Mar 152012
 

DTH (Direct to Home)

Direct to home (DTH)DTH means direct to home basically this service is alternative of cable TV. It is the distribution of television signals from high powered geo-stationary satellites through a small dish antenna & satellite receivers in home across the country. This service works on K U band (14/12 GHz)& are fully digital.

  • The main advantage of KU band satellite broadcasting is that it requires physically manageable smaller size of dish antenna compared to that of C band. C band requires 3.6m PDA (14db gain at 4GHz) while KU band requires 0.6m PDA (35db gain at 12GHz).

 

  • DTH reception requires a small dish antenna 60cm or 90cm, which can be easily mounted on the roof top. Feed along with low noise block converter & set top box (integrated receiver decoder, IRD) with CAS(conditional access system). A bouquet of 40to50 video programs can be simultaneously received in DTH mode.

 

  • DTH uplink chain the sources are feed to a router & output are divided into A, B&C group. Each group contain two video sources .All group are multiplexed digitally (QPSK) modulated individually at 70 MHz IF. Each group is further doubly up converted at L band (950-1450MHz) at KU band (12-14GHz). A, B&C group up converted to KU band streams RFA (13891MHz), RFB (13895MHz),& RPC(13839MHz)& are individually amplified using Klystron high power amplifier(KHPA).

 

  • DTH Downlink chain  consist of a dish, LNBF & RF cable, set top box. RF  waves (12/14GHz) from satellite are received up by a level converting into electric signal. The electric signal is amplified and down converted to L band (950-1450MHz) Feed & LNBC are combined in single unit called LNBF. The L band signal goes to indoor unit. The set top box or IRD (integrated receiver decoder) down converts the L band signal to 70MHz IF signal performs digital demodulator, demultiplexing, decoding and finally gives audio video output to TV for viewing.

 

  • The only drawback of KU band is that the signal attenuates during rains.

Direct to home (DTH)

Dec 142011
 

Technical Tips

TELEVISION TRANSMISSION– STANDARDS AND TECHNIQUESVSB reception

In a TV broadcast both the sound signal and the video signal are to be conveyed to the viewer using radio frequency.  These two signals have very distinct features. The audio signal is a symmetrical signal without continuous current but the frequency does not exceed 20 kHz.

 

 

 

 

 

 

 

 

 

Low Power TV TransmitterLPT

The transmitter design is based on solid state techniques and employs modular construction. The video and audio signals are processed in the exciter electronics and modulated at low level, at IF frequency of 38.9 MHz and 33.4 MHz respectively. The modulated IF signals are combined and passed through IF corrector and VSB filter.

 

 

 

 

 

 

 

 

 

THRULINE POWER METER

The Bird makes thru line power meter directional wattmeter is a portable insertion-type instrument for measuring forward and reflected power in coaxial transmission lines. It will accurately measure RF power under any load condition. Plug-in elements are available to fit your frequency and power needs.

 

 

 

 

 

 

 

Servo Voltage Stabilizers (AVR)

Servo Stabilizer is a revolutionary product in the field of voltage stabilization. It is specially built for constant voltage output for the areas where voltage supplies are fluctuating and damages the sophisticated equipment’s. This system keeps in check voltage variations by the fast rate of correction.

 

 

 

 

 

 

Diesel Generator

The diesel engine is a type of internal combustion engine, more specifically, a compression ignition engine, in which the fuel is ignited by the high temperature of compressed gas, rather than a separate source of energy (such as a spark plug).

 

 

 

 

 

 

WavefromThe Wavefrom Monitor is an operational monitor providing the signal monitoring features useful to the graphics workstation, telecine, all of the video features, Transmitter, or camera setup operator, for use by a person responsible for the look and continuity of the television picture.

Diesel Generator Maintenance – Minor

The following maintenance procedures must be followed to ensure a good performance of the equipment and extension of its lifetime.

CLEANING, LUBRICATING SYSTEM, FUEL SYSTEM, AIR INTAKE SYSTEM, EXHAUST SYSTEM

Aug 122011
 

Satellite Guide

The Satellite Transponder Guide is the Comprehensive Resource That Includes Everything You Need To Know About  Transponders.

