A television transmitter is a
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
that is used for
terrestrial (over-the-air)
television broadcasting
A television network or television broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid- ...
. It is an electronic device that radiates
radio wave
Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...
s that carry a
video signal
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) sy ...
representing moving images, along with a synchronized
audio channel, which is received by
television receivers ('televisions' or 'TVs') belonging to a public audience, which display the image on a screen. A television transmitter, together with the
broadcast studio which originates the content, is called a
television station
A television station is a set of equipment managed by a business, organisation or other entity, such as an amateur television (ATV) operator, that transmits video content and audio content via radio waves directly from a transmitter on the ea ...
. Television transmitters must be licensed by governments, and are restricted to a certain
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
channel and power level. They transmit on frequency
channels in the
VHF
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
and
UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
bands. Since radio waves of these frequencies travel by
line of sight, they are limited by the horizon to reception distances of 40–60 miles depending on the height of transmitter station.
Television transmitters use one of two different technologies: ''analog'', in which the picture and sound are transmitted by
analog signal
An analog signal or analogue signal (see spelling differences) is any continuous signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the instantaneous signal voltage varies ...
s
modulated onto the radio
carrier wave, and ''digital'' in which the picture and sound are transmitted by
digital signal
A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; a ...
s. The original television technology,
analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog ...
, began to be replaced in a
transition beginning in 2006 in many countries with
digital television
Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative adva ...
(DTV) systems. These transmit pictures in a new format called HDTV (
high-definition television
High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the g ...
) which has higher resolution and a wider screen
aspect ratio than analog. DTV makes more efficient use of scarce
radio spectrum
The radio spectrum is the part of the electromagnetic spectrum with frequencies from 0 Hz to 3,000 GHz (3 THz). Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particul ...
bandwidth, as several DTV channels can be transmitted in the same bandwidth as a single analog channel. In both analog and digital television, different countries use several incompatible
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...
standards to add the video and audio signals to the radio
carrier wave.
The principles of primarily analog systems are summarized as they are typically more complex than digital transmitters due to the multiplexing of VSB and FM modulation stages.
Types of transmitters
There are many types of transmitters depending on
*
The system standard
*
Output power
* Back up facility, usually the Modulator, Multiplexer and Power Amplifier
*
Stereophonic (or
dual sound) facility, for analogue TV systems
* Aural and visual power combining principal, for analogue TV systems
* Active circuit element in the final amplifier stage
The system standard
An international plan by ITU (
International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
) on broadcast standards which is usually known as
Stockholm plan (1961) defines standards used in
broadcasting
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began wi ...
. In this plan, most important figures for transmitters are
radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the up ...
, frequency separation between aural and visual carriers and
band width.
Analogue TV Broadcast Systems
by Paul Schlyter
Input stage of a transmitter
The audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
* Digital audio, representation of sou ...
(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
In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier sig ...
carrier (IF). The modulation technique is usually frequency modulation
Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing.
In analog ...
(FM) with a typical maximum deviation of 50 kHz (for 1 kHz. input at 0 dBm level).
The video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) sy ...
(VF) input is a composite video signal
Composite video is an analog video signal format that carries standard-definition video (typically at 525 lines or 625 lines) as a single channel. Video information is encoded on one channel, unlike the higher-quality S-Video (two channels) ...
(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 signals 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 an 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. (i.e. 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 suppression causes phase delay problems the VSB stage also includes correction circuits to equalise the phase.
Output stages
The modulated signal is applied to a mixer
Mixer may refer to:
Electronics
* DJ mixer, a type of audio mixing console used by disc jockeys
* Electronic mixer, electrical circuit for adding signal voltages
* Frequency mixer, electrical circuit that creates new frequencies from two signals ...
(also known as frequency converter). Another input to the mixer which is usually produced in a crystal oven oscillator
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
is known as subcarrier
A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...
. 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
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
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
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
s. But in older transmitters tetrodes or klystrons are also utilized.
In modern solid-state VHF and UHF transmitters, LDMOS
LDMOS (laterally-diffused metal-oxide semiconductor) is a planar double-diffused MOSFET (metal–oxide–semiconductor field-effect transistor) used in amplifiers, including microwave power amplifiers, RF power amplifiers and audio power amplifi ...
power transistors
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
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'.
Combining aural and visual signals
There are two methods:
* Split sound system: There are two parallel transmitters, one for aural signal and one for visual signal. The signals are mixed and amplified before being combined at the output via a high power combiner
Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are passive devices used mostly in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmiss ...
. This is the system used in most high power applications.
* Intercarrier system: There are two input stages, one for AF and one for VF respectively. The two signals are combined in low power IF circuits (i.e., after modulators). Because the mixer and amplifiers are common to both signals, the system needs no high power combiners and therefore the price and power consumption is considerably lower than that of split sound system of the same operational level. An adverse effect of the two signals passing through amplifiers is intermodulation products, so the intercarrier system is not suitable for high power applications. In the case of lower power transmitters, a notch filter to reject the cross modulation products must be used at the output.
Output power
The output power of the transmitter is defined as the power during sync pulse (Real output power is variable depending on the content). The quantifiable power from the transmitting equipment and antenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...
are different from one another. The output power of the antenna is known as ERP, which is represented by the formula
where Po represents output power, and Ga represents antenna gain.
See also
*Amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
*Broadcast relay station
A broadcast relay station, also known as a satellite station, relay transmitter, broadcast translator (U.S.), re-broadcaster (Canada), repeater ( two-way radio) or complementary station (Mexico), is a broadcast transmitter which repeats (or tr ...
*Broadcast television systems
Broadcast television systems (or terrestrial television systems outside the US and Canada) are the encoding or formatting systems for the transmission and reception of terrestrial television signals.
Analog television systems were standardized by ...
* Differential gain
*Differential phase
Differential phase is a kind of linearity distortion which affects the color hue in TV broadcasting.
Composite color video signal
Composite color video signal (CCVS) consists of three terms:
*Monochrome (luminance) signal
*Auxiliary signals ...
* Intercarrier method
*Studio/transmitter link
A studio transmitter link (or STL) sends a radio station's or television station's audio and video from the broadcast studio or origination facility to a radio transmitter, television transmitter or uplink facility in another location. This is ...
(STL)
*Transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
* Transmitter/studio link (TSL)
* Transmitter station
*Transposer
In broadcasting, a transposer or translator is a device in or beyond the service area of a radio or television station transmitter that rebroadcasts signals to receivers which can’t properly receive the signals of the transmitter because of a p ...
References
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,
FARWAY IRFC, TV and Radio Transmission , Radio Data System Encoders , Broadcasting Technologies
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Broadcast engineering
Broadcast transmitters
Television technology
Television terminology