doordarshan

IntroductionOur training was in Doordarshan maintenance centre at Rajahmundry. This

report contains a detailed study of the Doordarshan maintenance centre at Rajahmundry.

There are three divisions in television broadcasting

Studio

Transmitter

Earth station

1. Studio

Studio is the place where the real looked scenes created using artificial materials and computer graphics. Studio is used to shooting various programs and to edit and modify the actual videos

2. Transmitter

Here the transmission of both audio as well as video signals is being made .The transmission section does the function of modulation of signals, power amplification of signals and mixing of audio and video signals

At last these signals are transmitted to antenna and we get the signal at almost 65 to 75 kms of the distance of the antenna

3. Earth station

The main function of the earth station is to make contact with the satellite or to communicate with it .The signals from other transmitters are down linked here. Also the signals here are also up linked to send it to larger distance

Doordarshan maintenance centre Rajahmundry mainly concentrates on analog terrestrial television transmission.

Studio The studio has

· Camera and lights and other equipment required for production of a feed. · Camera control unit or CCU

The PCR is where the post production activities like minor editing and Management of feed during a live program takes place. The production Manager sits in the PCR and directs the camera men and selects the angles sound parameters etc during the production stage in the PCR. It is in the PCR that we can control all the studio lights and all the microphones and other aspects. The PCR has a vision mixer and an audio mixer. Its working and other aspects are discussed in detail in the following pages. The PCR is where the phone in console and other systems are also kept.

The VTR is the next section where copies of all programs are stored. All the programs shot in the camera are simultaneously recorded in the VTR. Also the VTR plays back all the videos as and when required. Videos of prerecorded events are queued up in the VTR and are played back without a break. Videos of famous people and important events are stored in the central film pool.

The MSR stores all the circuitry of the DD. All the camera base units, all the vision mixer base units and all the audio processor base units are kept in MSR. The audio chain and video chain of MSR is explained in detail. The monitoring and control of all activities takes place in MSR. It is the MSR which decides what is to go in air. The MSR also performs some additional functions like logo addition etc. The next station is the earth station which has an uplink chain, simulcast transmitters, audio processors video processors, up converters, modulators etc. The earth station is in fully digital domain. The last stage is the transmitter which has the antenna and facilities for terrestrial transmission.

FUNDAMENTAL OF MONOCHROME AND COLOUR TV SYSTEMPicture formation

A picture can be considered to contain a number of small elementary areas of light or shade which are called PICTUREELEMENTS. The elements thus contain the visual image of the scene.

In the case of a TV camera the scene is focused on the photosensitive surface of pick up device and a optical image is formed. The photoelectric properties of the pickup device convert the optical image to a electric charge image depending on the light and shade of the scene (picture elements). Now it is necessary to pick up this information and transmit it. For this purpose scanning is employed. Electron beam scans the charge image and produces optical image. The electron beam scans the image line by line and field by field to provide signal variations in a successive order.

The scanning is both in horizontal and vertical direction simultaneously.

The horizontal scanning frequency is 15,625 Hertz. The vertical scanning frequency is 50 Hz. The frame is divided in two fields. Odd lines are scanned first and then the even lines. The odd and even lines are interlaced. Since the frame is divided into 2 fields the flicker reduces. The field rate is 50 Hertz. The frame rate is 25 Hertz (Field rate is the same as power supply frequency).Number of TV Lines per Frame

If the number of TV lines is high larger bandwidth of video and hence larger R.F. channel width is required. If we go for larger RF channel width the number of channels in the R.F. spectrum will be reduced. However, with more no. of TV lines on the screen the clarity of the picture i.e. resolution improves. With lesser number of TV lines per frame the clarity (quality) is poor.

The capability of the system to resolve maximum number of picture elements along scanning lines determines the horizontal resolution. It means how many alternate black and white elements can be there in a line. Let us also take another factor. It is realistic to aim at equal vertical and horizontal resolution. Therefore, the number of alternate black and white dots on linecan be 575 x 0.69 x 4/3 which is equal to 528.

It means there are 528 divided by 2 cyclic changes i.e. 264 cycles. These 264 cycles are there during 52 micro seconds.Hence the highest frequency is 5 MHz.

