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. The scanning is both in horizontal and vertical direction simultaneously.e. The odd and even lines are interlaced. For this purpose scanning is employed.F.e. A similar calculation for 525 lines system limits the highest frequency to 4 MHz and hence the horizontal resolution of same value. Therefore. Since the frame is divided into 2 fields the flicker reduces. The electron beam scans the image line by line and field by field to provide signal variations in a successive order. The capability of the system to resolve maximum number of picture elements along scanning lines determines the horizontal resolution. However.625 Hertz. 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). It means how many alternate black and white elements can be there in a line. the number of alternate black and white dots on linecan be 575 x 0. Therefore the horizontal resolution of the system is 5 MHz. .FUNDAMENTAL OF MONOCHROME AND COLOUR TV SYSTEM Picture formation A picture can be considered to contain a number of small elementary areas of light or shade which are called PICTUREELEMENTS. The frame is divided in two fields. 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.F. The vertical scanning frequency is 50 Hz. It means there are 528 divided by 2 cyclic changes i. Odd lines are scanned first and then the even lines. Now it is necessary to pick up this information and transmit it. The frame rate is 25 Hertz (Field rate is the same as power supply frequency). The elements thus contain the visual image of the scene. resolution improves. Hence the highest frequency is 5 MHz. of TV lines on the screen the clarity of the picture i. spectrum will be reduced. Electron beam scans the charge image and produces optical image. Let us also take another factor.69 x 4/3 which is equal to 528. Number of TV Lines per Frame If the number of TV lines is high larger bandwidth of video and hence larger R. with more no. The field rate is 50 Hertz. 264 cycles. channel width is required. With lesser number of TV lines per frame the clarity (quality) is poor. These 264 cycles are there during 52 micro seconds. If we go for larger RF channel width the number of channels in the R. The horizontal scanning frequency is 15. It is realistic to aim at equal vertical and horizontal resolution. referred to as a frame. and referred to as fields. Picture Basics A television creates a continuous series of moving pictures on the screen. Aninterlaced picture is painted on the screen in two passes. Although they all utilize the same basic scanning system . They differ in the technique used to cover the area of the screen. Field 1 consists of lines 1 through 262 1/2. Starting at the top. which is the name of the organization that developed the standard. 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. Two fields make up a frame. or picture. one would need to be converted to the other before any common processing could be done. PAL (phase alternating line).In view of the above the horizontal bandwidth of signal in 625 lines system is 5 MHz. 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. NTSC stands for National Television Systems Committee. 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. At the end of each line. Only a few lines at the top and the bottom of each field are shown. Interlaced scanning is where each picture. This sequence is repeated at a fast enough rate so that the displayed images are perceived to have continuous motion. 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. One complete set of lines makes a picture. Television signals and compatible displays are typically interlaced. the NTSC format is used. Each of the formats is generally not compatible with the others.This section will describe in detail how pictures are created in a television. There are many different kinds of video signals. developed after NTSC. all of the lines on the display are scanned in this way. These two formats are incompatible with each other. It should be noted that there is a total of about 15 different sub-formats contained within these three general formats. is divided into two separate sub-pictures. Acamera works exactly on the same principle applied the other way round. not shown) that tells the scanning circuit to retrace to the top of the display and start scanning the next frame. Once the first complete picture is scanned. the PAL format is common. The format of television signals varies from country to country. and computer signals and compatible displays are typically progressive (non-interlaced). and field 2 consists of lines 262 1/2 through 525. In Europe. The interlaced principle is illustrated in Figure 2. SECAM is used in France and stands for sequential coleur avec memoire (with memory). is an improvement over NTSC. In the United States and Japan. which can be divided into either television or computer types. there is another portion of the waveform (vertical blanking interval. This is called a frame. In Europe. In India we use the PAL system. the NTSC format is used. It has 625 lines in each frame and uses interlaced scanning. among others. number of scan lines. and color modulation techniques. This compatibility is a major advantage for computer formats in that media. This compatibility is a major advantage for computer formats in that media. they differ in specific scanning frequencies. . It has 625 lines in each frame and uses interlaced scanning. and color modulation techniques. which is the name of the organization that developed the standard. which can be divided