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Asubcarrier is asideband 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.
Stereo broadcasting is made possible by using a subcarrier onFM radiostations, 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 carrierAM, 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 (whichranges 50 Hz ~ 15 kHz). A 19 kHzpilot 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 receiverdemodulates 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 alocal oscillator, the 19 kHz pilot tone provides an in-phasereference signal used to reconstruct the missingcarrier wave from 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.
Likewise, analogTV signals are transmitted with theblack and whiteluminance part as the main signal, and the colorchrominance 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 thehuman eye sees much more detail incontrast 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 theremainder. (See:YIQ,YCbCr,YPbPr) Variousbroadcast television systems use different subcarrier frequencies, in addition to differences inencoding.
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 forsecond audio programs (such asdescriptive 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 atechnician orbroadcast engineer at a remotetransmitter site to talk back to the studio), or any other use aTV station might see fit. (See alsoNICAM,A2 Stereo.)
In RF-transmittedcomposite video, subcarriers remain in the baseband signal after main carrierdemodulation 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, whileS-Video places the chrominance and luminance signals on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picturesharpness and brightness).
Beforesatellite,Muzak and similar services were transmitted todepartment stores on 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 parlors inNew York state to gethorse racing results from the state gaming commission via the same technology.[citation needed]
Manynon-commercial educational FM stations in the US (especiallypublic radio stations affiliated withNPR) broadcast aradio 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 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.
TheRDS/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 broadcastEAS messages, and has a station "format" name ALERT to automatically trigger radios to tune in for emergency info, even if aCD is playing. While it never really caught on inNorth America,European stations frequently rely on this system.[1] An upgraded version is built intodigital radio.
xRDS 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 and other information toGPS navigation devices,wristwatches, and other devices. Many of the subcarriers were from stations owned byClear Channel. The technology was known asDirectBand.
FMeXtra on FM uses dozens of smallCOFDM subcarriers to transmit digital radio in a fullyin-band on-channel manner. Removing other analog subcarriers (such as stereo) increases either the audio quality or channels available, the latter making it possible to send non-audiometadata along with it, such as album covers, song lyrics, artist info, concert data, and more.
Many stations use subcarriers for internal purposes, such as gettingtelemetry back 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 is wrong, as long as the station is on the air and they are within range. This is the essence of awirelesstransmitter/studio link.
On wirelessstudio/transmitter links (STLs), not only are the broadcast station's subcarriers transmitted, but otherremote controlcommands as well.
Interruptible foldback, such as forremote broadcasting, is also possible over subcarriers, though its role is limited.
Analogsatellite television and terrestrial analogmicrowaverelay communications rely on subcarriers transmitted with the video carrier on a satellitetransponder 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 asmultiple channel per carrier (MCPC).
This is now mostly superseded bydigital TV (usuallyDVB-S,DVB-S2 or anotherMPEG-2-based system), where audio and video data are packaged together (multiplexed) in a singleMPEG transport stream.
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