Therefore the horizontal resolution of the system is 5 MHz. A similar calculation for 525 lines system limits the highest frequency to 4 MHz and hence the horizontal resolution of same value.

In view of the above the horizontal bandwidth of signal in 625lines system is 5 MHz.

Picture Basics

A television creates a continuous series of moving pictures on the screen.This section will describe in detail how pictures are created in a television. Acamera works exactly on the same principle applied the other way round.

A picture is "drawn" on a television or computer display screen by sweepingan electrical signal horizontally across the display one line at a time. The amplitude of this signal versus time represents the instantaneous brightnessat that physical point on the display.

At the end of each line, there is a portion of the waveform (horizontal blanking interval) that tells the scanning circuit in the display to retrace to the left edge of the display and then start scanning the next line. Starting at the top, all of the lines on the display are scanned in this way. One complete set of lines makes a picture. This is called a frame. Once the first complete picture is scanned, there is another portion of the waveform (vertical blanking interval, not shown) that tells the scanning circuit to retrace to the top of the display and start scanning the next frame, or picture. Thissequence is repeated at a fast enough rate so that the displayed images are perceived to have continuous motion. This is the same principle as that behind the "flip books" that you rapidly flip through to see a moving picture or cartoons that are drawn and rapidly displayed one picture at a time.Interlaced versus Progressive Scans

These are two different types of scanning systems. They differ in the technique used to cover the area of the screen. Television signals and compatible displays are typically interlaced, and computer signals and compatible displays are typically progressive (non-interlaced). These two formats are incompatible with each other; one would need to be converted to the other before any common processing could be done. Interlaced scanning is where each picture, referred to as a frame, is divided into two separate sub-pictures, and referred to as fields. Two fields make up a frame. Aninterlaced picture is painted on the screen in two passes, by first scanning the horizontal lines of the first field and then retracing to the top of the screen and then scanning the horizontal lines for the second field in-between the first set. Field 1 consists of lines 1 through 262 1/2, and field 2 consists of lines 262 1/2 through 525. The interlaced principle is illustrated in Figure 2. Only a few lines at the top and the bottom of each field are shown.

There are many different kinds of video signals, which can be divided intoeither television or computer types. The format of television signals varies from country to country. In the United States and Japan, the NTSC format is used. NTSC stands for National Television Systems Committee, which is the name of the organization that developed the standard. In Europe, the PAL format is common. PAL (phase alternating line), developed after NTSC, is an improvement over NTSC. SECAM is used in France and stands for sequential coleur avec memoire (with memory). It should be noted that there is a total of about 15 different sub-formats contained within these three general formats. Each of the formats is generally not compatible with the others. Although they all utilize the same basic scanning system

and represent color with a type of phase modulation, they differ in specific scanning frequencies, number of scan lines, and color modulation techniques, among others. The various computer formats (such as VGA, XGA, and UXGA) also differ substantially, with the primary difference in the scan frequencies. These differences do not cause as much concern,because most computer equipment is now designed to handle variable scan rates. This compatibility is a major advantage for computer formats in that media, and content can be interchanged on a global basis. In India we use the PAL system. It has 625 lines in each frame and uses interlaced scanning.

There are many different kinds of video signals, which can be divided intoeither television or computer types. The format of television signals varies from country to country. In the United States and Japan, the NTSC format is used. NTSC stands for National Television Systems Committee, which is the name of the organization that developed the standard. In Europe, the PAL format is common. PAL (phase alternating line), developed after NTSC, is an improvement over NTSC. SECAM is used in France and stands for sequential coleur avec memoire (with memory). It should be noted that there is a total of about 15 different sub-formats contained within these three general formats. Each of the formats is generally not compatible with the others. Although they all utilize the same basic scanning system and represent color with a type of phase modulation, they differ in specific scanning frequencies, number of scan lines, and color modulation techniques, among others. The various computer formats (such as VGA, XGA, and UXGA) also differ substantially, with the primary difference in the scan frequencies. These differences do not cause as much concern, because most computer equipment is now designed to handle variable scan rates. This compatibility is a major advantage for computer formats in that media, and content can be interchanged on a global basis. In India we use the PAL system. It has 625 lines in each frame and uses interlaced scanning.

