signal). Composite color video signals are comprised of three components:

• The luminance (luma), Y, signal which contains the intensity information.

• The chrominance (chroma), C, signal which contains the color information.

• The synchronization (sync), S, signal which controls the horizontal line frequency.

Colour Encoding Standards

There are three color encoding standards that define how color information is modulated and combined with luminance:

1. NTSC (National Television Standard Committee) – used in North America, Central America, some parts of South America, Japan.

2. PAL (Phase Alternating Line) – used in Europe, Asia, Pacific, Africa, South America

3. SECAM (SÉquence de Colour À Mémoire) – used in France, Russia, Africa

Color Television:

A color TV screen differs from a black-and-white screen in three ways:

• There are three electron beams that move simultaneously across the screen. They are named the red, green and blue beams.

• The screen is not coated with a single sheet of phosphor as in a black-and-white TV. Instead, the screen is coated with red, green and blue phosphors arranged in dots or stripes. If you turn on your TV or computer monitor and look closely at the screen with a magnifying glass, you will be able to see the dots or stripes.

• On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen.

When a color TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue.

Concept of colors in TV:

The standard color wheel is the key to understanding many issues in color television.
Red, green and blue are TV's primary colors, and yellow, magenta, and cyan are considered secondary colors.
If any two colors exactly opposite each other on the color wheel are mixed, the result is white.
Note that instead of canceling each other as they did with subtractive colors, these complementary colors combine for an additive effect.
It may be obvious at this point that by combining the proper mixture of red, green and blue light any color of the rainbow can be produced.
Therefore, in color television only three colors (red, green and blue) are needed to produce a full range of colors in a color TV picture.

Television (TV) Camera Tube

A TV camera tube may be called the eye of a TV system or video camera. A TV camera tube is a transducer which converts the variations of light intensity into the variation of electrical current or voltage, known as video signals. It is an essential part of a video camera.

Basic Principle of Camera Tube:

An opto electrical converter is used to translate brightness variations into an electrical picture signal. Different converter systems are available, but only pickup tubes with a photosensitive semiconductor layer are really important for TV technology.

Any picture appears to be composed of small elementary areas of light or shade, which are known as picture elements. The elements thus contain the visual image of the scene.

The purpose of a TV pick-up tube is to sense each element independently and develop a signal in electrical form proportional to the brightness of each element. Light from the scene is focused on a photosensitive surface known as the image plate, and the optical image thus formed with a lens system represents light intensity variations of the scene. By making use of photoelectric properties, the image plate then converts different light intensities into corresponding electrical variations.

Types of Camera Tubes:

The first developed storage type of camera tube was ‘Iconoscope’ which has now been replaced by image orthicon camera tube. Because of its high light sensitivity, stability and high-quality picture capabilities. The light sensitivity is the ratio of the signal output to the incident illumination.

Next to be developed was the vidicon camera tube and is much simpler in operation. Similar to the vidicon camera tube is another tube known as plumbicon. The latest device in use for image scanning is the solid-state image scanner.

PAL TV Receiver:

Phase Alternating Line (PAL) is a colour encoding system for Analog television used in broadcast television systems in most countries broadcasting at 625-line / 50 field (25 frame) per second (576i). It was one of three major analogue colour television standards, the others being NTSC and SECAM.

The TV receiver has a VHF and UHF tuner on the front. It has a tuned circuit that allows you to choose the channel you want.

The antenna signal is amplified and transformed into an IF signal, which is then sent into the video IF amplifier.

Since the tuner's output signal isn't strong enough to drive the video detector, it is amplified to the appropriate level via cascaded IF amplifiers.

From the modulated composite video stream, the video detector recovers the original video signal.

The luminance signal ‘Y’ is applied to the Y amplifier which is a wideband video amplifier. It is further passed through a delay network providing 64 µsec delay and applied to the RGB matrix.

Despite the intensity of the input signal, the Automatic Gain Control (AGC) circuit keeps the output signal at consistent amplitude.

The detected U and V signals are applied to a resistive matrix to produce the (R-Y), (B-Y) and (G-Y) signal which are applied to RGB matrix along with luminance signal ‘Y’ to produce R, G and B signals.

