Apr 142013
 

 Safety Precautions use before TV servicing and repairing

 Safety Precautions use before TV servicing and repairing

1. Be sure that all of the built-in protective devices are replaced. Restore any missing protective shields.

2. When reinstalling the chassis and its assemblies, be sure to restore all protective devices, including: nonmetallic control knobs and compartment covers.

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Jan 272013
 

DD DIRECT DTH  ServiceDD DIRECT DTH SERVICE

What is DD DIRECT DTH SERVICE ?

Doordarshan, the national broadcaster in India, at present has a network of more than 1400 transmitters spread throughout the country and its signals are available to about 90% (DD1) and 43% (DD-News) population of country. The prime duty of any national public service broadcaster is to make the programs of national importance available to all its people and nations. It was estimated that the coverage of remaining 10 % population with terrestrial (single channel) broadcast would cost enormously. Besides that, setting up of terrestrial transmitters in the uncovered areas would have taken a number of years (10 to 15 years). Also, operation of terrestrial transmission would have required a huge manpower (a few thousand persons).

With the fast developments taking place in Satellite Broadcasting, it is but natural that Doordarshan has also come up with an alternative to get the required reach with an alternate technology option “Ku-band broadcasting” which is envisaged for the coverage of remaining populations. This is a much cheaper and economical option as compared to the coverage through Terrestrial transmitters.

Ku-band transmission will provide coverage in all uncovered areas including remote, border, tribal, hilly and inaccessible areas in one go within a short time. With this coverage, the national broadcaster proposes to meet its obligation of covering the whole nation and its people not only with national channels, but also make available popular Doordarshan and some other free-to-air channels on its platform. In order to meet its obligations, it has also been decided that 10,000 receive systems (Dish and Set Top Box) would be provided free of cost at public institutions like Anganwadis, Schools, Public Health Centers, Panchayats, Youth Clubs, Cooperative Societies etc. in the uncovered areas.

It may be pertinent to mention here that incidentally Doordarshan would be starting DTH service with Ku band broadcasting as not only uncovered areas are covered with its commencement, but also the whole country gets multi-channel service in one stroke. The signal will be available at each home directly without the use of any cable etc. and one will be able to receive the programs with the help of a small Receive segment.

This platform has been named as DD DIRECT+ or DD DIRECT DTH SERVICE DD DIRECT+

Presently the DD DIRECT+ is envisaged to telecast 50 free-to-air TV channels (containing both Doordarshan and private channels) Satellite Earth Station for uplink of signals has been set up in Delhi. Honorable Prime Minister of India inaugurated the service on 16/12/2004. The high power Ku-band transponders of Indian Satellite INSAT-4B at 93.5º E are being used for hosting the DD DIRECT+ services.

How to receive DD DIRECT DTH SERVICE ?

Receive System includes a small satellite dish (which is an antenna for receiving a satellite broadcast signal); a digital integrated receiver/decoder (IRD) also called STB (Set Top Box), which separates each channel and decompresses and translates the digital signal for viewing over a television; and a remote control.

Investment on the part of subscriber for receiving DD-DIRECT DTH SERVICE  signal is only one time on receive system and no recurring monthly expenditure will have to be incurred by the viewer. As compared to other DTH systems, where a huge activation fee and a monthly subscription fee is charged, the DD DIRECT+ can be received without payment of any activation fees. Price of a receive system is less than Rs 3000/-. Once the technology gets induced the demand for the receive systems will increase and the prices would further comedown.

Installation of the Receive System

Installation of the System is very easy and does not take much time. However the viewer has to take the services of skilled technical personnel to get the dish installed and oriented towards the Satellite, which is required to be carried out before the system starts receiving the DD DIRECT+ signals. Tuning/configuring the STB also is required to be done initially. Tuning procedure is normally supplied by the manufacturer along with the STB.

Some of the parameters which are required to be fed to the IRD are mentioned therein and are also given under the heading 'Satellite in use'.

Satellite in use

A powerful Satellite INSAT-4B is being used to uplink the DD DIRECT+ signal. Reception of the signal has been checked to be available throughout the country.

Main features of the satellite are listed below:

  • Orbit type: Geo-synchronous
  • Orbital location: 93.5° E

INSAT-4B is the latest Geo-synchronous Satellite of INSAT series with high power Ku Band Transponders launched by ISRO in the beginning of the year 2007. Service parameters of the bouquet of channels, which have been put into operation at present, are as follows:

  • In the beginning Doordarshan DTH started with three streams of 20 TV Channel, which is now upgraded to 5 streams of DTH bouquet, capable of transmitting 50 TV channels.

