Subcarrier
Encyclopedia
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. This is an early and simple method of multiplexing
.
s, which takes the left channel and "subtracts" the right channel from it — essentially by hooking up the right-channel wires backward (reversing polarity
) and then joining left and reversed-right. The result is modulated with suppressed carrier AM
, more correctly called sum and difference modulation or SDM, at 38 kHz
in the FM signal, which is joined at 2% modulation with the mono left+right audio (which ranges 50 Hz ~ 15 kHz). A 19 kHz pilot tone is also added at a 10% modulation to trigger radios to decode the stereo subcarrier, making FM stereo fully compatible with mono.
Once the receiver demodulates the L+R and L−R signals, it adds the two signals ([L+R] + [L−R] = 2L) to get the left channel and subtracts ([L+R] − [L−R] = 2R) to get the right channel. Rather than having a local oscillator
, the 19 kHz pilot tone provides an in-phase reference signal used to reconstruct the missing carrier wave
from the 38 kHz signal.
For AM broadcasting
, different analog (AM stereo
) and digital (HD Radio
) methods are used to produce stereophonic audio. Modulated subcarriers of the type used in FM broadcasting are impractical for AM broadcast due to the relatively narrow signal bandwidth allocated for a given AM signal. On standard AM broadcast radios, the entire 9 kHz to 10 kHz allocated bandwidth of the AM signal may be used for audio.
signals are transmitted with the black and white
luminance part as the main signal, and the color chrominance
as the subcarriers. A black and white TV simply ignores the extra information, as it has no decoder for it. To reduce the bandwidth of the color subcarriers, the sampling rate
for color information is reduced four-to-one by using half vertical resolution on every other scan line
. (This is made possible by the fact that the human eye
sees much more detail in contrast
than in color.) In addition, only blue and red are transmitted, with green being determined by subtracting the other two from the luminance and taking the remainder
. (See: YIQ
, YCbCr
, YPbPr
) Various broadcast television system
s use different subcarrier frequencies, in addition to differences in encoding
.
For the audio part, MTS
uses subcarriers on the video that can also carry three audio channels, including one for stereo (same left-minus-right method as for FM), another for second audio program
s (such as descriptive video service
for the vision-impaired, and bilingual programs), and yet a third hidden one for the studio to communicate with reporters or technicians in the field (or for a technician
or broadcast engineer at a remote transmitter
site to talk back to the studio), or any other use a TV station might see fit. (See also NICAM
, A2 Stereo.)
In RF-transmitted composite video
, subcarriers remain in the baseband signal after main carrier demodulation
to be separated in the receiver. The mono audio component of the transmitted signal is in a separate carrier and not integral to the video component. In wired video connections, composite video
retains the integrated subcarrier signal structure found in the transmitted baseband signal, while S-Video
places the chrominance and luminance subcarriers on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picture sharpness
and brightness).
, Muzak
was transmitted to department store
s on FM subcarriers. The United States Federal Communications Commission
(FCC) also allowed betting parlor
s in New York
state to get horse racing
results from the state gaming commission via the same technology.
Many non-commercial educational
FM stations in the US (especially public radio stations affiliated with NPR
) broadcast a radio reading service
for the blind, which reads articles in local newspapers and sometimes magazines. The vision-impaired can request a special radio, permanently tuned to hear audio on a particular subcarrier frequency (usually 67 kHz or 92 kHz), from a particular FM station.
Services like these and others on broadcast FM subcarriers are referred to as a Subsidiary Communications Authority
(SCA) service by the FCC in the United States
, and as Subsidiary Communications Multiplex Operations (SCMO) by the Canadian Radio-television and Telecommunications Commission (CRTC) in Canada
.
/RBDS subcarrier (57 kHz) allows FM radios to display what station they are on, pick another frequency on the same network or with the same format, scroll brief messages like station slogans, news, weather, or traffic—even activate pagers or remote billboards. It can also broadcast EAS
messages, and has a station "format
" name ALERT to automatically trigger radios to tune in for emergency info, even if a CD is playing. While it never really caught on in North America
, Europe
an stations frequently rely on this system. An upgraded version is built into digital radio
.
