Television transmitter
Encyclopedia
A television transmitter is a device which broadcasts an electromagnetic
signal to the television
receivers
. Television transmitter
s may be analog
or digital
.
) on broadcast standards which is usually known as Stockholm plan (1961) defines standards used in broadcasting
. In this plan, most important figures for transmitters are radio frequency
, frequency separation between aural and visual carriers and band width.
(AF) input (or inputs in case of stereophonic broadcasting) is usually a signal with 15 kHz maximum bandwidth and 0 dBm maximum level. Preemphasis
time constant is 50 µs. The signal after passing buffer stages is applied to a modulator where it modulates an intermediate frequency
carrier (IF
). The modulation technique is usually frequency modulation
(FM) with a typical maximum deviation
of 50 kHz (for 1 kHz. input at 0 dBm level).
The video
(VF) input is a composite video signal (video information with sync) of maximum 1 volt on 75 Ω impedance. (1 V limit is for luminance signal
. Some operators may accept superimposed color signal
s slightly over 1 V.) After buffer and 1 V clipping
circuits the signal is applied to the modulator where it modulates an intermediate frequency signal (which is different from the one used for aural signal.) The modulator is a amplitude modulator which modulates the IF signal in a manner where 1 V VF corresponds to low level IF and 0 volt VF corresponds to high level IF. AM modulator produces two symmetrical side bands in the modulated signals. Thus IF
band width is two times the video band width. (ie if the VF bandwidth is 4.2 MHz, the IF bandwidth is 8.4 MHz.) However, the modulator is followed by a special filter known as Vestigal sideband (VSB) filter. This filter is used to suppress a portion of one side band, thus bandwidth is reduced. (Since both side bands contain identical information, this suppression doesn't cause a loss in information .)Although the suppresion causes phase delay problems the VSB stage also includes correction circuits to equalise the phase.
(also known as frequency converter). Another input to the mixer which is usually produced in a crystal oven oscillator is known as subcarrier
. The two outputs of the mixer are the sum and difference of two signals. Unwanted signal (usually the sum) is filtered out and the remaining signal is the radio frequency (RF
) signal. Then the signal is applied to the amplifier
stages. The number of series amplifiers depends on the required output power
. The final stage is usually an amplifier consisting of many parallel power transistor
s. But in older transmitters tetrode
s or klystron
s are also utilized.
In modern solid-state VHF and UHF transmitters, LDMOS
power transistors are the device of choice for the output stage, with the latest products employing 50V LDMOS devices for higher efficiency and power density. Even higher energy efficiency is possible using Envelope Tracking
, which in the broadcast industry is often referred to as 'drain modulation'.
of the transmitter is defined as the power during sync pulse. But the output power of the transmitting equipment and the output power of the antenna
are two different quantities. The output power of the antenna is known as ERP
which is actually the transmitter power times the antenna gain.
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
signal to the television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. Television transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s may be analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
or digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
.
Types of transmitters
There are many types of transmitters depending on- The system standard
- Output power
- Back up facility
- Stereophonic (or dual sound) facility
- Aural and visual power combining principle
- Active circuit element in the final amplifier stage
The system standard
An international plan by ITU (International Telecommunication UnionInternational Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
) on broadcast standards which is usually known as Stockholm plan (1961) defines standards used in broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
. In this plan, most important figures for transmitters are radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
, frequency separation between aural and visual carriers and band width.
Input stage of a transmitter
The audioSound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
(AF) input (or inputs in case of stereophonic broadcasting) is usually a signal with 15 kHz maximum bandwidth and 0 dBm maximum level. Preemphasis
Preemphasis
In processing electronic audio signals, pre-emphasis refers to a system process designed to increase the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as...
time constant is 50 µs. The signal after passing buffer stages is applied to a modulator where it modulates an intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
carrier (IF
If
If is a conjunction that can introduce a conditional clause.If may also refer to:-Music:* If , a 1951 hit song by Perry Como.* If , 1970s British progressive jazz-rock band* If...
). The modulation technique is usually frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(FM) with a typical maximum deviation
Deviation
Deviation may refer to:* Deviation , the difference between the value of an observation and the mean of the population in mathematics and statistics** Standard deviation, which is based on the square of the difference...
of 50 kHz (for 1 kHz. input at 0 dBm level).
The video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
(VF) input is a composite video signal (video information with sync) of maximum 1 volt on 75 Ω impedance. (1 V limit is for luminance signal
Luma (video)
In video, luma, sometimes called luminance, represents the brightness in an image . Luma is typically paired with chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information...
. Some operators may accept superimposed color signal
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
s slightly over 1 V.) After buffer and 1 V clipping
White clipper
White clipper is a circuit in professional video products that limits the maximum amplitude of the luminance part of the analogue video signal to 1 volt...
circuits the signal is applied to the modulator where it modulates an intermediate frequency signal (which is different from the one used for aural signal.) The modulator is a amplitude modulator which modulates the IF signal in a manner where 1 V VF corresponds to low level IF and 0 volt VF corresponds to high level IF. AM modulator produces two symmetrical side bands in the modulated signals. Thus IF
If
If is a conjunction that can introduce a conditional clause.If may also refer to:-Music:* If , a 1951 hit song by Perry Como.* If , 1970s British progressive jazz-rock band* If...
band width is two times the video band width. (ie if the VF bandwidth is 4.2 MHz, the IF bandwidth is 8.4 MHz.) However, the modulator is followed by a special filter known as Vestigal sideband (VSB) filter. This filter is used to suppress a portion of one side band, thus bandwidth is reduced. (Since both side bands contain identical information, this suppression doesn't cause a loss in information .)Although the suppresion causes phase delay problems the VSB stage also includes correction circuits to equalise the phase.