Advantages :

-The phase error causing error in reproduction of colour is eliminated

-Bandwidth of U & V is same .This simplifies filtering action

-Studio mixing is easy as compared to SECAM

-Use of delay lines before demodulators isolates U and V signals from each other reduces crosstalk type of interference in colours better than NTSC-results in better picture quality.

Disadvantages:

-Design is complex as compared to others.

-Delay line technique reduces vertical resolution of chroma signal

Difference between PAL, NTSC and SECAM:

There are only three television standards in the world: NTSC, PAL, and SECAM

Plasma and conduction of charge:

A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Now a days Plasma TV lost nearly all market share due to competition from low-cost LCDs and more expensive but high-contrast OLED flat-panel displays. Plasma displays are bright, have a wide color range, and can be produced in fairly large sizes—up to 3.8 metres (150 in) diagonally.

Design:

A panel of a plasma display typically comprises millions of tiny compartments in between two panels of glass. These compartments, or "bulbs" or "cells", hold a mixture of noble gases and a minor amount of another gas (e.g., mercury vapor). Just as in the fluorescent lamps, when a high voltage is applied across the cell, the gas in the cells forms a plasma. Many tiny cells located between two panels of glass hold an inert mixture of noble gases (neon and xenon). The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light.

Working:

Essentially plasma is an electrically conductive gas that contains free-flowing ions (positively charged) and electrons (negatively charged). If you introduce more electrons by applying a voltage through the gas then they will begin to collide with atoms, knocking off electrons and turning them into ions. Then negatively charged particles will start to move towards the positively charged area, and vice versa. This causes the atomic equivalent of a motorway pile up, with particles smashing into each other and the xenon and neon gases used in plasma screens releasing photons of light. Most of this light is ultraviolet light which is invisible, but this is turned into visible light by painting the tiny cells with phosphoric material.

Advantages:

Color reproduction is very similar to that of CRTs.
Gives a superior contrast ratio than LCDs
Wider viewing angles than those of LCD
Faster response time
Gives good brightness level
They were less expensive for the buyer per square inch than LCD

Disadvantages:

Plasma displays are generally heavier than LCD
Does not work as well at high altitudes above 6,500 feet
Uses more electrical power
Signal processing in Plasma TV receivers: A PDP is built from two glass substrates, with a gas mixture between them. A ‘barrier’ structure between the glass plates divides the panel into separate cells or channels that are coated with phosphorescent material. In the gas, usually based on Neon and Xenon, an ion discharge can be induced by applying a voltage above the ‘ignition’ threshold (typically around 100V). This discharge emits UV-light, which is converted to visible light by phosphorescent materials.

Carbon Microphone

The carbon microphone is not widely used these days.

The carbon microphone was developed in the 1870s.

It was the first reliable form of microphone and it was widely used for many years before being supplanted by other types that gave much higher levels of performance.

The basic concept behind the carbon microphone is the fact that when carbon granules are compressed, their resistance decreases.

This occurs because the granules come into better contact with each other when they are pushed together by the higher pressure.

The carbon microphone comprises carbon granules that are contained within a small container that is covered with a thin metal diaphragm. A battery is also required to cause a current to flow through the microphone.

When sound waves strike the carbon microphone diaphragm it vibrates, exerting a varying pressure onto the carbon. These varying pressure levels are translated into varying levels of resistance, which in turn vary the current passing through the microphone.

The varying current can be passed through a transformer or a capacitor to enable it to be used within a telephone, or by some form of amplifier.

The frequency response of the carbon microphone, however, is limited to a narrow range, and the device produces significant electrical noise. Often the microphone would produce a form of crackling noise which could be eliminated by shaking it or giving it a small sharp knock. This would shake the carbon granules and enable them to produce a more steady current.

Applications

They were widely used in telephone

Radio

Carbon microphone advantages & disadvantages

As with any form of microphone there are advantages and disadvantages.

Carbon microphone advantages

• High output

• Simple principle & construction

• Cheap and simple to manufacture

Carbon microphone disadvantages

• Very noisy - high background noise and on occasions it would crackle

• Poor frequency response

• Requires battery or other supply for operation

MOVING COIL MICROPHONE:

Principle:

A moving coil microphone works on the principle of electromagnetic induction. Whenever a magnet is moved relative to a coil a current is produced in the coil.

