A subcarrier is a separate analog or
digital signalcarried on a main radio transmission, which carries extra information such as voiceor data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequencyand bandwidth. This is an early and simple method of multiplexing.
Stereo broadcasting is made possible by using a subcarrier on
FM radiostations, 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 AM, more correctly called sum and difference modulation or SDM, at 38 kHz in the FM signal, which is joined with the mono left+right audio (which ranges 50 Hz ~ 15 kHz). A 19 kHz low deviation pilot toneis also added at a low modulation percentage 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 signalused to reconstruct the missing carrier wavefrom the 38 kHz signal.
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.
Likewise, TV signals are transmitted with the black and white luminance part as the main signal, and the color
chrominanceas 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 ratefor color information is reduced four-to-one by using only every other pixelon every other scan line. (This is made possible by the fact that the human eyesees 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 systems use different subcarrier frequencies, in addition to differences in encoding.
For the audio part, MTS uses subcarriers on the video 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 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 a technicianor broadcast engineerat a remote transmittersite to talk back to the studio), or any other use a TV stationmight see fit. See also NICAM, A2 Stereo.
composite video, subcarriers remain in the baseband signal after main carrier demodulationto be separated in the receiver. The 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, while S-Videoplaces the chrominance and luminance subcarriers on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picture sharpnessand brightness).
Muzakwas transmitted to department stores on FM subcarriers. The United States Federal Communications Commission(FCC) also allowed betting parlors in New Yorkstate to get horse racingresults from the state gaming commission via the same technology.
non-commercial educationalFM stations in the US (especially public radiostations affiliated with NPR) broadcast a radio reading servicefor 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.
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 broadcast EAS messages, and has a station "format" name ALERT to automatically trigger radios to tune in for emergency info, even if a CDis playing. While it never really caught on in North America, European stations rely on it quite a bit. An upgraded version is built into digital radio. MSN Directuses subcarriers to transmit traffic, gas prices, movie times, weatherand other information to GPSnavigation devices, wristwatches, and other devices. Many of the subcarriers are from stations owned by Clear Channel. The technology is known as DirectBand. FMeXtraon FM uses dozens of small COFDMsubcarriers to transmit digital radio in a fully in-band on-channelmanner. Removing other analog subcarriers (such as stereo) increases the audio quality or channels available, and other non-audio metadatathat 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
telemetryback 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.
studio/transmitter links (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 televisionand terrestrial analog microwave relaycommunications rely on subcarriers transmitted with the video carrier on a satellite transponderor 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-S2or another MPEG-2-based system), where audio and video data are packaged together in a single transport stream.
Subsidiary Communications Authority(SCA)
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