To install the channels following procedure is to be followed which is only suggestive. Viewers are advised to consult the installation manual supplied with STB or contact the nearest STB Dealer

1. Satellite Name:

  • Go to Installation menu or setup program from remote.
  •  Select Satellite Edit menu.
  • Select Add New Satellite menu.
  • Select Sat Name Edit menu.
  • Enter Satellite Name as INSAT- 4B.
  • Enter Satellite Longitude as 93.5o E
  • Press Exit

2. LNB Configuration

  • Go to LNB Configuration menu. Ensure that satellite selected is INSAT- 4B only.
  • Set LNB Types as Universal.
  • Set LNB Power on.
  • Press Exit

3. Transponder (TP) Edit:

  • Go to Transponder Edit menu. Ensure that satellite selected is INSAT- 4B.
  • Select Add New TP menu.
  • Enter TP Frequency as 10990 MHz.
  • Select Symbol rate as 27500 ksps.
  • Select Polarization as Vertical.
  • Select Scan as FTA.
  • Press OK.
  • ‘22K’ OFF*
  • ‘Disc. Equal’. Off

After filling the above values Go to ‘search’

Press ‘OK’

The new TV and Radio services will be displayed. Similarly add all other four TPs by entering the frequency, symbol rate and polarization as given in Table above

Note: – The above-mentioned steps may be in, different order in some of the STBs.

In case there is any difficulty, viewers are advised to contact the nearest LPT (Doordarshan Relay Center) or STB Dealer.

 Posted by at 8:55 am
Jan 262013
 

Fire Extinguisher

Before using your fire extinguisher, be sure to read the instructions before it's too late. Although there are many different types of fire extinguishers, all of them operate in a similar manner.Fire Extinguisher

Types of Fire Extinguisher

Each fire extinguisher has its own symbolic notation, that is a special geometric symbol to make it easier for you to identify the extinguisher type. They also have some additional information necessary in case of this or that class of fire fighting.

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 Posted by at 10:57 pm
Jan 042013
 

Television system ( T V)

INTRODUCTION

The aim of a television system is to extend the sense of sight beyond its natural limits and to transmit sound associated with the scene. The picture signal is generated by a television camera and sound signal by a microphone. In the 625 line CCIR monochrome and PAL-B colour TV systems adopted by India, the picture signal is amplitude modulated and sound signal frequency modulated before transmission. The two carrier frequencies are suitably spaced and their modulation products radiated through a common antenna. As in radio communication, each television station is allotted different carrier frequencies to enable selection of the desired station at the receiving end.

The TV receiver has tuned circuits in its input section called ‘tuner’. It selects desired channel signal out of the many picked up by the antenna. The selected RF band is converted to a common fixed IF band for the convenience of providing large amplification to it. The amplified IF signals are detected to obtain video (picture) and audio (sound) signals. The video signal after large amplification drives the picture tube to reconstruct the televised picture on the receiver screen. Similarly, the audio signal is amplified and fed to the loudspeaker to produce sound output associated with the scene.

PICTURE TRANSMISSION

The picture information is optical in character and may be thought of as an assemblage of a large number of tiny areas representing picture details. These elementary areas into which picture details may be broken up are known as ‘picture elements’ or ‘pixels’, which when viewed together represent visual information of the scene. Thus, at any instant there are almost an infinite number of pieces of information that need to be picked up simultaneously for transmitting picture details. However, simultaneous pick-up is not practicable because it is not feasible to provide a separate signal path (channel) for the signal obtained from each picture element. In practice, this problem is solved by a method known as ‘scanning’ where conversion of optical information to electrical form is carried out element by element, one at a time and in a sequential manner to cover the entire picture. Besides, scanning is done at a very fast rate and repeated a large number of times per second to create an illusion (impression at the eye) of the simultaneous reception from all the elements, though using only one signal path.

Black and White Pictures

In a monochrome (black and white) picture, each element is either bright, some shade of grey or dark. A television camera, the heart of which is a camera tube, is used to convert this optical information into corresponding electrical signals, the amplitude of which varies in accordance with variations of brightness.

Fig. 1 shows very elementary details of one type of camera tube (vidicon) and associated components to illustrate the principle. An optical image of the scene to be transmitted is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner side of the glass face-plate has a transparent conductive coating on which is laid a very thin layer of photoconductive material. The photolayer has very high resistance when no light falls on it, but decreases depending on the intensity of light falling on it. Thus depending on light intensity variations in the focused optical image, the conductivity of each element of photo layer changes accordingly. An electron beam is used to pick-up picture information now available on the target plate in terms of varying resistance at each point.