Transponders Insat 4B 93.5° East
P/Channel Name Frequency S/R VID Type Mode
F.E.C Polar
Doordarshan
3725 27500 DVB TV FTA
4-Mar H
   
DD National       TV FTA
DD News       TV FTA
DD Bharati       TV FTA
DD Sports       TV FTA
DD Urdu       TV FTA
DD Bangla       TV FTA
Rajya Sabha TV       TV FTA
DD Dehradun
3750 4250 DVB TV FTA
4-Mar H
   
DD Raipur
3762 4250 DVB TV FTA
4-Mar H
   
DD Ranchi
3768 4250 DVB TV FTA
4-Mar H
   
DD Jammu
3774 4250 DVB TV FTA
4-Mar H
   
DD Hissar
3802 4250 DVB TV FTA
4-Mar H
   
DD Chandigarh
3808 4250 DVB TV FTA
4-Mar H
   
DD Port Blair
3822 4250 DVB TV FTA
4-Mar H
   
DD Bihar
3832 6250 DVB TV FTA
4-Mar H
   
DD North-East 
3841 6250 DVB TV FTA
4-Mar H
   
 Doordarshan
3925 27500 DVB TV FTA
4-Mar H
   
DD National       TV FTA
DD News       TV FTA
DD Sports        TV FTA
DD Bharati       TV FTA
DD Bangla       TV FTA
DD Podhigai       TV FTA
DD Saptagiri       TV FTA
DD Punjab       TV FTA
DD Malayalam       TV FTA
DD Oriya       TV FTA

 

Transponders @ Insat 3C at 74.0° East
T.P/Channel Name Frequency S/R VID Type Mode
F.E.C Polar
DD Feeds 3740 2500 DVB    
4-Mar V
Feeds 3745 2500 DVB    
4-Mar V
DD Feeds 3752 2500 DVB    
4-Mar V
DD Feeds 3756 2500 DVB    
4-Mar V
DD Feeds 3776 3800 DVB    
4-Mar V
DD National
3780 6250 DVB TV FTA
4-Mar H
   
Feeds 3781 2000 DVB    
4-Mar V
DD Feeds 3788 3800 DVB    
4-Mar V
DD Feeds 3796 3800 DVB    
4-Mar V
DD Feeds 3801 3800 DVB    
4-Mar V
Feeds 3868 2250 DVB    
4-Mar H
DD Feeds 3871 2250 DVB    
4-Mar H
DD Feeds 3874 2250 DVB    
4-Mar H
Feeds 3879 2200 DVB    
4-Mar H
Feeds 3884 2250 DVB    
4-Mar H
Feeds 3889 2250 DVB    
4-Mar H
DD Feeds 3895 2000 DVB    
4-Mar H
Feeds 3900 2250 DVB    
4-Mar H
Gyandarshan 4165 26000 DVB TV FTA
2-Jan H
Gyandarshan         FTA
DD Technology         FTA
CEC-UGC Education Channel         FTA
DD Technology         FTA
Gyandarshan 5 Test Card         FTA

 

Transponders Insat 2E 83.00 ° East 
P/Channel Name Frequency S/R VID Type Mode
F.E.C Polar
DD National
4071 5000 DVB TV FTA
  V
4-Mar  

 

Transponders Insat 4A 83.00 ° East 
P/Channel Name Frequency S/R VID Type Mode
F.E.C Polar
            Lok Sabha TV
4170 4650 DVB TV FTA
4-Mar
   
May 282011
 

TELEVISION TRANSMISSION

In TV broadcast both the sound signal and the video signal are to be conveyed to the viewer using radio frequency.  These two signals have very distinct features. The audio signal is a symmetrical signal without continuous current but the frequency does not exceed 20 kHz. The video signal consists of a logical component, the sync and the field sync and an analogue part according to the line picture scanning.  This unsymmetrical signal thus has a continuous component.  The frequency bandwidth also extends from 0 to 5 MHz. The two signals modulate the carrier waves whose frequencies and types of modulations are as per established standards.  These modulated carriers are further amplified and then diplexed for transmission on the same line and antenna.  This technique is used with High Power TV Transmitters.  However for LPTs i.e. transmitters operating at sync peak power less than 1 kW, both the signals (video and audio) are modulated separately (In most of the present day TV transmitters the picture signal is amplitude modulated while the audio signal is frequency modulated) but amplified jointly using common vision and aural amplifiers.  Both of these systems have merits and demerits.  In the first case (separate amplification) special group delay equalisation circuit is needed because of errors caused by diplexer while in the second case inter modulation products are more prominent and special filters for suppressing them are required.  Hence technique of joint amplification is suitable only for LPTs and not for HPTs.

Though frequency modulation has certain advantages over amplitude modulation, its use for picture transmission is not permitted due to large bandwidth requirements, which is not possible due to very limited channel space available in VHF/UHF bands.  Secondly as the power of the carrier and side band components go on varying with modulation in the case of FM, the signal with frequency modulation after reflection from nearby structures at the receiving end will cause variable multiple ghosts, which will be very disturbing.  Hence use of FM for terrestrial transmission of picture signal is not permitted.