into either television or computer types. among others. is an improvement over NTSC. These differences do not cause as much concern. Each of the formats is generally not compatible with the others.because most computer equipment is now designed to handle variable scan rates. with the primary difference in the scan frequencies. PAL (phase alternating line). It should be noted that there is a total of about 15 different sub-formats contained within these three general formats. These differences do not cause as much concern. and content can be interchanged on a global basis. with the primary difference in the scan frequencies. The various computer formats (such as VGA. The format of television signals varies from country to country. In the United States and Japan. because most computer equipment is now designed to handle variable scan rates. XGA. and UXGA) also differ substantially. and UXGA) also differ substantially.and represent color with a type of phase modulation. developed after NTSC. the PAL format is common. and content can be interchanged on a global basis. The various computer formats (such as VGA. NTSC stands for National Television Systems Committee. they differ in specific scanning frequencies. Although they all utilize the same basic scanning system and represent color with a type of phase modulation. SECAM is used in France and stands for sequential coleur avec memoire (with memory). In India we use the PAL system. XGA. number of scan lines. There are many different kinds of video signals. In order of .Typical Frequencies for Common TV and Computer Video Formats There are three basic levels of baseband signal interfaces. The brightness information is the instantaneous amplitude at any point in time. blanking. they are composite (or CVBS). which uses one wire pair. This figure depicts the portion of the signal that represents one horizontal scan line. 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 colorburst reference phase. This is the same connector as that used for standard line level audio connections. From the figure. Each wire pair consists of a signal and a ground. and the synchronizing signals on just one cable. These three interfaces differ in their level of information combination (or encoding). Composite/CVBS Interface Composite signals are the most commonly used analog video interface. or composite video baseband signal. Component has the least amount of encoding. The amplitude of the modulation is proportional to the amount of color (or . which uses three wire pairs. video. It combines the brightness information (luma).increasing quality. which uses two wire pairs. whereas the horizontal blanking portion would be displayed as black and therefore not beseen on the screen. The active video portion contains the picture brightness (luma) and color (chroma) information. the color information (chroma). Each line is made up of the active video portion and the horizontal blanking portion. Composite video is also referred to as CVBS. and component. which stands for color. The connector is typically an RCA jack.Y/C (or S-video). it can be see that the voltage during the active video portion would yield a brightwhite picture for this horizontal scan line. and sync. More encoding typically degrades the quality but allows the signal to be carried on fewer wires. A typical waveform of an all-white NTSC composite video signal is shown in Figure. and composite the most. because they have the least encoding. The Y signal contains the brightness (luma) and synchronizing information. These are intermediate. R-Y: Luma and color-difference signals.33). Horizontal blanking interval is the time period allocated for retrace of the signal from the right edge of the . Commonly found in high-end consumer equipment. which is the Y signal. Pr. Y/C Interfaces The Y/C signal is a video signal with less encoding. and the color (chroma). Y. whereas colordifference formats are generally used in television applications. 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"). Sometimes incorrectly referred to as a "component interface. green.85:1 or 2.78). and the phase information denotes the tint (or hue) of the color. Cb: Digital-signal equivalent to Y. Component Interfaces Component signal interfaces are the highest performance. Blanking Interval There are horizontal and vertical blanking intervals. Pb: Pr and Pb are scaled versions of B-Y and R-Y. U. Pb.saturation). The theory behind this combination is that each of the base R. and B components can be derived from these difference signals.35:1 are used in cinema . Standard television and computers have an aspect ratio of 4:3(1. Brightness (luma). are carried on two separate sets of wires. blue (RGB) format. and the colordifferencesignals contain the red (R) minus the Y signal and the blue (B) minus the Y signal. Y. They always utilize three pairs of wires that are typically in either a luma (Y) and twocolor-differencesignals format or a red.Common variations of these signals are as follows: Y. V: Not an interface standard. Pr. Sometimes incorrectly used in place of Y. B-Y. quadrature signals used in the formation of composite and Y/C signals. G. HDTV has aspects ratios of either 4:3 or 16:9(1. Additional aspect ratios like 1." 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. Cr. the C signal. RGB formats are almost always used in computer applications. Pb. The signals exist in a nearly native format. Y. Pr. 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. Blanking Level Used to describe a voltage level (blanking level). 