CRT

The beam is formed by an electron gun in the television camera tube. On its way to the inner side of glass face-plate, it is deflected by a pair of deflecting coils mounted on the glass envelope and kept mutually perpendicular to each other to achieve scanning of the entire target area. Scanning is done in the same way as one reads a written page to cover all the words in one line and all the lines on the page (see Fig. 2). To achieve this, the deflecting coils are fed separately from two sweep oscillators which continually generate suitable waveform voltages, each operating at a different desired frequency.

Magnetic deflection caused by the current in one coil gives horizontal motion to the beam from left to right at uniform rate and then brings it quickly to the left side to commence trace of the next line. The other coil is used to deflect the beam from top to bottom at a uniform rate and for its quick retrace back to the top of the plate to start this process over again. Two simultaneous motions are thus given to the beam, one from left to right across the target plate and the other from top to bottom thereby covering entire area on which electrical image of the picture is available. As the beam moves from element to element, it encounters a different resistance across the target-plate, depending on the resistance of photoconductive coating. The result is a flow of current which varies in magnitude as the elements are scanned. This current passes through a load resistance RL connected to the conductive coating on one side and to a dc supply source on the other. Depending on the magnitude of current, a varying voltage appears across resistance RL and this corresponds to optical information of the picture.

If the scanning beam moves at such a rate that any portion of the scene content does not have time to change perceptibly in the time required for one complete scan of the image, the resulting electrical signal contains true information existing in the picture during the time of scan. The desired information is now in the form of a signal varying with time and scanning may thus be identified as a particular process which permits conversion of information existing in space and time co-ordinates into time variations only. The electrical information thus obtained from the TV camera tube is generally referred to as video signal (video is Latin for ‘see’).

scanning beam

Colour Pictures

It is possible to create any colour including white by additive mixing of red, green and blue colour lights in suitable proportions. For example, yellow can be obtained by mixing red and green colour lights in the intensity ratio of 30 : 59. Similarly, light reflected from any colour picture element can be synthesized (broken up) into red, green and blue colour light constituents. This forms the basis of colour television where Red (R), Green (G) and Blue (B) colours are called primary colours and those formed by mixing any two of the three primaries as complementary colours. A colour camera, the elements of which are shown in Fig. 3, is used to develop signal voltages proportional to the intensity of each primary colour light.

Colour Pictures

It contains three camera tubes (vidicons) where each pick-up tube receives light of only one primary colour. Light from the scene falls on the focus lens and through that on special mirrors. Colour filters that receive reflected light via relay lenses split it into R, G and B colour lights.Thus, each vidicon receives a single colour light and develops a voltage proportional to the intensity of one of the primary colours. If any primary colour is not present in any part of the picture, the corresponding vidicon does not develop any output when that picture area is scanned. The electron beams of all the three camera tubes are kept in step (synchronism) by deflecting them horizontally and vertically from common driving sources.

Any colour light has a certain intensity of brightness. Therefore, light reflected from any colour element of a picture also carries information about its brightness called luminance. A signal voltage (Y) proportional to luminance at various parts of the picture is obtained by adding definite proportions of VR , V G and V B (30:59:11). This then is the same as would be developed by a monochrome (black and white) camera when made to scan the same colour scene. This i.e., the luminance (Y) signal is also transmitted alongwith colour information and used at picture tube in the receiver for reconstructing the colour picture with brightness levels as in the televised picture.

TELEVISION TRANSMITTER

television transmitter
An oversimplified block diagram of a monochrome television transmitter is shown in Fig. 4. The luminance signal from the camera is amplified and synchronizing pulses added before feeding it to the modulating amplifier. Synchronizing pulses are transmitted to keep the camera and the picture tube beams in step. The allotted picture carrier frequency is generated by a crystal controlled oscillator. The continuous wave (CW) sine wave output is given large amplification before feeding to the power amplifier where its amplitude is made to vary (AM) in accordance with the modulating signal received from the modulating amplifier. The modulated output is combined (see Fig. 4) with the frequency modulated (FM) sound signal in the combining network and then fed to the transmitting antenna for radiation.