Typical Frequencies for Common TV and Computer Video Formats

There are three basic levels of baseband signal interfaces. In order of

increasing quality, they are composite (or CVBS), which uses one wire pair;Y/C (or S-video), which uses two wire pairs; and component, which uses three wire pairs. Each wire pair consists of a signal and a ground. These three interfaces differ in their level of information combination (or encoding). More encoding typically degrades the quality but allows the signal to be carried on fewer wires. Component has the least amount of encoding, and composite the most.

Composite/CVBS Interface

Composite signals are the most commonly used analog video interface. Composite video is also referred to as CVBS, which stands for color, video, blanking, and sync, or composite video baseband signal. It combines the brightness information (luma), the color information (chroma), and the synchronizing signals on just one cable. The connector is typically an RCA jack. This is the same connector as that used for standard line level audio connections. A typical waveform of an all-white NTSC composite video signal is shown in Figure.

This figure depicts the portion of the signal that represents one horizontal scan line. Each line is made up of the active video portion and the horizontal blanking portion. The active video portion contains the picture brightness (luma) and color (chroma) information. The brightness information is the instantaneous amplitude at any point in time. From the figure, it can be see that the voltage during the active video portion would yield a brightwhite picture for this horizontal scan line, whereas the horizontal blanking portion would be displayed as black and therefore not beseen on the screen. Color information is added on top of the luma signal and is a sine wave with the colors identified by a specific phase difference between it and the colorburstreference phase.

The amplitude of the modulation is proportional to the amount of color (or

saturation), and the phase information denotes the tint (or hue) of the color. The horizontal blanking portion contains the horizontal synchronizing pulse (sync pulse) as well as the color reference (color burst) located just after the rising edge of the sync pulse (called the "back porch"). It is important to note here that the horizontal blanking portion of the signal is positioned in time such that it is not visible on the display screen.

Y/C Interfaces

The Y/C signal is a video signal with less encoding. Brightness (luma),which is the Y signal, and the color (chroma), the C signal, are carried ontwo separate sets of wires.

Component Interfaces

Component signal interfaces are the highest performance, because they have the least encoding. The signals exist in a nearly native format. They always utilize three pairs of wires that are typically in either a luma (Y) and twocolor-difference-signals format or a red, green, blue (RGB) format. RGB formats are almost always used in computer applications, whereas colordifference formats are generally used in television applications. The Y signal contains the brightness (luma) and synchronizing information, and the colordifferencesignals contain the red (R) minus the Y signal and the blue (B) minus the Y signal. The theory behind this combination is that each of the base R, G, and B components can be derived from these difference signals.Common variations of these signals are as follows:

Y, B-Y, R-Y: Luma and color-difference signals.Y, Pr, Pb: Pr and Pb are scaled versions of B-Y and R-Y. Commonly found in high-end consumer equipment.

Y, Cr, Cb: Digital-signal equivalent to Y, Pr, Pb. Sometimes incorrectly used in place of Y, Pr, Pb.

Y, U, V: Not an interface standard. These are intermediate, quadrature signals used in the formation of composite and Y/C signals. Sometimes incorrectly referred to as a "component interface." Some important terms and their meanings in this context are listed below

Aspect Ratio

Aspect ratio is the ratio of the visible-picture width to the height. Standard television and computers have an aspect ratio of 4:3(1.33). HDTV has aspects ratios of either 4:3 or 16:9(1.78). Additional aspect ratios like 1.85:1 or 2.35:1 are used in cinema

.Blanking Interval

There are horizontal and vertical blanking intervals. Horizontal blanking interval is the time period allocated for retrace of the signal from the right edge of the

display back to the left edge to start another scan line. Vertical blanking interval is the time period allocated for retrace of the signal from the bottom back to the top to start another field or frame. Synchronizing signals occupy a portion of the blanking interval.