MSN Direct
uses subcarriers to transmit traffic, gas prices, movie times, weather
and other information to GPS navigation devices, wristwatches, and other devices. Many of the subcarriers are from stations owned by Clear Channel
. The technology is known as DirectBand
.
FMeXtra
on FM uses dozens of small COFDM subcarriers to transmit digital radio in a fully in-band on-channel
manner. Removing other analog subcarriers (such as stereo) increases the audio quality or channels available, and other non-audio metadata
that can be sent along with it such as album covers, song lyrics, artist info, concert data, and more.
back from a remote transmitter
, often located in a difficult-to-access area at the top of a mountain. A station's engineer can carry a decoder around with him and know anything that's wrong, as long as the station is on the air and he is within range. This is the essence of a wireless
transmitter/studio link
.
On wireless studio/transmitter link
s (STLs), not only are the broadcast station's subcarriers transmitted, but other remote control
commands
as well. Thus, the STL's total bandwidth may actually be even wider than the station's. This is also used sometimes when transmitting more than one station at a time.
Interruptible foldback, such as for remote broadcasting, is also possible over subcarriers, though its role is limited.
satellite television
and terrestrial analog microwave
relay
communications rely on subcarriers transmitted with the video carrier on a satellite transponder
or microwave channel for the audio channels of a video feed. There are usually at frequencies of 5.8, 6.2, or 6.8 MHz (the video carrier usually resides below 5 MHz on a satellite transponder or microwave relay). Extra subcarriers are sometimes transmitted at around 7 or 8 MHz for extra audio (such as radio stations) or low-to-medium-speed data. This is referred to as multiple channel per carrier (MCPC).
This is now mostly superseded by digital TV (usually DVB-S
, DVB-S2
or another MPEG-2
-based system), where audio and video data are packaged together (multiplex
ed) in a single MPEG transport stream.
Digital signal
A digital signal is a physical signal that is a representation of a sequence of discrete values , for example of an arbitrary bit stream, or of a digitized analog signal...
carried on a main radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
transmission, which carries extra information such as voice
Voice
Voice may refer to:* Human voice* Voice control or voice activation* Writer's voice* Voice acting* Voice vote* Voice message-In film:* Voice , a 2005 South Korean film* The Voice , a 2010 Turkish horror film directed by Ümit Ünal...
or data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
. More technically, it is an already-modulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
signal, which is then modulated into another signal of higher frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and bandwidth. This is an early and simple method of multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
.
FM stereo
Stereo broadcasting is made possible by using a subcarrier on FM radio stationRadio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
s, which takes the left channel and "subtracts" the right channel from it — essentially by hooking up the right-channel wires backward (reversing polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
) and then joining left and reversed-right. The result is modulated with suppressed carrier AM
Amplitude modulation
Amplitude 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...
, more correctly called sum and difference modulation or SDM, at 38 kHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
in the FM signal, which is joined at 2% modulation with the mono left+right audio (which ranges 50 Hz ~ 15 kHz). A 19 kHz pilot tone is also added at a 10% modulation to trigger radios to decode the stereo subcarrier, making FM stereo fully compatible with mono.
Once the receiver demodulates the L+R and L−R signals, it adds the two signals ([L+R] + [L−R] = 2L) to get the left channel and subtracts ([L+R] − [L−R] = 2R) to get the right channel. Rather than having a local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
, the 19 kHz pilot tone provides an in-phase reference signal used to reconstruct the missing carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
from the 38 kHz signal.
For AM broadcasting
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
, different analog (AM stereo
AM stereo
AM stereo is a term given to a series of mutually incompatible techniques for wireless radio broadcasting stereo audio in the AM band in a manner that is compatible with standard AM receivers...