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. In interlaced scanning systems. two fields make a frame.43362MHz. 50Hz scanning systems with a subcarrier frequency of 4. Color Burst The color burst. This term is sometimes incorrectly referred to as "chrominance. This term is sometimes incorrectly called "luminance. predates current color television. Fields and Frames A frame is one complete scan of a picture." which refers to the actual displayed brightness. PAL Phase alternate line. The blanking level is the nominal voltage of a video waveform during the horizontal and vertical periods. Pixel . Commonly used with 626-line." is 8 to 10 cycles of the color reference frequency. excluding the more negative voltage sync tips. Monochrome.display back to the left edge to start another scan line. a field is half of a frame. commonly known as black-and-white. also commonly called the "color subcarrier." which is the actual displayed color information. Synchronizing signals occupy a portion of the blanking interval. Similar to NTSC but uses subcarrier phase alternation to reduce the sensitivity to phase errors that would be displayed as color errors. Luma The monochrome or black-and-white portion of a video signal. It is positioned between the rising edge of sync and the start of active video for a composite video signal. PAL is used to refer to systems and signals that are compatible with this specific modulation technique. Chroma The color portion of a video signal. Monochrome The luma (brightness) portion of a video signal without the color information. thus. In NTSC it consists of 525 horizontal scan lines. " . 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 is the luma (brightness) portion. RGB Stands for red. A pixel is the smallest piece of display detail that has a unique brightness and color. Y Cr Cb A digital component video interface. represented by a certain number of bits to indicate the brightness. Y/C An analog video interface in which the chroma (color) information is carried separately from the luma (brightness) and sync information. Often incorrectly referred to as "S-video. green. It is a component interface typically used in computer graphics systems. and blue. denoted Y and C or Y/C.Picture element. Sync Signals/Pulses Sync signals. also known as sync pulses. Two wire pairs are used. and Cr and Cb are the color-difference portions of the signal. In a digital image. a pixel is an individual point in the image. 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.vertical . Earth station of the doordarshan uplinks the digital signal of channel towards satellite and the people using DTH can directly receive signals from satellite.3749.5MHz Down link frequency . Doordarshan uses 12 frame GOP. 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.5974. So For the up linking it required compression of the signals.5 MHz Symbol rate Uplink polarization .horizontal Downlink polarization . 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 .6.Earth station Earth 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 .5 Mbps for transmission over digitally modulation technique used is QPSK ( quadrature phase shift keying ).25MBPS . MPEG-2 format with sampling parameter of 4:2:0 with 10 bit quantization and the bit rate is 4. • • • • Satellite FEC .insat-3A in geosynchronous orbit -¾ Compression format .MPEG2 .4:2:2 Coded standard . the automatic phase control (APC) circuit is provided. Exciter is said to be the heart of the transmission section. so that after passing through all subsequent transmitter stages. They consist of an exciter followed by power amplifiers which boost the exciter power to the required level. 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. It contains precorrector units both at base band as well as at IF stage. the characteristics of the pre-correction circuits can be varied over a wide range. may differ according to the required output power. The unit fault status is displayed with a light emitting diode when a unit output fault or an APC fault has occurred.HIGH POWER TV TRANSMITTER All the TV transmitters have the same basic design. To fix the average frequency of the average frequency of the oscillator at the reference input. pre-emphasis of either 50 microseconds or 75 microseconds can be selected. Since the number and type of amplifier stages. 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. EXCITER The exciter stage determines the quality of a transmitter. It consists of different sections. For 600 ohms balanced line. an acceptable signal is available. . 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. 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. High level correction of amplitude is performed by the use of a saturated class A transistor amplifier. In the low level circuit.MOD unit. In which some corrections of IM due to the non-linearity of the PA stage can be carried out.MOD unit). IM CORRECTOR The HPB-3112 IM CORR (DS) UNIT is used for dual sound having 2 carriers. High level correction of phase is performed by the use of a class C transistor amplifier. SC signal and V pulse signal that the DVC unit need. as the result the phase combining of the linear signal with the non-linear signal produces amplitude correction. for measuring the peak value of the output signal. The output is applied to a VHF mixer unit.Auxiliary circuits are provided for the monitoring the output signal. Clamps the pedestal potential of the video signal. correction can be performed by combining a non-linear signal generated by a class B amplifier with the linear signal. . Furthermore. and for measuring the frequency deviation. This unit contains a low level and high level correction circuit each having correction for amplitude and phase. this unit also supplies the 4fsc clock signals. • • • • Carries out synchronous of the main input video signal. Furthermore.up type that can compensate for variations in the characteristics of the power amplifier by APL. 