A subcarrier is a
sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands ...
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 broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.
FM stereo
Stereo broadcasting is made possible by using a subcarrier on
FM radio
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...
stations, 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
Reduced-carrier transmission is an amplitude modulation (AM) transmission in which the carrier signal level is reduced to reduce wasted electrical power. Suppressed-carrier transmission is a special case in which the carrier level is reduced below ...
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 9% 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 (
+R+
−R= 2L) to get the left channel and subtracts (
+R−
−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
AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") trans ...
, different analog (
AM stereo) and digital (
HD Radio
HD Radio (HDR) is a trademark for an in-band on-channel (IBOC) digital radio broadcast technology. It generally simulcasts an existing analog radio station in digital format with less noise and with additional text information. HD Radio is used ...
) 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, analog
TV signals are transmitted with the
black and white
Black-and-white (B&W or B/W) images combine black and white in a continuous spectrum, producing a range of shades of grey.
Media
The history of various visual media began with black and white, and as technology improved, altered to color. ...
luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls with ...
part as the main signal, and the color
chrominance
Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represen ...
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, they are filtered to remove higher frequencies. This is made possible by the fact that the
human eye
The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm.
...
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
YIQ is the color space used by the analog NTSC color TV system, employed mainly in North and Central America, and Japan.
''I'' stands for ''in-phase'', while ''Q'' stands for ''quadrature'', referring to the components used in quadrature amplitud ...
,
YCbCr,
YPbPr) Various
broadcast television systems 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 programs (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
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 ...
site to talk back to the studio), or any other use a
TV 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 eart ...
might see fit. (See also
NICAM,
A2 Stereo
Zweikanalton ("two-channel sound") or A2 Stereo, is an analog television sound transmission system used in Germany, Austria, Australia, Switzerland, Netherlands and some other countries that use or used PAL-B or PAL-G. TV3 Malaysia formerly us ...
.)
In RF-transmitted
composite video
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 channe ...
, subcarriers remain in the baseband signal after main carrier
demodulation
Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulate ...
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 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 channe ...
retains the integrated subcarrier signal structure found in the transmitted baseband signal, while
S-Video
S-Video (also known as separate video, Y/C, and erroneously Super-Video ) is an analog video signal format that carries standard-definition video, typically at 525 lines or 625 lines. It encodes video luma and chrominance on two separate chann ...
places the chrominance and luminance signals on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picture
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 ( typical s ...
and brightness).
Private audio
Before
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
,
Muzak
Muzak is an American brand of background music played in retail stores and other public establishments. The name has been in use since 1934, and has been owned by a division or subsidiary of one or another company ever since. In 1981, Westingh ...
and similar services were transmitted to
department store
A department store is a retail establishment offering a wide range of consumer goods in different areas of the store, each area ("department") specializing in a product category. In modern major cities, the department store made a dramatic appe ...
s on FM subcarriers. The fidelity of the subcarrier audio was limited compared to the primary FM radio audio channel. The United States
Federal Communications Commission
The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdicti ...
(FCC) also allowed
betting parlors in
New York state to get
horse racing
Horse racing is an equestrian performance sport, typically involving two or more horses ridden by jockeys (or sometimes driven without riders) over a set distance for competition. It is one of the most ancient of all sports, as its basic pr ...
results from the state gaming commission via the same technology.
Many
non-commercial educational
A non-commercial educational station (NCE station) is a radio station or television station that does not accept on-air advertisements ( TV ads or radio ads), as defined in the United States by the Federal Communications Commission (FCC) and was o ...
FM stations in the US (especially
public radio
Public broadcasting involves radio, television and other electronic media outlets whose primary mission is public service. Public broadcasters receive funding from diverse sources including license fees, individual contributions, public financing ...
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 receive 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
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
, and as Subsidiary Communications Multiplex Operations (SCMO) by the
Canadian Radio-television and Telecommunications Commission
The Canadian Radio-television and Telecommunications Commission (CRTC; french: Conseil de la radiodiffusion et des télécommunications canadiennes, links=) is a public organization in Canada with mandate as a regulatory agency for broadcast ...