Output stages
The modulated signal is applied to a mixerElectronic mixer
An electronic mixer is a device that combines two or more electrical or electronic signals into one or two composite output signals. There are two basic circuits that both use the term mixer, but they are very different types of circuits: additive mixers and multiplying mixers...
(also known as frequency converter). Another input to the mixer which is usually produced in a crystal oven oscillator is known as subcarrier
Subcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
. The two outputs of the mixer are the sum and difference of two signals. Unwanted signal (usually the sum) is filtered out and the remaining signal is the radio frequency (RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
) signal. Then the signal is applied to the amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
stages. The number of series amplifiers depends on the required output power
Transmitter power output
In radio transmission, transmitter power output is the actual amount of power of radio frequency energy that a transmitter produces at its output....
. The final stage is usually an amplifier consisting of many parallel power transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s. But in older transmitters tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
s or klystron
Klystron
A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...
s are also utilized.
In modern solid-state VHF and UHF transmitters, LDMOS
LDMOS
LDMOS transistors are used in RF/microwave power amplifiers. These transistors are fabricated via an epitaxial silicon layer on a more highly doped silicon substrate....
power transistors are the device of choice for the output stage, with the latest products employing 50V LDMOS devices for higher efficiency and power density. Even higher energy efficiency is possible using Envelope Tracking
Envelope Tracking
Envelope tracking describes an approach to RF amplifier design in which the power supply voltage applied to the power amplifier is constantly adjusted to ensure that the amplifier is operating at peak efficiency for the given instantaneous output power requirements....
, which in the broadcast industry is often referred to as 'drain modulation'.
Combining aural and visual signals
There are two methods:- Split sound systemSplit sound systemSplit sound is an old system in analog television transmitters. It has long been superseded, but transmitters working on this principle are still in use. In this system there are two almost independent transmitters, one for sound and one for picture...
: Actually there are two parallel transmitters one for aural and one for visual signal. The two signals are combined at the output via a high power combiner. In addition to a combiner, this system requires separate mixer and amplifiers for aural and visual signals. This is the system used in most high power applications.
- Intercarrier systemIntercarrier methodThe intercarrier method is a system in television that reduces the cost of transmitters and receiver sets by processing audio and video signals together and minimizing the number of separate stages for audio and video signals....
: There are two input stages one for AF and one for VF. But the two signals are combined in low power IF circuits (ie, after modulators) The mixer and the amplifiers are common to both signals and the system needs no high power combiners. So both the price of the transmitter and the power consumption is considerably lower than that of split sound system of the same power level. But two signals passing through amplifiers produce some intermodulationIntermodulationIntermodulation or intermodulation distortion is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities...
products. So intercarrier system is not suitable for high power applications and even at lower power transmitters a notchNotchNotch may refer to:* The nock of an arrow* Notch , a Hip hop, R&B, reggae, dancehall and reggaeton artist* Notch signaling pathway, a cell signaling system present in most multicellular organisms...
filter to reject the cross modulation products must be used at the output.
The output power
The output powerOutput power of an analog TV transmitter
Output power of a TV transmitter is the electric power applied to antenna system. There are two definitions: nominal and thermal.- Power defined in terms of voltage :The average power for a sinusoidal drive is...
of the transmitter is defined as the power during sync pulse. But the output power of the transmitting equipment and the output power of the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
are two different quantities. The output power of the antenna is known as ERP
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
which is actually the transmitter power times the antenna gain.
See also
- Amplitude ModulationAmplitude modulationAmplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
- Broadcast relay stationBroadcast relay stationA broadcast relay station, relay transmitter, broadcast translator , rebroadcaster , or repeater is a broadcast transmitter which relays, repeats, or reflects the signal of another radio station or television station, usually to an area not covered by the signal of the originating station...
- Broadcast television systems
- Differential gainDifferential gainDifferential gain is a kind of linearity distortion which affects the color saturation in TV broadcasting.- Composite color video signal :Composite color video signal consists of three terms:*Luminance signal...
- Differential phaseDifferential phaseDifferential phase is a kind of linearity distortion which affects the color hue in TV broadcasting.- Composite color video signal :Composite color video signal consists of three terms:*Monochrome signal...
- Intercarrier methodIntercarrier methodThe intercarrier method is a system in television that reduces the cost of transmitters and receiver sets by processing audio and video signals together and minimizing the number of separate stages for audio and video signals....
- Studio/transmitter linkStudio/transmitter linkA studio-transmitter link sends a radio station's or television station's audio and video from the broadcast studio to a radio transmitter or television transmitter in another location....
(STL) - TransmitterTransmitterIn electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
- Transmitter/studio linkTransmitter/studio linkThe transmitter/studio link of a radio station or television station is a return link which sends telemetry data from the remotely-located radio transmitter or television transmitter back to the studio for monitoring purposes...
(TSL) - Transmitter stationTransmitter stationTransmitter station refers to terrestrial infrastructure for transmitting radio frequency signals. The station maybe used for, wireless communication, broadcasting, microwave link, mobile telephone etc.-Choice of location:...
- TransposerTransposerIn broadcasting, a transposer is a device in the service area of a transmitter which rebroadcasts signals to the receivers which can’t properly receive the signals of the transmitter because of a physical obstruction . A transposer receives the signals of the transmitter and rebroadcasts the...
Further reading
- Bernard Grob,Charles E.Herndon: Television and video systems, Glencoe McGraw-Hill
- Reference data for Radio Engineers, Chapter 30, Howard W.Sams Co Inc., Indianapolis,1977, ISBN 0672212188
- FARWAY IRFC, TV and Radio Transmission , Radio Data System Encoders , Broadcasting Technologies