Thus amplitude modulation is invariably used for picture transmission while frequency modulation is generally used for sound transmission due to its inherent advantages over amplitude modulation.  As the picture signal is unsymmetrical, two types of modulation is possible.

i)  Positive modulation

Wherein the increase in picture brightness causes increase in the amplitude of the modulation envelope.

 ii) Negative modulation

The increase in picture brightness causes reduction in carrier amplitude i.e. the carrier amplitude will be maximum corresponding to sync tip and minimum corresponding to peak white.

In television though positive modulation was adopted in initial stages, negative modulation is generally adopted (PAL’B uses negative modulation) now a days, as there are certain advantages over positive modulation.

 Advantages of Negative Modulation

 i)Impulse noise peaks appear only in black region in negative modulation.  This black noise is less objectionable compared to noise in white picture region.

ii) Best linearity can be maintained for picture region and any non-linearity affects only sync which can be corrected easily.

iii) The efficiency of the transmitter is better as the peak power is radiated during sync duration only (which is about 12% of total line duration).

iv) The peak level representing the blanking or sync level may be maintained constant, thereby providing a reference for AGC in the receivers.

v)  In negative modulation, the peak power is radiated during the sync-tip.  As such even in case of fringe area reception, picture locking is ensured, and derivation of inter carrier is also ensured.

VESTIGIAL SIDE BAND TRANSMISSION

Another feature of present day TV Transmitters is vestigial side band transmission. If normal amplitude modulation technique is used for picture transmission, the minimum transmission channel bandwidth should be around 11 MHz taking into account the space for sound carrier and a small guard band of around 0.25 MHz.  Using such large transmission BW will limit the number of channels in the spectrum allotted for TV transmission.  To accommodate large number of channels in the allotted spectrum, reduction in transmission BW was considered necessary.  The transmission BW could be reduced to around 5.75 MHz by using single side band (SSB) AM technique, because in principle one side band of the double side band (DSB) AM could be suppressed, since the two side bands have the same signal content.

It was not considered feasible to suppress one complete side band due to difficulties in ideal filter design in the case of TV signal as most of the energy is contained in lower frequencies and these frequencies contain the most important information of the picture.  If these frequencies are removed, it causes objectionable phase distortion at these frequencies which will affect picture quality.  Thus as a compromise only a part of lower side band is suppressed while taking full advantage of the fact that:

 i) Visual disturbance due to phase errors are severe and unacceptable where large picture areas are concerned (i.e. at LF) but

ii) Phase errors become difficult to see on small details (i.e. in HF region) in the picture.  Thus low modulating frequencies must minimize phase distortion where as high frequencies are tolerant of phase distortions as they are very difficult to see.

The radiated signal thus contains full upper side band together with carrier and the vestige (remaining part) of the partially suppressed LSB.  The lower side band contains frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is at 1.25 MHz.

 RECEPTION OF VESTIGIAL SIDE BAND SIGNALS

Corresponding to the VSB characteristics used in transmission an amplitude versus frequency response results.  When the radiated signal is demodulated with an idealized detector, the response is not flat.  The resulting signal amplitude during the double sideband portion of VSB is exactly twice the amplitude during the SSB portion.

 This characteristic is shown in Fig.

 

 
Fig- Response for VSB reception

 

In order to equalize the amplitude, the receiver response is designed to have an attenuation characteristics over the double side band region appropriate to compensate for the two to one relationship.

This attenuation characteristic, the so-called nyquist slope, is assumed to be in the form of a linear slope over the + 750 kHz (DSB region) with the visual carrier located at the mid point (-6 dB point) relative to SSB portion of the band.  Such a characteristic exactly compensates the amplitude response non-symmetry due to VSB.  Fig..

 

 

 

Typical receiver characteristics

 

 

 

Modern practice for purposes of circuit and IF filter design simplification, also provides an attenuation of the upper end of the channel such that colour sub carrier is also attenuated by 6 dB as shown in Fig.

Typical receiver IF characteristics

TV receivers have Nyquist characteristics for reception which introduces group delay errors in the low frequency region.  Notch filters are used in receivers as aural traps in the vision IF and Video amplifier stages.  These filters introduce GD errors in the high frequency region of the video band.  These GD errors are pre-corrected in the TV transmitters (using RX pre corrector) so that economical receiver filter design is possible.  The group delays of the RX and TX with pre-correction are shown in Fig.