(CRTC) in
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
.
Datacasting
The
RDS/
RBDS
Radio Data System (RDS) is a communications protocol standard for embedding small amounts of digital information in conventional FM radio broadcasts. RDS standardizes several types of information transmitted, including time, station identificati ...
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
Format may refer to:
Printing and visual media
* Text formatting, the typesetting of text elements
* Paper formats, or paper size standards
* Newspaper format, the size of the paper page
Computing
* File format, particular way that informatio ...
" 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 is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...
,
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
an stations frequently rely on this system.
[The radio station Heart FM used this service to cause people's radio sets to switch over to adverts for commercial vehicles during the morning and evening rush hours] An upgraded version is built into
digital radio
Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services.
Types
In digital broadcasting s ...
.
xRDS
Background
The XML format used by XRDS was originally developed in 2004 by the OASIS XRI ( extensible resource identifierTechnical Committeeas the resolution format for XRIs. The acronym XRDS was coined during subsequent discussions between XRI ...
is a system with which broadcasters can multiply the speed of data transmission in the FM channel by using further normal RDS subcarriers, shifted into the higher frequencies of the FM multiplex. The extra RDS subcarriers are placed in the upper empty part of the multiplex spectrum and carry the extra data payload. xRDS has no fixed frequencies for the additional 57 kHz carriers.
Until 2012,
MSN Direct used subcarriers to transmit traffic, gas prices, movie times,
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
and other information to
GPS navigation devices,
wristwatch
A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached ...
es, and other devices. Many of the subcarriers were from stations owned by
Clear Channel. The technology was known as
DirectBand.
FMeXtra on FM uses dozens of small
COFDM
In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital commun ...
subcarriers to transmit digital radio in a fully
in-band on-channel
In-band on-channel (IBOC) is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency. The name refers to the new digital signals being broadcast in the same AM or FM band (in-band) ...
manner. Removing other analog subcarriers (such as stereo) increases either the audio quality or channels available, the latter making it possible to send non-audio
metadata
Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including:
* Descriptive metadata – the descriptive ...
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
telemetry
Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", an ...
back from a remote
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 ...
, 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 communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
transmitter/studio link.
On wireless
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 ...
s (STLs), not only are the broadcast station's subcarriers transmitted, but other
remote control
In electronics, a remote control (also known as a remote or clicker) is an electronic device used to operate another device from a distance, usually wirelessly. In consumer electronics, a remote control can be used to operate devices such ...
commands as well.
Interruptible foldback, such as for
remote broadcasting, is also possible over subcarriers, though its role is limited.
MCPC satellites
Analog satellite television
Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna com ...
and terrestrial analog
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
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
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 advanc ...
(usually
DVB-S
Digital Video Broadcasting – Satellite (DVB-S) is the original DVB standard for Satellite Television and dates from 1995, in its first release, while development lasted from 1993 to 1997. The first commercial applications was by Star TV in Asia ...
,
DVB-S2
Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) 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 Digital Video Broadcasting Projec ...
or another
MPEG-2
MPEG-2 (a.k.a. H.222/H.262 as was defined by the ITU) 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, ...
-based system), where audio and video data are packaged together (
multiplex
Multiplex may refer to:
* Multiplex (automobile), a former American car make
* Multiplex (comics), a DC comic book supervillain
* Multiplex (company), a global contracting and development company
* Multiplex (assay), a biological assay which measu ...
ed) in a single
MPEG transport stream
MPEG transport stream (MPEG-TS, MTS) or simply transport stream (TS) is a standard digital container format for transmission and storage of audio, video, and Program and System Information Protocol (PSIP) data. It is used in broadcast syste ...
.
See also
*
Subsidiary Communications Authority (SCA)
*
xRDS
Background
The XML format used by XRDS was originally developed in 2004 by the OASIS XRI ( extensible resource identifierTechnical Committeeas the resolution format for XRIs. The acronym XRDS was coined during subsequent discussions between XRI ...
References
External links
FM Receiver with subcarrier decoding circuit United States Patent 4476581
United States Patent 3974520
{{Audio broadcasting
Broadcast engineering