Blanking Level

Used to describe a voltage level (blanking level). The blanking level is the nominal voltage of a video waveform during the horizontal and vertical periods, excluding the more negative voltage sync tips.Chroma

The color portion of a video signal. This term is sometimes incorrectly referred to as "chrominance," which is the actual displayed color information.

Color Burst

The color burst, also commonly called the "color subcarrier," is 8 to 10 cycles of the color reference frequency. It is positioned between the rising edge of sync and the start of active video for a composite video signal.

Fields and Frames

A frame is one complete scan of a picture. In NTSC it consists of 525 horizontal scan lines. In interlaced scanning systems, a field is half of a frame; thus, two fields make a frame.Luma

The monochrome or black-and-white portion of a video signal. This term is sometimes incorrectly called "luminance," which refers to the actual displayed brightness.Monochrome

The luma (brightness) portion of a video signal without the color information. Monochrome, commonly known as black-and-white, predates current color television.

PAL

Phase alternate line. PAL is used to refer to systems and signals that are compatible with this specific modulation technique. Similar to NTSC but uses subcarrier phase alternation to reduce the sensitivity to phase errors that would be displayed as color errors. Commonly used with 626-line, 50Hz scanning systems with a subcarrier frequency of 4.43362MHz.

Pixel

Picture element. A pixel is the smallest piece of display detail that has a unique brightness and color. In a digital image, a pixel is an individual point in the image, represented by a certain number of bits to indicate the brightness.

RGB

Stands for red, green, and blue. It is a component interface typically used incomputer graphics systems.

Sync Signals/Pulses

Sync signals, also known as sync pulses, are negative-going timing pulses in video signals that are used by video-processing or display devices to synchronize the horizontal and vertical portions of the display.

Y Cr Cb

A digital component video interface. Y is the luma (brightness) portion, andCr and Cb are the color-difference portions of the signal.

Y/C

An analog video interface in which the chroma (color) information is carriedseparately from the luma (brightness) and sync information. Two wire pairsare used, denoted Y and C or Y/C. Often incorrectly referred to as "S-video."

Earth stationEarth station is a main part which communicate with satellite in which up

linking the signal in to /from the satellite

Earth station is a purely digital version

In MSR [ master switching room] whatever the signal it is in analog form which is converted into digital version

Digital version of audio and video are standard form which is known as

SDI - serial digital interface for video

AES -audio engineering society for audio

Signal from MSR

In MSR whatever the signals are converted into digital form video in SDI and aural in AES form. SDI and AES is embedded in MSR and converted into SDI embedded signal in which the audio is inserted in the video signal

In earth station all signals are in digital form which require high data rate. So

For the up linking it required compression of the signals. Doordarshan uses 12 frame GOP. MPEG-2 format with sampling parameter of 4:2:0 with 10 bit quantization and the bit rate is 4.5 Mbps for transmission over digitally modulation technique used is QPSK ( quadrature phase shift keying ). Earth station of the doordarshan uplinks the digital signal of channel towards satellite and the people using DTH can directly receive signals from satellite. Many HPTs and LPTs spread over to cover the large guiarate for terrestrial transmission also uses downlink from satellite and after converting it to analog they transmits terrestrially

Specifications of doordarshan earth station

Up link frequency - 5974.5MHz

Down link frequency - 3749.5 MHz

Symbol rate - 6.25MBPS

Uplink polarization - horizontal

Downlink polarization - vertical

Satellite - insat-3A in geosynchronous orbit

FEC - ¾

Compression format - 4:2:2

Coded standard - MPEG2

HIGH POWER TV TRANSMITTERAll the TV transmitters have the same basic design. They consist of an exciter

followed by power amplifiers which boost the exciter power to the required level.

EXCITER

The exciter stage determines the quality of a transmitter. It contains pre-corrector units both at base band as well as at IF stage, so that after passing through all subsequent transmitter stages, an acceptable signal is available. Since the number and type of amplifier stages, may differ according to the required output power, the characteristics of the pre-correction circuits can be varied over a wide range.