 

 

Group delay curves

 

 

Depth of Modulation

Care must be taken to avoid over-modulation at peak-Luminance signal values to avoid picture distortions and interruptions in vision carrier. The peak white levels when over modulated tend to reduce the vision carrier power or even cause momentary interruptions of vision carrier.  These periodic interruptions due to accidental over modulation result in interruptions of the sound carrier in inter carrier receiver systems which produces undesired sound buzz in the receiver output.

 Therefore, to prevent this effect, the maximum depth of modulation of the visual carrier by peak white signal values is specified as being 87.5%.  This 12.5% residual carrier (white level) is required because of the inter-carrier sound method used in TV receiver .

Carrier signal and modulation envelop

The depth of modulation is set by using a ramp signal or step signal as given in the manual.  It should be 87.5% for 100% modulation (i.e. m = 1).

Inter Carrier

 The TV receivers incorporate inter carrier principle.  According to our system, the inter-carrier i.e. the difference between the vision transmitter frequency and sound transmitter frequency is 5.5 MHz.  Hence it is to be ensured that even when the modulating video signal is at white peak, 12.5% of residual carrier is left so that sound can be extracted even at the peak white level, where the carrier power is minimum.

Power Output

 The peak power radiated during the sync. tip or sometimes the carrier power corresponding to black level is designated as the vision transmitter power.  This power is measured by using a thruline power meter after isolating the aural carrier.  The power read on thruline meter is multiplied by a factor of 1.68 to get the peak power (vision) radiated.  As transmitter output is connected to an antenna, having a finite gain, the effective radiated power (ERP) is obtained by multiplying the peak power by the antenna gain (w.r.t a half wave dipole).  Hence a 100 W LPT using transmitting antenna having a gain of 3 dB w.r.t a half wave dipole will have an ERP of 200 W or 53 dBm or 23 dBW.

 In TV broadcasting, the sound signal is transmitted by frequency modulating the RF sound carrier in accordance with the standards. The sound carrier is 5.5 MHz above the associated vision carrier. The maximum frequency deviation is + 50 kHz which is defined as 100 per cent modulation in PAL-B system.  In the case of NTSC, the maximum deviation permissible is + 25 kHz.

Standards

The characteristics of the TV signal in sections 1 and 2 refer to CCIR B/G standards.  Various other standards are given in Table .

 Table 1

 

Vision/sound carrier spacing
channel width

Frequency Range

Vision sound carrier spacing

5.5 MHz

Channel width

7 MHz (B) VHF

8 MHz (G) UHF

Sound Modulation

FM

FM deviation (maximum)

+ 50 kHz

 

Standards

American

European PAL

No. of lines per frame

525

625

No. of frames per second

30

25

Field Frequency Hz

60

50

Line Frequency Hz

15750

15625

Channel  MHz

6

7

Video BW, MHz

4.2

5

Colour subcarrier, MHz

3.58

4.43

Sound System

FM

FM

Maximum sound deviation kHz

25

50

Intercarrier frequency MHz

4.5

5.5

 

May 152011
 

Preventive maintenance of Diesel Generator

Because of the durability of diesel engines, most maintenance is preventive in nature. Preventive diesel engine maintenance consists of the following operations:

• General inspection
• Lubrication service
• Cooling system service
• Fuel system service
• Servicing and testing starting batteries
• Regular engine exercise

CHARTGeneral inspection

When the generator set is running, operators need to be alert for mechanical problems that could create unsafe or hazardous conditions. Following are several areas that should be inspected frequently to maintain safe and reliable operation.

• Exhaust system: With the generator set operating, inspect the entire exhaust system including the exhaust manifold, muffler and exhaust pipe. Check for leaks at all connections, welds, gaskets and joints, and make sure that the exhaust pipes are not heating surrounding areas excessively. Repair any leaks immediately.

• Fuel system: With the generator set operating, inspect the fuel supply lines, return lines, filters and fittings for cracks or abrasions. Make sure the lines are not rubbing against anything that could cause an eventual breakage. Repair any leaks or alter line routing to eliminate wear immediately.

• DC electrical system: Check the terminals on the starting batteries for clean and tight connections. Loose or corroded connections create resistance which can hinder starting.

• Engine: Monitor fluid levels, oil pressure and coolant temperatures frequently. Most engine problems give an early warning. Look and listen for changes in engine performance, sound, or appearance that will indicate that service or repair is needed. Be alert for misfires, vibration, excessive exhaust smoke, loss of power or increases in oil or fuel consumption.

Lubrication service

Check the engine oil level when the engine is shut down at the interval specified in CHART. For accurate readings on the engine’s dipstick, shut off the engine and wait approximately 10 minutes to allow the oil in the upper portions of the engine to drain back into the crankcase. Follow the engine manufacturer’s recommendations for API oil classification and oil viscosity. Keep the oil level as near as possible to the “full” mark on the dipstick by adding the same quality and brand of oil.