Colour Transmitter

A colour television transmitter is essentially the same as the monochrome transmitter except for the additional need that colour (chroma) information is also to be transmitted. Any colour system is made compatible with the corresponding monochrome system. Compatibility means that the colour TV signal must produce a normal black and white picture on a monochrome receiver and a colour receiver must be able to produce a normal black and white picture from a monochrome television signal. For this, the luminance (brightness) signal is transmitted in a colour system in the same way as in the monochrome system and with the same bandwidth. However, to ensure compatibility, the colour camera outputs are modified to obtain (B-Y) and (R-Y) signals. These are modulated on the colour sub-carrier, the value of which is so chosen that on combining with the luminance signal, the sidebands of the two do not interfere with each other i.e., the luminance and colour signals are correctly interleaved. A colour sync signal called ‘colour burst’ is also transmitted for correct reproduction of colours.

Sound Transmission

There is no difference in sound transmission between monochrome and colour television systems. The microphone converts the sound associated with the picture being televised into proportionate electrical signal, which is normally a voltage. This electrical output, regardless of the complexity of its waveform, is a single valued function of time and so needs a single channel for its transmission. The audio signal from the microphone after amplification is frequency modulated, employing the assigned carrier frequency. In FM, the amplitude of carrier signal is held constant, whereas its frequency is varied in accordance with amplitude variations of the modulating signal. As shown in Fig..4, output of the sound FM transmitter is finally combined with the AM picture transmitter output, through a combining network, and fed to a common antenna for radiation of energy in the form of electromagnetic waves.

TELEVISION RECEIVER

TELEVISION RECEIVER

A simplified block diagram of a black and white television receiver is shown in Fig. 5. The receiving antenna intercepts radiated RF signals and the tuner selects desired channel’s frequency band and converts it to the common IF band of frequencies. The receiver employs two or three stages of intermediate frequency (IF) amplifiers. The output from the last IF stage is demodulated to recover the video signal. This signal that carries picture information is amplified and coupled to the picture tube which converts the electrical signal back into picture elements of the same degree of black and white. The picture tube shown in Fig. 6 is very similar to the cathode-ray tube used in an oscilloscope. The glass envelope contains an electron-gun structure that produces a beam of electrons aimed at the fluorescent screen. When the electron beam strikes the screen, light is emitted. The beam is deflected by a pair of deflecting coils mounted on the neck of picture tube in the same way as the beam of camera tube scans the target plate. The amplitudes of currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast rate of scanning.

Elements of picture tube

The video signal is fed to the grid or cathode of the picture tube. When the varying signal voltage makes the control gridless negative, the beam current is increased, making the spot of light on the screen brighter. More negative grid voltage reduces brightness. If the grid voltage is negative enough to cut-off the electron beam current at the picture tube, there will be no light. This state corresponds to black. Thus the video signal illuminates the fluorescent screen from white to black through various shades of grey depending on its amplitude at any instant. This corresponds to brightness changes encountered by the electron beam of the camera tube while scanning picture details element by element. The rate at which the spot of light moves is so fast that the eye is unable to follow it and so a complete picture is seen because of storage capability of the human eye.

Sound Reception

The path of a sound signal is common with the picture signal from the antenna to a video detector section of the receiver. Here the two signals are separated and fed to their respective channels. The frequency modulated audio signal is demodulated after at least one stage of amplification. The audio output from the FM detector is given due amplification before feeding it to the loudspeaker.

Colour Receiver

Colour Receiver
A colour receiver is similar to the black and white receiver as shown in Fig. 7. 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. 7, the colour picture tube has three guns corresponding to the three pick-up 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 information 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.

SYNCHRONIZATION

It is essential that the same co-ordinates be scanned at any instant both at the camera tube target plate and at the raster of picture tube, otherwise, the picture details would split and get distorted. To ensure perfect synchronization between the scene being televised and the picture produced on the raster, synchronizing pulses are transmitted during the retrace, i.e., fly-back intervals of horizontal and vertical motions of the camera scanning beam. Thus, in addition to carrying picture details, the radiated signal at the transmitter also contains synchronizing pulses. These pulses which are distinct for horizontal and vertical motion control, are processed at the receiver and fed to the picture tube sweep circuitry thus ensuring that the receiver picture tube beam is in step with the transmitter camera tube beam.

As stated earlier, in a colour television system additional sync pulses called colour burst are transmitted along with horizontal sync pulses. These are separated at the input of chroma section and used to synchronize the colour demodulator carrier generator. This ensures the correct reproduction of colours in the otherwise black and white picture.

RECEIVER CONTROLS

Most black and white receivers have on their front panel (i ) channel selector, ( ii ) fine tuning, ( iii ) brightness, ( iv ) contrast, ( v) horizontal hold and (vi ) volume controls besides an ON-OFF switch. Some receivers also provide a tone control. The channel selector switch is used for selecting the desired channel. The fine tuning control is provided for obtaining best picture details in the selected channel.