Exciter is said to be the heart of the transmission section. It consists of different sections. They are:

AD & DA convertor Digital video compensator Visual modulator Aural modulator Visual mixer Aural mixer Synthesizer IF corrector IM corrector

AURAL MODULATOR

The HBP-3101 aural modulator unit generates a frequency modulated aural if signal by modulating a voltage controlled oscillator with an audio input. Two sets of audio inputs are provided: one for 600 ohms balanced line and the other for 75 ohms unbalanced line used for sound multiplex broadcasting. For 600 ohms balanced line, pre-emphasis of either 50 microseconds or 75 microseconds can be selected. To fix the average frequency of the average frequency of the oscillator at the reference input, the automatic phase control (APC) circuit is provided.

The unit fault status is displayed with a light emitting diode when a unit output fault or an APC fault has occurred.

Auxiliary circuits are provided for the monitoring the output signal, for measuring the peak value of the output signal, and for measuring the frequency deviation.

The output is applied to a VHF mixer unit.

IM CORRECTOR

The HPB-3112 IM CORR (DS) UNIT is used for dual sound having 2 carriers. In which some corrections of IM due to the non-linearity of the PA stage can be carried out.

This unit contains a low level and high level correction circuit each having correction for amplitude and phase. In the low level circuit, correction can be performed by combining a non-linear signal generated by a class B amplifier with the linear signal, as the result the phase combining of the linear signal with the non-linear signal produces amplitude correction.

High level correction of amplitude is performed by the use of a saturated class A transistor amplifier. High level correction of phase is performed by the use of a class C transistor amplifier.

AD-DA (A/D Convertor D/A convertor)

The HPB-30102A AD-DA unit has functions that convert the video input signal applied to the exciter into a PCM signal and sends the PCM signal into a unit for digital correction (HPB-3103 DVC unit) and which converts the video PCM signal after the digital correction into analog video signal and supplied the analog video signal to a visual modulator unit (HPB-3101V.MOD unit).

Furthermore, this unit also supplies the 4fsc clock signals, SC signal and V pulse signal that the DVC unit need.

The functions exercised by the AD-DA unit are as follows:

Allows switching between input video signal from the main line and that from a feedback line.

Converts the analog signal fed to the exciter into a 10 bit PDM signal (referred to as video data in this manual) and supplies the video data to the DVC unit at the next stage.

Converts the 12 bit video data from the DVC unit into an analog video signal and supplies the signal to V.MOD unit.

Clamps the pedestal potential of the video signal.

Carries out synchronous of the main input video signal, the feedback input video signal and that of input synchronous signal when scrambling is used.

Generates 4fsc clocks locked to the input video signal or F/B video signal (BCO circuit).

Reconstruct the sync pulses of the main input video signal (sync reform function).

Changes the clamp potential in the visual blanking period corresponding to video-level in version scramble, similarly for the visual period.

DVC (Digital Video Compensator)

The HPB-30103 DVC unit compensates by the use of digital signal processing technology, distortion of input video signals and different types of distortions (linear and non-linear distortion) produced in a transmitter and receiver.

This unit composed of a non-linear compensating circuit, linear distortion compensating circuit, control circuit etc., receives the demodulated output signal of the transmitter and automatically compensates for the distortion in the output signal.

The non-linear distortion components circuit compensates as an auxiliary circuit to the IF corrector, the non-linear distortion caused by the power amplifier of the transmitter. Furthermore, this circuit is an APL follow- up type that can compensate for variations in the characteristics of the power amplifier by APL.

The linear distortion compensating circuit performs high accuracy compensation by using a 128 tap digital filter.

The non-linear and linear distortion compensating circuit respectively can be by-passed.

The control signal is provided with a digital signal processor that analyzes the demodulation output of the transmitter and automatically compensation values.

This compensation unit has the following functions:

Automatic compensation of the non linear distortions (DG, DP and luminance linearity).

Automatic compensation of the graph delay characteristics.

Automatic compensation of the frequency characteristics.

White clip (manual).

Pre-compensation of the graph delay characteristics of the receiver (fixed/manual).

Synchronous expansion (manual/auto).

Manual adjustment of the distortion compensation.

Automatic fault detection by means of self check.

Visual Modulator

The HPB-3104 visual modulator is intended to convert a baseband video signal into a modulated IF signal with ring modulator and the IF carrier is also phase modulated by a processed video signal to pre-correct the incidental carrier phase modulation.