Change the oil and filter at the intervals recommended in CHART. Check with the engine manufacturer for procedures for draining the oil and replacing the oil filter. Used oil and filters must be disposed of properly to avoid environmental damage or liability.

Cooling system service

Check the coolant level during shutdown periods at the interval specified in CHART. Remove the radiator cap after allowing the engine to cool and, if necessary, add coolant until the level is about 3/4-inch below the radiator cap lower sealing surface. Heavy duty diesel engines require a balanced coolant mixture of water, antifreeze and coolant additives. Use a coolant solution as recommended by the engine manufacturer. Inspect the exterior of the radiator for obstructions and remove all dirt or foreign material with a soft brush or cloth. Use care to avoid damaging the fins. If available, use low pressure compressed air or a stream of water in the opposite direction of normal air flow to clean the radiator. Check the operation of the coolant heater by verifying that hot coolant is being discharged from the outlet hose.

Fuel system service

Diesel fuel is subject to contamination and deterioration over time, and one reason for regular generator set exercise is to use up stored fuel over the course of a year before it degrades. In additional to other fuel system service recommended by the engine manufacturer, the fuel filters should be drained at the interval indicated in CHART. Water vapor accumulates and condenses in the fuel tank and must also be periodically drained from the tank along with any sediment present.

The charge-air piping and hoses should be inspected daily for leaks, holes, cracks or loose connections. Tighten the hose clamps as necessary. Also, inspect the charge-air cooler for dirt and debris that may be blocking the fins. Check for cracks, holes or other damage.

The engine air intake components should be checked at the interval indicated in CHART. The frequency of cleaning or replacing air cleaner filter elements is primarily determined by the conditions in which the generator set operates. Air cleaners typically contain a paper cartridge filter element which can be cleaned and reused if not damaged. Starting batteries Weak or undercharged starting batteries are the most common cause of standby power system failures. Even when kept fully charged and maintained, lead-acid starting batteries are subject to deterioration over time and must be periodically replaced when they no longer hold a proper charge. Only a regular schedule of inspection and testing under load can prevent generator starting problems. See CHART for the recommended inspection interval for the batteries and charging system.

Testing batteries: Merely checking the output voltage of the batteries is not indicative of their ability to deliver adequate starting power. As batteries age, their internal resistance to current flow goes up, and the only accurate measure of terminal voltage must be done under load.

• Cleaning batteries: Keep the batteries clean by wiping them with a damp cloth whenever dirt appears excessive. If corrosion is present around the terminals, remove the battery cables and wash the terminals with a solution of baking soda and water (1/4-pound baking soda to one quart of water).
Be careful to prevent the solution from entering the battery cells, and flush the batteries with clean water when done. After replacing the connections, coat the terminals with a light application of petroleum jelly.

• Checking specific gravity: Use a battery hydrometer to check the specific gravity of the electrolyte in each battery cell. A fully charged battery will have a specific gravity of 1.260. Charge the battery if the specific gravity reading is below1.215.

• Checking electrolyte level: Check the level of the electrolyte in the batteries at least every 200 hours of operation. If low, fill the battery cells to the bottom of the filler neck with distilled water.

Generator set exercise

Generator sets on continuous standby must be able to go from a cold start to being fully operational in a matter of seconds. This can impose a severe burden on engine parts. However, regular exercising keeps engine parts lubricated, prevents oxidation of electrical contacts, uses up fuel before it deteriorates, and, in general, helps provide reliable engine starting. Exercise the generator set at least once a month for a minimum of 30 minutes loaded to no less than one-third of the nameplate rating. Periods of no-load operation should be held to a minimum, because unburned fuel tends to accumulate in the exhaust system. If connecting to the normal load is not convenient for test purposes, the best engine performance and longevity will be obtained by connecting it to a load bank of at least one-third the nameplate rating.

Conclusion

Preventive maintenance of diesel generators plays a critical role in maximizing reliability, minimizing repairs and reducing long term costs. By following generally recognized diesel maintenance procedures and specific manufacturer recommendations for your application, you’ll be assured that your standby power system will start and run when you need it most.

