Subcarrier

Asubcarrieris asidebandof 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 onFM radio stations,which takes the left channel and "subtracts" the right channel from it — essentially by hooking up the right-channel wires backward (reversingpolarity) and then joining left and reversed-right. The result is modulated withsuppressed carrier AM,more correctly called sum and difference modulation or SDM, at 38kHzin the FM signal, which is joined at 2% modulation with the mono left+right audio (whichranges50 Hz ~ 15 kHz). A 19 kHzpilot toneis also added at a 9% modulation to trigger radios to decode the stereo subcarrier, making FM stereo fully compatible with mono.

Once the receiverdemodulatesthe 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 alocal oscillator,the 19 kHz pilot tone provides an in-phasereference signalused to reconstruct the missingcarrier wavefrom the 38 kHz signal.

ForAM 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.

Television

Likewise, analogTVsignals are transmitted with theblack and white luminancepart as the main signal, and the colorchrominanceas 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 thehuman eyesees much more detail incontrastthan in color. In addition, only blue and red are transmitted, with green being determined by subtracting the other two from the luminance and taking theremainder.(See:YIQ,YCbCr,YPbPr) Variousbroadcast television systemsuse different subcarrier frequencies, in addition to differences inencoding.

For the audio part,MTSuses subcarriers on the video that can also carry three audio channels, including one for stereo (same left-minus-right method as for FM), another forsecond audio programs(such asdescriptive video servicefor 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 atechnicianorbroadcast engineerat a remotetransmittersite to talk back to the studio), or any other use aTV stationmight see fit. (See alsoNICAM,A2 Stereo.)

In RF-transmittedcomposite video,subcarriers remain in the baseband signal after main carrierdemodulationto 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 videoretains the integrated subcarrier signal structure found in the transmitted baseband signal, whileS-Videoplaces the chrominance and luminance signals on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picturesharpnessand brightness).

Private audio

Beforesatellite,Muzakand similar services were transmitted todepartment storeson FM subcarriers. The fidelity of the subcarrier audio was limited compared to the primary FM radio audio channel.^{[citation needed]}The United StatesFederal Communications Commission(FCC) also allowedbetting parlorsinNew Yorkstate to gethorse racingresults from the state gaming commission via the same technology.^{[citation needed]}

Manynon-commercial educationalFM stations in the US (especiallypublic radiostations affiliated withNPR) broadcast aradio reading servicefor 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 aSubsidiary Communications Authority(SCA) service by the FCC in theUnited States,and as Subsidiary Communications Multiplex Operations (SCMO) by theCanadian Radio-television and Telecommunications Commission(CRTC) inCanada.

Datacasting

TheRDS/RBDSsubcarrier (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 broadcastEASmessages, and has a station "format"name ALERT to automatically trigger radios to tune in for emergency info, even if aCDis playing. While it never really caught on inNorth America,Europeanstations frequently rely on this system.^[1]An upgraded version is built intodigital radio.

xRDSis 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 Directused subcarriers to transmit traffic, gas prices, movie times,weatherand other information toGPSnavigation devices,wristwatches,and other devices. Many of the subcarriers were from stations owned byClear Channel.The technology was known asDirectBand.

FMeXtraon FM uses dozens of smallCOFDMsubcarriers to transmit digital radio in a fullyin-band on-channelmanner. Removing other analog subcarriers (such as stereo) increases either the audio quality or channels available, the latter making it possible to send non-audiometadataalong 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 gettingtelemetryback from a remotetransmitter,often located in a difficult-to-access area at the top of a mountain. A station's engineer can carry a decoder around with them and know anything that's wrong, as long as the station is on the air and they are within range. This is the essence of awireless transmitter/studio link.

On wirelessstudio/transmitter links(STLs), not only are the broadcast station's subcarriers transmitted, but otherremote control commandsas well.

Interruptible foldback,such as forremote broadcasting,is also possible over subcarriers, though its role is limited.

MCPC satellites

Analog satellite televisionand terrestrial analogmicrowave relaycommunications rely on subcarriers transmitted with the video carrier on a satellitetransponderor 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 asmultiple channel per carrier(MCPC).

This is now mostly superseded bydigital TV(usuallyDVB-S,DVB-S2or anotherMPEG-2-based system), where audio and video data are packaged together (multiplexed) in a singleMPEG transport stream.

References

^The radio stationHeart FMused this service to cause people's radio sets to switch over to adverts for commercial vehicles during the morning and evening rush hours

External links

FM Receiver with subcarrier decoding circuit;United States Patent 4476581
Chrominance subcarrier phase control system;United States Patent 3974520

[1] The radio stationHeart FMused this service to cause people's radio sets to switch over to adverts for commercial vehicles during the morning and evening rush hours

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