) and digital (HD Radio
HD Radio
HD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
) methods are used to produce stereophonic audio. Modulated subcarriers of the type used in FM broadcasting are impractical for AM broadcast due to the relatively narrow signal bandwidth allocated for a given AM signal. On standard AM broadcast radios, the entire 9 kHz to 10 kHz allocated bandwidth of the AM signal may be used for audio.
Television
Likewise, TVTelevision
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
signals are transmitted with the black and white
Monochrome
Monochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
luminance part as the main signal, and the color chrominance
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'...
as the subcarriers. A black and white TV simply ignores the extra information, as it has no decoder for it. To reduce the bandwidth of the color subcarriers, the sampling rate
Sampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...
for color information is reduced four-to-one by using half vertical resolution on every other scan line
Scan line
A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....
. (This is made possible by the fact that the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
sees much more detail in contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
than in color.) In addition, only blue and red are transmitted, with green being determined by subtracting the other two from the luminance and taking the remainder
Remainder
In arithmetic, the remainder is the amount "left over" after the division of two integers which cannot be expressed with an integer quotient....
. (See: YIQ
YIQ
YIQ is the color space used by the NTSC color TV system, employed mainly in North and Central America, and Japan. It is currently in use only for low-power television stations, as full-power analog transmission was ended by the U.S. Federal Communications Commission on 12 June 2009...
, YCbCr
YCbCr
YCbCr or Y′CbCr, sometimes written or , is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the luma component and CB and CR are the blue-difference and red-difference chroma components...
, YPbPr
YPbPr
' is a color space used in video electronics, in particular in reference to component video cables. is the analog version of the YCBCR color space; the two are numerically equivalent, but YPBPR is designed for use in analog systems whereas YCBCR is intended for digital video. cables are also...
) Various broadcast television system
Broadcast television system
Broadcast television systems are encoding or formatting standards for the transmission and reception of terrestrial television signals. There are three main analog television systems in current use around the world: NTSC, PAL, and SECAM...
s use different subcarrier frequencies, in addition to differences in encoding
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
.
For the audio part, MTS
Multichannel television sound
Multichannel television sound, better known as MTS , is the method of encoding three additional channels of audio into an NTSC-format audio carrier.- History :...
uses subcarriers on the video that can also carry three audio channels, including one for stereo (same left-minus-right method as for FM), another for second audio program
Second audio program
Second audio program , also known as secondary audio programming, is an auxiliary audio channel for analog television that can be broadcast or transmitted both over the air and by cable TV.-Usage:...
s (such as descriptive video service
Descriptive Video Service
The Descriptive Video Service is a major United States producer of video description, which makes visual media, such as television programs, feature films, and home videos, more accessible to people who are blind or otherwise visually impaired...
for the vision-impaired, and bilingual programs), and yet a third hidden one for the studio to communicate with reporters or technicians in the field (or for a technician
Technician
A technician is a worker in a field of technology who is proficient in the relevant skills and techniques, with a relatively practical understanding of the theoretical principles. Experienced technicians in a specific tool domain typically have intermediate understanding of theory and expert...
or broadcast engineer at a remote 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...
site to talk back to the studio), or any other use a TV station might see fit. (See also NICAM
NICAM
Near Instantaneous Companded Audio Multiplex is an early form of lossy compression for digital audio. It was originally developed in the early 1970s for point-to-point links within broadcasting networks...
, A2 Stereo.)
In RF-transmitted composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
, subcarriers remain in the baseband signal after main carrier demodulation
Demodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
to be separated in the receiver. The mono audio component of the transmitted signal is in a separate carrier and not integral to the video component. In wired video connections, composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
retains the integrated subcarrier signal structure found in the transmitted baseband signal, while S-Video
S-Video
Separate Video, more commonly known as S-Video and Y/C, is often referred to by JVC as both an S-VHS connector and as Super Video. It is an analog video transmission scheme, in which video information is encoded on two channels: luma and chroma...
places the chrominance and luminance subcarriers on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picture sharpness
Sharpness
Sharpness is an English port in Gloucestershire, one of the most inland in Britain, and eighth largest in the South West. It is on the River Severn at , at a point where the tidal range, though less than at Avonmouth downstream , is still large .The village of Sharpness is pronounced with the...
and brightness).