Apr 202011
 

LOW POWER TV TRANSMITTERS

VHF Low Power TV Transmitter

(VHF range : 54 to 216 MHz)

The transmitter design is based on solid state techniques and employs modular construction.  The video and audio signals are processed in the exciter electronics and modulated at low level, at IF frequency of 38.9 MHz and 33.4 MHz respectively.  The modulated IF signals are combined and passed through IF corrector and VSB filter.  SAW filter is used for vestigial sideband shaping.  The combined signal is upconverted to desired channel frequency and amplified in linear power amplifier to obtain 100 watt (sync peak) visual power and 10 watt aural power.  RF is finally routed to antenna through channel filter and directional coupler.  A leveled block schematic of such transmitters available in the network is shown in Figs. A brief description of the exciter and power amplifier is given in the following paragraphs.

 

 

Exciter

 

Video Signal Processing and Modulation

 

The leveled block schematic of old and new generation exciter is given in figure 2 (a) and 2 (b).  The  1VPP input video signal is limited to 5 MHz in low pass filter and is compensated for group delay in delay equalizer and receiver precorrector unit.  The resulting signal is subjected to DC restoration by clamping at back porch, amplified and inverted in video processor.  The output of the video processor is fed to visual modulator where the same is amplitude modulated with negative polarity at 38.9 MHz IF and amplified.

In the new generation exciter, the electronics for base-band processing (LPF, DEq and V. processor) is integrated, resulting into a single and compact video processor unit.  The output of later has normal polarity.  The modulator used is a ring modulator instead of double diode balance modulator.  The video signal is reversed in polarity and modulates the vision IF.  The IF at 38.9 MHz is generated in the modulator unit using a transistorized crystal oscillator (TCXO).  The SAW filter in the unit shapes the modulated IF to vestigial sideband format.

The audio signal is frequency modulated at IF of 33.4 MHz in a varactor VCO modulator.  The VCO oscillates at centre frequency of 33.4 MHz.  The incoming audio is passed through a balanced to unbalanced transformer and pre-emphasised in a 50 micro second pre-emphasis network.  Signal is then amplified and applied to varactor diodes.  The information  contained in the amplitude variation of audio is converted into frequency variation in the VCO.  The VCO frequency deviates about centre frequency in proportion to audio amplitude.  The centre frequency of VCO is maintained at 33.4 MHz which is below vision IF by 5.5 MHz.  This is achieved by a phase locked loop (PLL).  The concept of the same is given in figure

 

 

 

Output from VCO (33.4 MHz FM) is mixed with vision IF of 38.9 MHz (taken from IF oscillator unit) using a transistorized mixer.  The resulting difference signal of 5.5 MHz is shaped for square pulses and using suitable dividers the frequency is reduced to a low value.  In most of the transmitter the dividers used are   5,   16   and    128.  Thus the 5.5 MHz square shaped wave is reduced to 537 Hz.  A crystal oscillator of suitable frequency (normally 1.1 MHz) is used as standard oscillator.  The output of crystal osc after getting shaped suitably is also passed through dividers, so that a frequency similar to above value is obtained.  Normally the o/p is divided by 16 & 128 so that a reference frequency of 537 Hz is obtained.  The above two outputs are fed to a comparator circuit, so that an error signal is generated whenever there is a drift of centre frequency of VCO from its nominal value of 33.4 MHz.  The frequency of the VCO is kept under control using the error signal.  The same error signal is used for providing visual indications through LED and a DC ammeter to indicate the deviation of the center frequency of the VCO.  

In some of the exciters, the audio is first modulated at an inter carrier frequency of 5.5 MHz and then converted to standard IF of 33.4 MHz by mixing with vision IF in a mixer.  New generation exciters have additional circuitry in audio modulator.  This includes a test tone for test purpose and dual sound/stereo coder.  

 
Apr 102011
 

AVR

Servo Stabilizer is a revolutionary product in the field of voltage stabilization. It is specially built for constant voltage output for the areas where voltage supplies is fluctuating and damages the sophisticated equipment’s. This system keeps in check voltage variations by fast rate of correction. It is designed for working on single phase as well as three phase AC supply. These stabilizers are manufactured for continuous use at full load, and are suitable for Balanced & Unbalanced load.

 

 

 

(AVR) consists of the followings:-

  • Continuously variable Voltage Regulator.
  • Electrolytic Copper Wound Buck Boost Transformers duly vacuum impregnated.
  • Electronic Control Circuit and meter panel.
  • Synchronous motor is controlled by thyristors which are itself controlled by CMOS integrated circuit. Thus the whole system is quite solid state.avr Ckt

Advantages of AVR

  • Energy Saving.
  • Improved Power Factor.
  • Lower Break Down for Electrical Equipment.
  • 100% Depreciation.
  • End product with uniform quality.
  • By Pass Switch.
  • Reverse phase Preventer.
  • Single Phase Preventer.
  • Overload Protection.
 Posted by at 5:23 am
Mar 272011
 