The video signal for the IF modulation is arbitrarily sliced into three regions of sync, black and white in which each signal amplitude is individually expanded or compressed, then summed into the video signal by which the carrier for the ring modulator is phase modulated.

The ring modulator is followed by a pin diode circuitry via the harmonic filter, and then the signal passes through the vestigial sideband filter (VSBF) which uses a surface acoustic wave (saw) filter to achieve the Nyquist shaping.

The HPB-3104 visual modulator performs three functions: to modulate the IF carrier with the ring modulator, to remove one side band with the VSBF and to pre-correct the ICPM with the IF phases modulator.

IF CORRECTOR

This IF corrector unit generally used for the correction of non linear distortion generated in the PA stage, enables correction of DG and DP characteristics of visual signal. This unit also contains a means to combine two modulated IF carriers of the visual and aural allowing multiplex operation of the transmitter.

VHF MixerIn this unit, the IF signal applied at the input is converted to an RF signal by a

DBM and the RF signal is passed through filters (BPF and BEF) to separate out only the specified band and amplified to obtain an RF signal of +20 dBm. By applying AGC to the IF signal, the output power of the transmitter is maintained at a constant level.

The BPF and BEF are all installed for adjustment from the front side.

SYNTHESIZER

Synthesizer is a device which generates the intermediate frequency for audio and video modulation (for aural mixer and visual mixer). Synthesizer changes from exciter to exciter. It depends on the channel frequencies allocated by the F.C.C.

TRANSMITTING ANTENNAA transmitter is an electronic device which, usually with the aid of an antenna,

propagates an electromagnetic signal such as radio, television, or other telecommunications. In other applications signals can also be transmitted using an analog 0/4-20 mA current loop signal.

In radio electronics and broadcasting, a transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio frequency (AF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and radio receiver, with the combined unit referred to as a transceiver. A common consumer electronics device is a Personal FM transmitter, a very low power transmitter generally designed to take a simple audio source like an iPod, CD player, etc. and transmit it a few feet to standard radio receiver. In the USA, most personal FM transmitters fall under part 15% of the FCC regulations to avoid any user licensing requirements.

In amateur radio, a transmitter can be a separate piece of electronic gear or a set of a transceiver, and often referred to using an abbreviated form: “XMTR”.

In industrial process control, a “transmitter” is any device which converts measurements from a sensor into a signal to be received, usually sent via wires, by some display or control device located a distance away.

Typically in process control applications the “transmitter” will output an analog 4-20 mA current loop or digital protocol to represent a measured variable within a range. For example, a pressure transmitter might use 4 mA as a representation for 50 psig of pressure and 20 mA as 1000 psig of pressure and any value in between proportionally ranged between 3 to 15 psig to represent a process variable.

Generally and in communication and information processing, a transmitter is any object which sends information to an observer. When used in this more general sense, vocal cords may also be considered as example of a transmitter.

History in the early days of radio engineering, radio frequency was generated using arcs known as Alexranderson alternator or mechanic alternates (of which a rare example survives at the SAQ transmitter in Grineton, Sweden). In the 1920s electronic transmitters, based on vaccum tubes, began to be used.

Power Output

In broadcasting, and telecommunication, the part which contains the oscillator, modulator and sometimes audio processor, is called the exciter. Confusingly, the high-power amplifier which

The exciter then feeds into is often called the “transmitter” y broadcast engineers. The final output is given as transmitter power output (TPO), although this is not what most stations are rated by.

Effective radiated power (ERP) is used when calculating station coverage, even for mot non-broadcast stations. It is the TPO, minus any attenuation or radiated loss in the line to the antenna, multiplied by the gain (magnification) which the antenna provides toward the horizon. This is important, because the electric utility bill for the transmitter would be enormous otherwise, as would the cost of a transmitter. For most large stations in the VHF and UHF-range, the transmitter power is no more than 20% of the ERP.

For VLF, LF, MF and HF the ERP is typically not determined separately. In most cases the transmission power found in lists of transmitters is the value for the output of the transmitter. This is the only correct for Omni directional aerials with a length of a quarter wavelengths or shorter.