Private audio
Before satelliteCommunications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
, Muzak
Muzak
Muzak Holdings LLC is a company based in metro Fort Mill, South Carolina, United States, just outside of Charlotte, North Carolina. Founded in 1934, Muzak Holdings is best known for distribution of background music to retail stores and other companies....
was transmitted to department store
Department store
A department store is a retail establishment which satisfies a wide range of the consumer's personal and residential durable goods product needs; and at the same time offering the consumer a choice of multiple merchandise lines, at variable price points, in all product categories...
s on FM subcarriers. The United States Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) also allowed betting parlor
Off-track betting
Off-track betting refers to sanctioned gambling on horse racing outside a race track.-US history:...
s in New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
state to get horse racing
Horse racing
Horse racing is an equestrian sport that has a long history. Archaeological records indicate that horse racing occurred in ancient Babylon, Syria, and Egypt. Both chariot and mounted horse racing were events in the ancient Greek Olympics by 648 BC...
results from the state gaming commission via the same technology.
Many non-commercial educational
Non-commercial educational
The term non-commercial educational applies to a radio station or TV station that does not accept on air advertisements , as defined in the United States by the Federal Communications Commission . NCE stations do not pay broadcast license fees for their non-profit uses of the radio spectrum...
FM stations in the US (especially public radio stations affiliated with NPR
NPR
NPR, formerly National Public Radio, is a privately and publicly funded non-profit membership media organization that serves as a national syndicator to a network of 900 public radio stations in the United States. NPR was created in 1970, following congressional passage of the Public Broadcasting...
) broadcast a radio reading service
Radio reading service
A radio reading service or reading service for the blind is a service of many universities, community groups and public radio stations, where a narrator reads books, newspapers and magazines aloud for the benefit of the blind and vision-impaired. It is most often carried on a subcarrier, with...
for the blind, which reads articles in local newspapers and sometimes magazines. The vision-impaired can request a special radio, permanently tuned to hear audio on a particular subcarrier frequency (usually 67 kHz or 92 kHz), from a particular FM station.
Services like these and others on broadcast FM subcarriers are referred to as a Subsidiary Communications Authority
Subsidiary Communications Authority
Subsidiary Communications Authorization in the United States, and Subsidiary Communications Multiplex Operation in Canada, is a subcarrier on a radio station, allowing the station to broadcast additional services as part of its signal.-Background:"Subsidiary Communications Authorization" is the...
(SCA) service by the FCC in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, and as Subsidiary Communications Multiplex Operations (SCMO) by the Canadian Radio-television and Telecommunications Commission (CRTC) in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
Datacasting
The RDSRadio Data System
Radio Data System, or RDS, is a communications protocol standard for embedding small amounts of digital information in conventional FM radio broadcasts. RDS standardises several types of information transmitted, including time, station identification and programme information.Radio Broadcast Data...
/RBDS subcarrier (57 kHz) allows FM radios to display what station they are on, pick another frequency on the same network or with the same format, scroll brief messages like station slogans, news, weather, or traffic—even activate pagers or remote billboards. It can also broadcast EAS
Emergency Alert System
The Emergency Alert System is a national warning system in the United States put into place on January 1, 1997, when it superseded the Emergency Broadcast System , which itself had superseded the CONELRAD System...
messages, and has a station "format
Radio format
A radio format or programming format not to be confused with broadcast programming describes the overall content broadcast on a radio station. Radio formats are frequently employed as a marketing tool, and constantly evolve...
" name ALERT to automatically trigger radios to tune in for emergency info, even if a CD is playing. While it never really caught on in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an stations frequently rely on this system. An upgraded version is built into digital radio
Digital radio
Digital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
.