The hold control is used to get a steady picture in case it rolls up or down. The brightness control varies the beam intensity of the picture tube and is set for optimum average brightness of the picture. The contrast control has actually gained control of the video amplifier. This can be varied to obtain desired contrast between white and black contents of the reproduced picture. The volume and tone controls form part of the audio amplifier in sound section, and are used for setting volume and tonal quality of the sound output from the loudspeaker.

In colour receivers there is an additional control called ‘colour’ or ‘saturation’ control. It is used to vary the intensity or amount of colours in the reproduced picture. In modern colour receivers that employ integrated circuits in most sections of the receiver, the hold control is not necessary and hence usually not provided.

Sep 092012
 

SWITCH MODE POWER SUPPLY – SMPS

The use of ICs and modular construction is very common is modern television receivers, Transmitter, Computer and many Electronics Equipment’s. This has led to the introduction of switching mode power supplies to meet the dc requirements of such receivers. These are smaller, lighter and dissipate less power than equivalent series regulated supplies.

Basic Principle

SMPS

In a switched mode supply the regulating elements consist of series connected transistors that act as rapidly opening and closing switches. The input ac is first converted to unregulated dc, which, in turn is chopped by the switching elements operating at a rapid rate, typically 20 KHz. The resultant 20 KHz pulse train is transformer coupled to an output network which provides final rectification and smoothing of the dc output. Regulation is accomplished by control circuits which vary the duty cycle (on-off periods) of the switching elements if the output voltage tends to vary.

Operating Advantages and Disadvantages of SMPS

The advantages of a SMPS over a conventional regulated supply are:

( i ) The switching transistors are basically on-off devices and hence dissipate very little power when either on (saturated) or off (non-conducting). Efficiencies ranging from 65 to 85 percent are typical of such supplies as compared to 30 to 45 percent efficiencies for linear supplies.

( ii ) On account of the higher switching rate (20 KHz) the power transformer, inductor and filter capacitors are much smaller and lighter than those required for operation at power line frequencies. Typically a switching power supply is less than one third in size and weight of a comparable series regulated supply.

( iii) A switched-mode supply can operate under low ac input voltage. It has a relatively long hold-up period if input power is lost momentarily. This is so because more energy can be stored in its input filter capacitors.

Disadvantages of SMPS. Although the advantages are impressive a SMPS has the following inherent disadvantages:

( i ) Electromagnetic interference (EMI) is a natural by-product of the on-off switching within these supplies. This interference can get coupled to various sections of the receiver and hinder their normal operation. For this reason, switching supplies have built-in shields and filter networks which substantially reduce EMI and also control output ripple and noise. In addition, special shields are provided around those sections of the receiver circuitry which are highly susceptible to electromagnetic interference.

( ii ) The control circuitry is expensive, quite complex and somewhat less reliable.

Typical Circuit of a SMPS

Figure shows simplified circuit and associated waveforms of a typical switching mode power supply. Regulation is achieved by a pair of push-pull switching transistors (Q1 and Q2) operating under the control of a feedback network consisting of a pulse-width modulator and a voltage comparison amplifier. The waveforms illustrate the manner in which the duty-cycle is controlled to deliver a constant dc output voltage. The voltage comparison amplifier continuously compares a fraction of the output voltage with a stable reference source Vr 1 and develops a

RECEIVER POWER SUPPLIES

Control voltage ( V control ) for the turn-on comparator. The comparator compares V control with a triangular ramp waveform occurring at a frequency of 40 KHz. When the ramp voltage is more positive than the control level, a turn-on signal is generated. As shown in the waveforms, any increase or decrease in the control voltage (V control) will very width of the turn-on voltage and this in turn will alter the width of drive pulses to both  Q 1 and  Q 2. The drive pulses pass through steering logic which ensures alternate switching of Q1 and Q 2. Thus each switch operates at 20 KHz, i.e., one-half of the ramp frequency. When Q1  is on, current flows in the upper half of the primary winding of transformer T1 and completes its path through its Centre tap. Similarly when Q 2 conducts current flows in opposite direction through the lower half of the same winding to complete its return path thus providing transformer action.