For other aerial types there are gain factors, which can reach values until 50 for shortwave directional beams in the direction of maximum earn intensity.

Since some authors take account of gain factors of aerials of transmitters for frequencies below 30 MHz and others not, there are often discrepancies of the values of transmitted powers.

Power Supply

Transmitters are sometimes fed from a higher voltage level of the power supply grid than necessary in order to improve security of supply. For example, the Alouis, Konstantynow and Roumoules transmitters are fed from the high-voltage network (110 kV in Alouis and Konstantynow, 150 kV in Roumoules) even though a power supply from the medium-voltage level of the power grid (about 20 kV) would be able to deliver enough energy.

Cooling of final stages

Low-power transmitters do not require special cooling equipment. Modern transmitters can be incredibly efficient, with efficiencies exceeding 98 percent. However, a broadcast transmitter with a megawatt power stage transferring 98% of that into the antenna can also be viewed as a 20 kilowatt electric heater.

For medium-power transmitter, up to a few hundred watts, air cooling with fans is used. At power levels over a few kilowatts, the output stage is cooled by a forced liquid cooling system analogous to an automobile cooling system. Since the coolant directly touches the high-voltage anodes of the tubes, only distilled, deionised water or a special dielectric coolant can also be used in the cooling circuit. This high purity coolant is in turn cooled by a heat exchanger, where the second cooling circuit can use water of ordinary quality because it not in contact with energized parts. Very

high power tubes of small physical size may use evaporative cooling by water in contact with anode. The production of steam allows a high heat flow in a small space.

Protection equipment

The high voltages required used in high power transmitters (up to 40 kV) require extensive equipment. Also, transmitters are exposed to damage from lightening. Transmitters may be damaged if operated without an antenna, so protection circuits must detect the loss of the antenna and switch off the transmitter immediately. Tube based transmitters must have power applied the proper sequence, with the filament voltage applied before the anode voltage; otherwise the tubes can be damaged. The output stage must be monitored for standing waves, which indicate that generated power is not being radiated but reflected back into transmitter.

Lightning protection is required between the transmitter and antenna. This consists of spark gaps and gas-filled surge arresters to limit the voltage that appears on the transmitter terminals. The control instrument that measures the voltage standing wave ratio switches the transmitter of briefly if a higher voltage standing wave ratio is detected after lightning strikes, as the reflections are probably due to lightning damage. If this does not succeed after several attempts, the antenna may be damaged and the transmitter should remain switched off. In some transmitting plants UV detectors are fitted in critical places, frequency and other transmitter parameters are monitored for protection and diagnostic purposes, and may be displayed locally and/or at a remote control room.

Building

A commercial transmitter site will usually have a control building to shelter the transmitter components and control devices. This is usually a purely functional building, which may constant apparatus for both radio and television transmitters. To reduce transmission line loss the transmitter building is usually immediately adjacent to the antenna for VHF and UHF sites, but for lowest frequencies it may be desirable to have a distance of a few score or several hundred meters between building and the antenna. Some transmitting towers have enclosures built into the tower to house radio relay link transmitters or other, relatively low-power transmitters.

Legal and regulatory aspects

Since radio waves go over borders, international agreements control radio transmissions. In European countries like Germany often the national Post Offices is the regulating authority. In the United States broadcast and industrial transmitters are regulated by the Federal Communications Commission (FCC). The International Telecommunication Union (ITU) helps managing the radio frequency spectrum internationally.

Main and relay (repeater) transmitters

Transmitting stations are usually either classified as main stations or relay stations (also known as repeaters or translators).

Main stations are defined as those that generate their own modulated output signal from a baseband (unmodulated) input. Usually main stations operate at high power and large areas.

Relay stations take an already modulated input signal usually by direct reception of a parent station (off-air) and simply shift (translate) its frequency before rebroadcasting. Usually relay stations operate at medium or low power, and are used to fill in pockets of poor reception within, or at the fringe of, the service area of a parent main station.

Note that a main station may also take its input signal directly off-air from another station, however this signal would be fully demodulated to baseband first, processed, and then remodulated for transmission.