IRD OPERATIONS 

IRD (Scopus) FRONT PANEL OPERATION

Controls and Displays

IRD

IRD Front Panel Screen Types
The IRD-2900 display has access to the following five screen types:

• Menu navigation screen
• Edit Menu screen
• Table Menu screen
• Edit value screen
• Select value screen 


Scopus

1. Menu Navigation Screen

The Menu Navigation screen enables navigating through the tree structure of
the IRD-2900 menu.
In this example, the Menu Navigation screen displays the following items:

IRD Pennal

A.Top line indicates the menu name (Configuration) and the menu hierarchal
position ( 1-2, for example Configuration under the Root menu).
[ Up]/ [ Up/Down]/ [ Down ] symbols indicate that up or down scrolling
is possible.
B. Next up to four displayed lines is a list of numbered items.
C. Additional Available Items
The list can include more than four items, but only four items are visible
at a time. When more than four items are enabled, you can scroll using
the [UP]/[Down] arrow keys on the IRD-2900 front panel.
The currently selected option is displayed with white characters over a
black background (see Item #3 in the example).
To enter the next menu level press [Enter] to select the marked item
(either another Menu Navigation screen or an Edit Menu screen).

2. Edit Menu Screen

The Edit menu screen enables you to select, change or display the value of a
parameter or set of parameters.
In this example, the Edit Menu screen displays the following items:

IRD Edit menu

A. Top line indicates the menu name (Stream) and the menu hierarchal
position (1-2-2, for example Root-Configuration-Stream) in the
IRD-2900 Menu Tree. [ Up]/[ Up/Down]/ [ Down ] symbols indicate
that up or down scrolling is possible.
B. Next up to four displayed lines is a list of numbered items relevant to the
menu and their current values. The information provided for each item in the
list is:
• Left-aligned column displays a numbered list of parameters.
• Right-aligned column displays the value of the parameter.
– Editable parameters have a pencil icon next to them.
– Parameters without the pencil icon are informative only.
C. Additional Available Items
The list can include more than four items, but only four items are visible
at a time. When more than four items are enabled, you can scroll the list
using the [UP]/[Down] arrow keys.
The currently selected option is displayed with white characters over a
black background (see Item #1 in the example).
Press [ESC] to abort the selection or to return to the menu’s previous
level.
Press [Enter] to select the pointed editable option; a parameter-editing screen is displayed. This can be a Table Menu screen, an Edit
Value screen, or a Select Value screen.

3. Table Menu Screen

The Table menu screen displays information about various parameters using a
table format.
In this example, the Table Menu screen displays the following columns:

IRD Menu

A. Top line displays the headers for each table column.
B. Next up to four displayed lines is a numbered list of parameters relevant
to the menu and their current values. A radio button indicates which
parameter is currently active (€ currently enabled and { currently disabled)
C. Additional Available Items
The list can include more than four items, but only four items are visible
at a time. When more than four items are available, you can scroll the list
using the [UP]/[Down] arrow keys.
The currently selected option is displayed with white characters over a
black background (see Item #2 in the example).
Press [ESC] to abort the selection and return to the Edit Menu screen
without changing the parameters.
Press [Enter] to select the currently enabled button €. The selected
option becomes enabled and the former active option is disabled.

4. Edit Value Screen

The Edit Value screen enables setting up a parameter value. The parameter
value can be a number or a string of characters. Each digit or character is set up
individually.
In this example, the Edit Value screen displays the following information:

IRD Pcr

A. Top line displays the parameter name (Pcr1 ). The pencil icon
indicates that the parameter value is editable.
B. Second line displays the current value of the parameter. Changing the
value of the parameter is performed using the arrow keys:
• [Left] and [Right] arrow keys are used to mark a digit or a character
for change. The marked digit or a character is displayed with white
character over black background (see example above)
• [Up] and [Down] arrow keys are used to scroll up or down the digits
(range 0 thru 9) or the characters (a to z, A to Z, 0 to 9 and ctr.). The
scroll range can be limited to prevent setting up values out of range.
C. Third line displays the allowed range of values for the parameter.
Press [ESC] to abort the setup and return one level up to the Edit Value
Screen without changing the parameters.
Press [Enter] to accept the value. The display returns one level up to the
Edit Value Screen and the new value is displayed as the current value of the
parameter.