SMPS

Since the dc output voltage is proportional to the duty-cycle of current through the transformer, increasing the ‘on’ periods of the switching transistor will increase the output voltage and vice-versa. Thus the control voltage automatically monitors the duty-cycle to maintain a constant dc output voltage despite any input voltage or load current variations. In some such supplies only one transistor is used as the chopping and control element. In such designs a 20 KHz clock pulse is used to time the on-off periods. The comparator, ramp generator and steering or control logic usually form part of a dedicated IC. Though, not shown, in the modular chip it contains additional circuits for over voltage protection, over-current protection and prevention of any inrush of ac current.

 Posted by at 3:32 pm
Mar 192012
 

Block Diagram of Colour Television

According to Block Diagram of Colour Television Sets In a colour television receiver, additional circuits are provided to deal with the colour.

The only difference between black and White Television set and colour Television set is the IF circuit is the importance of bandwidth for colour receivers. Remember that video frequencies around 3.58 MHz just show details in monochrome, but these frequencies are essential for colour information. Without them, there is no colour. This is why the fine tuning control on colour television sets must be tuned exactly, or else the colour disappears, along with the higher resolution.

The sound is usually taken off before the video detector in colour sets, and a separate converter is used for it, instead of taking it from the video detector. The reason that this is done is to minimize a 920 KHz beat signal that can result between the 3.58 MHz colour subcarrier and the sound carrier signal. This signal would show up as interference in the television picture.

Block Diagram of Colour Television Sets


The output from the video detector is sent to two places: a series of colour circuits, and a luminance output amplifier.

The luminance amplifier also serves as a cutoff filter for frequencies above 3.2 MHz, thus removing all colour information from the luminance video signal and, alas, some of the sharpness and detail. On this amplifier is where you will find your brightness and contrast controls.

In the colour recovery circuits, several things happen. First, the video detector's output is sent through a colour "band pass" filter, which leaves us with just the chrominance information – the luminance has been removed. This chroma output contains both the colour information for the picture, and the colour burst. It is then sent to a burst separator to detect the phase and level of the colour burst. This is where you’ll find your "colour" control. Now we'll have a reference for the colours within the picture, which is sent to a crystal oscillator which generates constant 3.58 MHz subcarrier of the correct phase. This oscillator’s phase can be adjusted – this is your hue control. The oscillator is used with two colour demodulators to recover the R-Y and B-Y colour difference signals. The continuous wave subcarrier is delayed by 90 degrees of phase before it enters the R-Y demodulator. The R-Y and B-Y signals are combined further to recover the G-Y signal.

All three signals are then sent to the colour picture tube's grids. There, they are combined with three luminance drive signals in the correct proportions, giving us our familiar RGB signals for driving the electron guns within the picture tube to re-create the colour television picture.

When satellite television first hit the market in the early 1990s, home dishes were expensive metal units that took up a huge chunk of yard space. In these early years, only the most die-hard TV fans would go through all the hassle and expense of putting in their own dish. Satellite TV was a lot harder to get than broadcast and cable TV.

Today, you see compact satellite dishes perched on rooftops all over the United States. Drive through rural areas beyond the reach of the cable companies, and you'll find dishes on just about every house. The major satellite TV companies are luring in more consumers every day with movies, sporting events and news from around the world and the promise of movie-quality picture and sound.

Satellite TV offers many solutions to broadcast and cable TV problems. Though satellite TV technology is still evolving, it has already become a popular choice for many TV viewers.

In this article, we'll find out how satellite TV works, from TV station to TV set. We'll also learn about the changing landscape of TV viewing and some basic differences that distinguish satellite TV from cable and over-the-air broadcast TV.

The output from the video detector is sent to two places: a series of colour circuits, and a luminance output amplifier.

The luminance amplifier also serves as a cutoff filter for frequencies above 3.2 MHz, thus removing all colour information from the luminance video signal and, alas, some of the sharpness and detail. On this amplifier is where you will find your brightness and contrast controls.

In the colour recovery circuits, several things happen. First, the video detector's output is sent through a colour "band pass" filter, which leaves us with just the chrominance information – the luminance has been removed. This chroma output contains both the colour information for the picture, and the colour burst. It is then sent to a burst separator to detect the phase and level of the colour burst. This is where you’ll find your "colour" control. Now we'll have a reference for the colours within the picture, which is sent to a crystal oscillator which generates constant 3.58 MHz subcarrier of the correct phase. This oscillator’s phase can be adjusted – this is your hue control. The oscillator is used with two colour demodulators to recover the R-Y and B-Y colour difference signals. The continuous wave subcarrier is delayed by 90 degrees of phase before it enters the R-Y demodulator. The R-Y and B-Y signals are combined further to recover the G-Y signal.