MSN Direct
MSN Direct
MSN Direct is an FM radio-based digital service which allows 'SPOT' portable devices to receive information from MSN services. Devices that support MSN Direct include wristwatches, desktop clocks, in-car GPS satellite navigation units, and even small appliances such as coffee makers...
uses subcarriers to transmit traffic, gas prices, movie times, weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
and other information to GPS navigation devices, wristwatches, and other devices. Many of the subcarriers are from stations owned by Clear Channel
Clear channel
A clear-channel station is an AM band Radio station in North America that has the highest protection from interference from other stations, particularly concerning night-time skywave propagation. Usually known as class A stations since 1982, they are occasionally still referred to by their former...
. The technology is known as DirectBand
DirectBand
DirectBand is a North American wireless datacast network owned and operated by Microsoft. It uses FM radio broadcasts in over 100 cities to constantly transmit data to a variety of devices, including portable GPS devices, wristwatches and home weather stations....
.
FMeXtra
FMeXtra
FMeXtra, now marketed as VuCast and oriented at the broadcast data market like MSN Direct, is an in-band on-channel digital radio broadcasting technology created by Digital Radio Express, now VuCast Media. Unlike iBiquity's HD Radio system, it uses any FM radio station's existing equipment and...
on FM uses dozens of small COFDM subcarriers to transmit digital radio in a fully in-band on-channel
In-band on-channel
In-band on-channel is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency....
manner. Removing other analog subcarriers (such as stereo) increases the audio quality or channels available, and other non-audio metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
that can be sent along with it such as album covers, song lyrics, artist info, concert data, and more.
Telemetry and foldback
Many stations use subcarriers for internal purposes, such as getting telemetryTelemetry
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
back from a remote 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...
, often located in a difficult-to-access area at the top of a mountain. A station's engineer can carry a decoder around with him and know anything that's wrong, as long as the station is on the air and he is within range. This is the essence of a wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
transmitter/studio link
Transmitter/studio link
The 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...
.
On wireless studio/transmitter link
Studio/transmitter link
A 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....
s (STLs), not only are the broadcast station's subcarriers transmitted, but other remote control
Remote control
A remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...
commands
Command (computing)
In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is a directive to some kind of command line interface, such as a shell....
as well. Thus, the STL's total bandwidth may actually be even wider than the station's. This is also used sometimes when transmitting more than one station at a time.
Interruptible foldback, such as for remote broadcasting, is also possible over subcarriers, though its role is limited.
MCPC satellites
AnalogAnalog television
Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
satellite television
Satellite television
Satellite television is television programming delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an...
and terrestrial analog microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
communications rely on subcarriers transmitted with the video carrier on a satellite transponder
Transponder
In telecommunication, the term transponder has the following meanings:...
or microwave channel for the audio channels of a video feed. There are usually at frequencies of 5.8, 6.2, or 6.8 MHz (the video carrier usually resides below 5 MHz on a satellite transponder or microwave relay). Extra subcarriers are sometimes transmitted at around 7 or 8 MHz for extra audio (such as radio stations) or low-to-medium-speed data. This is referred to as multiple channel per carrier (MCPC).
This is now mostly superseded by digital TV (usually DVB-S
DVB-S
DVB-S is an abbreviation for Digital Video Broadcasting — Satellite; it is the original Digital Video Broadcasting forward error coding and demodulation standard for satellite television and dates from 1994, in its first release, while development lasted from 1993 to 1997...
, DVB-S2
DVB-S2
Digital Video Broadcasting - Satellite - Second Generation is a digital television broadcast standard that has been designed as a successor for the popular DVB-S system. It was developed in 2003 by the , an international industry consortium, and ratified by ETSI in March 2005...
or another MPEG-2
MPEG-2
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...
-based system), where audio and video data are packaged together (multiplex
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
ed) in a single MPEG transport stream.
External links
- FM Receiver with subcarrier decoding circuit; United States Patent 4476581
- Chrominance subcarrier phase control system; United States Patent 3974520