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. receives the demodulated output signal of the transmitter and automatically compensates for the distortion in the output signal. 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. White clip (manual). circuit performs high accuracy The non-linear and linear distortion compensating circuit respectively can be by-passed. Automatic compensation of the graph delay characteristics. the non-linear distortion caused by the power amplifier of the transmitter. Generates 4fsc clocks locked to the input video signal or F/B video signal (BCO circuit). The linear distortion compensating compensation by using a 128 tap digital filter. this circuit is an APL follow. This unit composed of a non-linear compensating circuit. The non-linear distortion components circuit compensates as an auxiliary circuit to the IF corrector.. Automatic compensation of the frequency characteristics. similarly for the visual period. linear distortion compensating circuit. DP and luminance linearity). 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. . the feedback input video signal and that of input synchronous signal when scrambling is used. control circuit etc. Pre-compensation of the graph delay characteristics of the receiver (fixed/manual). This unit also contains a means to combine two modulated IF carriers of the visual and aural allowing multiplex operation of the transmitter. VHF Mixer In this unit. The HPB-3104 visual modulator performs three functions: to modulate the IF carrier with the ring modulator. the output power of the transmitter is maintained at a constant level. . IF CORRECTOR This IF corrector unit generally used for the correction of non linear distortion generated in the PA stage. Manual adjustment of the distortion compensation.C. to remove one side band with the VSBF and to precorrect the ICPM with the IF phases modulator. enables correction of DG and DP characteristics of visual signal. Automatic fault detection by means of self check. By applying AGC to the IF signal. 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. then summed into the video signal by which the carrier for the ring modulator is phase modulated. The BPF and BEF are all installed for adjustment from the front side.• • • Synchronous expansion (manual/auto). Synthesizer changes from exciter to exciter. and then the signal passes through the vestigial sideband filter (VSBF) which uses a surface acoustic wave (saw) filter to achieve the Nyquist shaping.C. SYNTHESIZER Synthesizer is a device which generates the intermediate frequency for audio and video modulation (for aural mixer and visual mixer). black and white in which each signal amplitude is individually expanded or compressed. It depends on the channel frequencies allocated by the F. The video signal for the IF modulation is arbitrarily sliced into three regions of sync. 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. The ring modulator is followed by a pin diode circuitry via the harmonic filter. by some display or control device located a distance away. Sometimes a device (for example. In radio electronics and broadcasting. propagates an electromagnetic signal such as radio. a modulator. vocal cords may also be considered as example of a transmitter. In amateur radio. an oscillator. and transmit it a few feet to standard radio receiver. a “transmitter” is any device which converts measurements from a sensor into a signal to be received. 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. A common consumer electronics device is a Personal FM transmitter. When used in this more general sense. CD player. which is then broadcast. For example. television. or other telecommunications. In industrial process control. 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. with the combined unit referred to as a transceiver.TRANSMITTING ANTENNA A transmitter is an electronic device which. In the USA. Generally and in communication and information processing. etc. usually with the aid of an antenna. 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. a transmitter is any object which sends information to an observer. and often referred to using an abbreviated form: “XMTR”. a transmitter can be a separate piece of electronic gear or a set of a transceiver. . a very low power transmitter generally designed to take a simple audio source like an iPod. most personal FM transmitters fall under part 15% of the FCC regulations to avoid any user licensing requirements. a transmitter usually has a power supply. a cell phone) contains both a transmitter and radio receiver. In other applications signals can also be transmitted using an analog 0/4-20 mA current loop signal. usually sent via wires. the Alouis. and telecommunication. 