All three signals are then sent to the colour picture tube's grids. There, they are combined with three luminance drive signals in the correct proportions, giving us our familiar RGB signals for driving the electron guns within the picture tube to re-create the colour television picture.

When satellite television first hit the market in the early 1990s, home dishes were expensive metal units that took up a huge chunk of yard space. In these early years, only the most die-hard TV fans would go through all the hassle and expense of putting in their own dish. Satellite TV was a lot harder to get than broadcast and cable TV.

Today, you see compact satellite dishes perched on rooftops all over the United States. Drive through rural areas beyond the reach of the cable companies, and you'll find dishes on just about every house. The major satellite TV companies are luring in more consumers every day with movies, sporting events and news from around the world and the promise of movie-quality picture and sound.

Satellite TV offers many solutions to broadcast and cable TV problems. Though satellite TV technology is still evolving, it has already become a popular choice for many TV viewers.

In this article, we'll find out how satellite TV works, from TV station to TV set. We'll also learn about the changing landscape of TV viewing and some basic differences that distinguish satellite TV from cable and over-the-air broadcast TV.

Mar 192012
 

Block Diagram of Black and White Television 

Block Diagram of B & W Television Sets

According to the Block Diagram of Black and White Television Sets In a typical black and white television receiver, the signal from the antenna is fed to the tuner. Two channel selector switches – one for the VHF (very-high-frequency) channels 2-13 and the other for the UHF (ultra-high-frequency) channels 14-69 -are used. They connect circuits that are tuned to the desired channels and, also discriminate against signals from undesired channels. These circuits also form part of an amplifier, designed to add as little snow to the signal as possible.

The amplified signals from the desired channel are then passed to the mixer, which transposes all the signal frequencies in the channel to different values, called intermediate frequencies. The output of the tuner consists of all the signals in the desired channel, but the intermediate channel is fixed in the frequency band from 41 to 47 MHz, no matter what channel is tuned in. This is kind of like those cable television "set top" converters, that, regardless of what channel you’re watching, always convert it to "channel 3" for your TV set.

From the tuner, the 41-47 MHz channel with all picture and sound information present is passed successively through several additional amplifiers (from two to four intermediate frequency, or IF, amplifiers), which provide most of the amplification in the receiver. Their amplification is automatically adjusted, being maximum on a weak signal and less on a strong signal. So far the receiver handles the signals in the channel just like they would be received from the transmitter, except for the shift to intermediate frequencies and the amplification.

The next stage is the video detector, which removes the high frequency carrier signal and recovers the video signal. The detector also reproduces (at a lower frequency) the sound carrier and its frequency variations. The sound signal is then separated from the picture signal and passes through a frequency detector, which recovers the audio signal. This signal is amplified further and fed to the loudspeaker, where it re-creates the accompanying sound. The picture signal from the video detector is used in the normal fashion for display on the CRT of the television receiver.

Mar 152012
 

WORLD TELEVISION SIGNAL GUIDEWORLD TELEVISION SIGNAL GUIDE

The table below lists broadcast standards by country.

There are three main television standards used throughout the world.

NTSC – National Television Standards Committee

Developed in the US and first used in 1954, NTSC is the oldest existing broadcast standard. It consists of 525 horizontal lines of display and 60 vertical lines. Only one type exists, known as NTSC M. It is sometimes irreverently referred to as "Never Twice the Same Color."

SECAM – Système Électronique pour Couleur avec Mèmoire.

Developed in France and first used in 1967. It uses a 625-line vertical, 50-line horizontal display. Different types use different video bandwidth and audio carrier specifications. Types B and D are usually used for VHF. Types G, H, and K are used for UHF. Types I, N, M, K1 and L are used for both VHF and UHF. These different types are generally not compatible with one another. SECAM is sometimes irreverently referred to as "Something Essentially Contrary to the American Method" or "SEcond Color Always Magenta."

PAL – Phase Alternating Line

Developed in Germany and first used in 1967. A variant of NTSC, PAL uses a 625/50-line display. Different types use different video bandwidth and audio carrier specifications. Common types are B, G, and H. Less common types include D, I, K, N, and M. These different types are generally not compatible with one another. Proponents of PAL irreverently call it "Perfection At Last," while critics of its enormous circuit complexity call it "Pay A Lot" or "Picture Always Lousy."