Construction and Working

: A moving coil microphone has three main parts: a diaphragm, a moving coil and a permanent magnet. The diaphragm is a thin piece of metal, plastic or aluminium that vibrates when it is struck by sound waves. It is attached to the moving coil, which vibrates in response to the incoming sound waves. That is, the coil moves back and forth around the permanent magnet. This movement is converted into electrical signals, which are directed towards the loudspeaker through the wires.

The moving coil microphone is one of the most widely used forms of free standing microphones. It is widely used for vocals for musical performances as well as for many other applications.

The dynamic microphone is also simple in its design and as a result good microphones offer good value for money.

The dynamic or moving coil microphone relies on the fact that if a wire held within a magnetic field is moved then an electric current is induced. This is the same effect as seen in an electric generator and many other items

The assembly is held in place by an outer casing and the coil can move freely over the magnet.

As sound waves hit the diaphragm, this causes the coil to move backwards and forwards within the magnetic field

Cordless Microphone:

A wireless microphone, or cordless microphone, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. Also known as a radio microphone, it has a small, battery-powered, radio transmitter in the microphone body, which transmits the audio signal from the microphone by radio waves to a nearby receiver unit, which recovers the audio

A wireless microphone system is made up of the following 3 pieces:

1. Microphone

2. Transmitter

3. Receiver

Practically all wireless microphone systems use FM (frequency modulation) and need roughly 200 kHz bandwidth.

Loudspeaker:

A loudspeaker is a transducer that converts an electrical audio signal into a corresponding sound.

There are several different technologies and approaches used within loudspeakers. As a result there are several different types of loudspeaker

Moving Coil Loudspeaker:

It consists of a cone attached to a coil that is held within a magnetic field. It basically consists of a diaphragm, typically attached to a coil though which the audio is passed.

When a current flows in a wire, a magnetic field appears around it. When the wire is wound into a coil, the effect is increased.

If the coil is placed into a steady magnetic field created by a fixed magnet, then the two magnetic fields will interact. Opposite poles attract and like poles repel. This means that the current flowing in the coil can cause the coil to be attracted or repelled from the fixed magnetic field - the degree of the force being proportional to the current flowing. If the coil is attached to a large diaphragm, then the sound waves will be more effectively transferred to the air.

The coil is suspended within a magnetic field and this means that the variations in current flow resulting from the electrical audio signal cause the coil, and hence the cone to move. This results in the loudspeaker converting the electrical audio signal into sound.

The moving coil loudspeaker is the most widely known and used form of loudspeaker. It can be found in many electronic items from radios to Bluetooth speakers and in public address systems - in fact anywhere that electrical waveforms need to be turned into audible sound.

The whole paper in cone type loudspker acts as a diaphragm and causes pressure variation direct in the listeners area . Hence it is called "Direct radiating type loudspeaker

Horn loudspeaker:

It uses the same electromagnetic effect as the moving coil loudspeaker, a diaphragm held within a magnetic field that is varied in line with the audio. This causes the diaphragm to vibrate and these vibrations are then magnified by a horn.

The use of Horn loudspeakers can provide higher efficiency and more directionality

The main advantage of horn loudspeakers is they are more efficient; they can typically produce approximately 3 times (10 dB) more sound power than a cone speaker from a given amplifier output.

Therefore, horns are widely used in public address systems, megaphones, and sound systems for large venues like theatres, auditoriums, and sports stadiums

Multispeaker systems

In order for a speaker to efficiently produce sound, especially at lower frequencies, the speaker driver must be baffled, this generally takes the form of a speaker enclosure or speaker cabinet.

Hi-fi speaker system for home use with three types of dynamic drivers

1. Mid-range driver

2. Tweeter

3. Woofers

Number of speakers tells the way of system. A system having one driver speaker is known as one way system. If it has the three drivers, one to handle h.f, 2nd to handle mid range frequency and 3rd to handle low frequencies then it is called three way system

The smaller drivers capable of reproducing the highest audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers and those for low frequencies are called woofers.

A typical three way system may cover the following frequency ranges:

Low Freq. Range 30-800Hz Woofer

Mid frequency range 800 Hz-6KHz Mid range

HF Range above 6KHz Tweeter

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