5. Select Value Screen (Multiple Choices)

The Select Value screen displays a list of selectable items.
In this example, the Select Value screen displays the following information:
Format

IRD Formet

A. Top line displays the parameter name (Format). The pencil icon
indicates that the items are selectable from a list of displayed options.
[ Up]/ [ Up/Down]/ [ Down ] symbols indicate that up or down scrolling
is possible.
B. Next up to four displayed lines is a numbered list of parameters
relevant to the menu and their current values. A radio button indicates
which parameters are currently activated (€ currently enabled and {
currently disabled)
C. Additional Available Items
The list can include more than four items, but only four items are visible
at a time. When more than four items exist, you can scroll the list using
the [UP]/[Down] arrow keys.
The currently selected option is displayed with white characters over a
black background (see Item #4 in the example).
Press [ESC] to abort setup and return one level up of the Select Value
Screen without changing the parameters.
Press [Enter] to select the marked option (the selected option becomes
enabled € and the former enabled option is disabled { ). The display
returns up one level to the Select Value screen; the new option is
displayed as the current option of the parameter.

 Posted by at 7:34 am
Mar 272011
 

 

 

 

 

 

Wave from Monitor

 

The Waveform Monitor is an operational monitor providing the signal monitoring features useful to the graphics workstation, telecine, all of the video features, Transmitter, or camera setup operator, for use by a person responsible for the look and continuity of the television picture. It instills confidence that creative adjustments can be made without violating transmission standards, thus assuring trouble-free distribution throughout a facility.

The  wavefrom Monitor  to provide a more comprehensive evaluation of the digital transport layer and is used in digital production and master control operating centers, and provides data analysis capabilities for the installation and maintenance engineer.

 

 

 

 

 

Composite Video Signal

 

Features & Benefits

  • Two 270 MB Serial Component Loopthrough Inputs
  • Real-time CRT Display Suitable for Live Monitoring
  • Waveform Parade and Overlay Displays
  • Waveform Display in RGB or Y-Pb-Pr Levels
  • Bright Line Select Display
  • Component Vector Display
  • Tektronix Lightning, Bowtie, and Diamond Displays
  • Picture Display of Y Channel
  • Identification of Embedded Audio Channels
  • RGB or Y-Pb-Pr Analog Picture Monitor Outputs
  • Switched 270 MB Serial Component Picture Monitor Output
  • Waveform Cursors and Markers
  • SMPTE RP-165 Digital Error Detection and Reporting
  • Tektronix Arrowhead Display of NTSC/PAL Gamut Limit
  • Nine User Front-panel Presets
  • Environmental, Safety, and EMI Certified
  • Three-year Tektronix Warranty
  • CE Marking
Wave From Monitor

 

 

 

 

 

WaveFrom Monitor

 

Wave From Monitor

 

 

 

 

 

WaveFrom Monitor

If you described in excruciating detail the nature of the video signal. It's now time to use a piece of equipment that will allow us to see these precise wave formations. We need a "wave-form monitor"

For so long now, we've been describing CRTs as having screens that are scanned from left to right, and top to bottom. Nobody says we have to do it this way. In fact, with the exception of television picture tubes, CRTs are rarely scanned this way.

 

A waveform monitor is a sort of "programmable CRT." Its left-to-right scanning pattern is very similar to that of a picture monitor, but we can change the rate of that scanning. The vertical scanning pattern is not our usual top-to-bottom sawtooth, but is a representation of the voltage present at the input terminal – normally 1 volt of video (from the bottom of sync to 100 units of white level). The brighter the video level is, the higher it appears on the face of the waveform monitor screen. We can adjust how sensitive the waveform monitor's amplifiers will be in the vertical dimension (the "gain" of the displayed image).

As well, the frequency response characteristics of the amplifier can be changed so we can see just the luminance, just the chrominance, or both components of the composite video signal. Finally, we can synchronize the waveform monitor's sweep to the video we're looking at, or to an external reference. 

 Posted by at 6:03 am
Mar 162011
 

Introduction to Basic television system

INTRODUCTION

The aim of a basic 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 TV 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 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 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 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 TV camera, the heart of which is a camera tube, is used to convert this optical information into corresponding electrical signal, 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 photolayer 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.

TV camera tube

The beam is formed by an electron gun in the TV 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 continuously 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 resultant 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’).

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 intensity ratio of 30 : 59. Similarly, light reflected from any colour picture element can be synthesised (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.

Block Diagram of colour camera

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 pres ent 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 TV 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 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 TV 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 TV 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 TV 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 TV 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 picture tube. When the varying signal voltage makes the control grid less 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 sound signal is common with the picture signal from antenna to 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 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.

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 TV 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 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 beam intensity of the picture tube and is set for optimum average brightness of the picture. The contrast control is actually gain 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 intensity or amount of colours in the reproduced picture. In modern colour receivers that employ integ-rated circuits in most sections of the receiver, the hold control is not necessary and hence usually not provided