Television Standards by Country

Country

Signal Type

Afghanistan

PAL B, SECAM B

Albania

PAL B/G

Algeria

PAL B/G

Angola

PAL I

Antarctica

NTSC M

Antigua & Barbuda

NTSC M

Argentina

PAL N

Armenia

SECAM D/K

Aruba

NTSC M

Australia

PAL B/G

Austria

PAL B/G

Azerbaijan

SECAM D/K

Azores

PAL B

Bahamas

NTSC M

Bahrain

PAL B/G

Bangladesh

PAL B

Barbados

NTSC M

Belarus

SECAM D/K

Belgium

PAL B/H

Belgium (Armed Forces Network)

NTSC M

Belize

NTSC M

Benin

SECAM K

Bermuda

NTSC M

Bolivia

NTSC M

Bosnia/Herzegovina

PAL B/H

Botswana

SECAM K, PAL I

Brazil

PAL M

British Indian Ocean Territory 

NTSC M

Brunei Darussalam

PAL B

Bulgaria

PAL

Burkina Faso

SECAM K

Burundi

SECAM K

Cambodia

PAL B/G, NTSC M

Cameroon

PAL B/G

Canada

NTSC M

Canary Islands

PAL B/G

Central African Republic

SECAM K

Chad

SECAM D

Chile

NTSC M

China (People'S Republic)

PAL D

Colombia

NTSC M

Congo (People'S Republic)

SECAM K

Congo, Dem. Rep. (Zaire)

SECAM K

Cook Islands

PAL B

Costa Rica

NTSC M

Cote D'Ivoire (Ivory Coast)

SECAM K/D

Croatia

PAL B/H

Cuba

NTSC M

Cyprus

PAL B/G

Czech Republic

PAL B/G (cable), PAL D/K (broadcast)

Denmark

PAL B/G

Diego Garcia

NTSC M

Djibouti

SECAM K

Dominica

NTSC M

Dominican Republic

NTSC M

East Timor

PAL B

Easter Island

PAL B

Ecuador

NTSC M

 

Mar 152012
 

 SPECTRUM CHART

Radio Frequency Bandwidth
The Allocated Radio Spectrum is located between 9 KHz and 300 GHz


Bandwidth DESCRIPTION

FREQUENCY RANGE

Extremely Low Frequency (ELF)

 

 

0

 

to

3

KHz

Very Low Frequency (VLF)

 

 

3

KHz

to

30

KHz

Radio Navigation &
maritime/aeronautical mobile

 

 

9

KHz

to

540

KHz

Low Frequency (LF)

 

 

30

KHz

to

300

KHz

Medium Frequency (MF)

 

 

300

KHz

to

3000

KHz

AM Radio Broadcast

 

 

540

KHz

to

1630

KHz

Travellers Information Service

 

 

1610

KHz

 

 

 

High Frequency (HF)

 

 

3

MHz

to

30

MHz

 Shortwave Broadcast Radio

 

 

5.95

MHz

to

26.1

MHz

Very High Frequency (VHF)

 

 

30

MHz

to

300

MHz

 Low Band: TV Band 1 – Channels 2-6

 

 

54

MHz

to

88

MHz

Mid Band: FM Radio Broadcast

 

 

88

MHz

to

174

MHz

High Band: TV Band 2 – Channels 7-13

 

 

174

MHz

to

216

MHz

Super Band (mobile/fixed radio & TV)

 

 

216

MHz

to

600

MHz

Ultra-High Frequency (UHF)

 

 

300

MHz

to

3000

MHz

Channels 14-70

 

 

470

MHz

to

806

MHz

L-band:

 

 

500

MHz

to

1500

MHz

Personal Communications Services (PCS)

 

 

1850

MHz

to

1990

MHz

Unlicensed PCS Devices

 

 

1910

MHz

to

1930

MHz

Superhigh Frequencies (SHF)
   (Microwave)

 

 

3

GHz

to

30.0

GHz

C-band

 

 

3600

MHz

to

7025

MHz

X-band:

 

 

7.25

GHz

to

8.4

GHz

Ku-band

 

 

10.7

GHz

to

14.5

GHz

Ka-band

 

 

17.3

GHz

to

31.0

GHz

Extremely High Frequencies (EHF)
(Millimeter Wave Signals)

 

 

30.0

GHz

to

300

GHz

Additional Fixed Satellite

 

 

38.6

GHz

to

275

GHz

Infrared Radiation

 

 

300

GHz

to

430

THz

Visible Light

 

 

430

THz

to

750

THz

Ultraviolet Radiation

 

 

1.62

PHz

to

30

PHz

X-Rays

 

 

30

PHz

to

30

EHz

Gamma Rays

 

 

30

EHz

to

3000

EHz