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. air cooling with fans is used. the highpower amplifier which The exciter then feeds into is often called the “transmitter” y broadcast engineers. It is the TPO. although this is not what most stations are rated by. Sweden). LF. At power levels over a few kilowatts. For other aerial types there are gain factors. began to be used. However.History in the early days of radio engineering. Since some authors take account of gain factors of aerials of transmitters for frequencies below 30 MHz and others not. based on vaccum tubes. even for mot non-broadcast stations. multiplied by the gain (magnification) which the antenna provides toward the horizon. Confusingly. This is the only correct for Omni directional aerials with a length of a quarter wavelengths or shorter. In the 1920s electronic transmitters. modulator and sometimes audio processor. Cooling of final stages Low-power transmitters do not require special cooling equipment. For example. the output stage is cooled by a . minus any attenuation or radiated loss in the line to the antenna. For VLF. This is important. 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. MF and HF the ERP is typically not determined separately. 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. The final output is given as transmitter power output (TPO). For most large stations in the VHF and UHF-range. Effective radiated power (ERP) is used when calculating station coverage. because the electric utility bill for the transmitter would be enormous otherwise. In most cases the transmission power found in lists of transmitters is the value for the output of the transmitter. as would the cost of a transmitter. Power Output In broadcasting. Konstantynow and Roumoules transmitters are fed from the high-voltage network (110 kV in Alouis and Konstantynow. the transmitter power is no more than 20% of the ERP. which can reach values until 50 for shortwave directional beams in the direction of maximum earn intensity. Modern transmitters can be incredibly efficient. 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. up to a few hundred watts. there are often discrepancies of the values of transmitted powers. the part which contains the oscillator. with efficiencies exceeding 98 percent. is called the exciter. Lightning protection is required between the transmitter and antenna. In some transmitting plants UV detectors are fitted in critical places. To reduce transmission line loss the transmitter building is usually immediately adjacent to the antenna for VHF and UHF sites. In European countries like Germany often the national Post Offices is the regulating authority. Building A commercial transmitter site will usually have a control building to shelter the transmitter components and control devices. Tube based transmitters must have power applied the proper sequence. deionised water or a special dielectric coolant can also be used in the cooling circuit. 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. If this does not succeed after several attempts. The production of steam allows a high heat flow in a small space. as the reflections are probably due to lightning damage. This is usually a purely functional building. the antenna may be damaged and the transmitter should remain switched off. Protection equipment The high voltages required used in high power transmitters (up to 40 kV) require extensive equipment. Some transmitting towers have enclosures built into the tower to house radio relay link transmitters or other. and may be displayed locally and/or at a remote control room. Very high power tubes of small physical size may use evaporative cooling by water in contact with anode. Legal and regulatory aspects Since radio waves go over borders. so protection circuits must detect the loss of the antenna and switch off the transmitter immediately. international agreements control radio transmissions. which indicate that generated power is not being radiated but reflected back into transmitter. This consists of spark gaps and gas-filled surge arresters to limit the voltage that appears on the transmitter terminals. only distilled. which may constant apparatus for both radio and television transmitters. The output stage must be monitored for standing waves. Transmitters may be damaged if operated without an antenna. Since the coolant directly touches the high-voltage anodes of the tubes. where the second cooling circuit can use water of ordinary quality because it not in contact with energized parts. Main and relay (repeater) transmitters . In the United States broadcast and industrial transmitters are regulated by the Federal Communications Commission (FCC). Also. otherwise the tubes can be damaged. This high purity coolant is in turn cooled by a heat exchanger. transmitters are exposed to damage from lightening. relatively low-power transmitters. frequency and other transmitter parameters are monitored for protection and diagnostic purposes. 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. with the filament voltage applied before the anode voltage.forced liquid cooling system analogous to an automobile cooling system. The International Telecommunication Union (ITU) helps managing the radio frequency spectrum internationally. however this signal would be fully demodulated to baseband first. Main stations are defined as those that generate their own modulated output signal from a baseband (unmodulated) input. processed. or at the fringe of. and are used to fill in pockets of poor reception within. Note that a main station may also take its input signal directly off-air from another station. . Usually main stations operate at high power and large areas. and then remodulated for transmission. Usually relay stations operate at medium or low power. 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.Transmitting stations are usually either classified as main stations or relay stations (also known as repeaters or translators). the service area of a parent main station.