Subcarrier
From Wikipedia, the free encyclopedia
A subcarrier is 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.
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[edit] FM stereo
Stereo broadcasting is made possible by using a subcarrier on FM radio 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 AM, more correctly called sum and difference modulation or SDM, at 38kHz in the FM signal, which is joined with the mono left+right audio (which ranges 50Hz~15kHz). A 19kHz low deviation pilot tone is 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 has the L+R and L−R signals demodulated, it adds the two ((L+R) + (L−R) = 2L) to get the left channel and subtracts ((L+R) − (L−R) = 2R) to get the right. Rather than having a local oscillator, the 19KHz tone also provides an in-phase reference signal that is used to reconstruct the missing carrier wave from the 38kHz signal.
Note that higher frequencies drop off much more rapidly, which explains in large part why FM stereo gets noisy at a distance, while switching to mono is still perfectly clear and easy to listen to even in areas of "fringe" reception. This is also explained by the use of lower modulation levels for the stereophonic subchannel.
For AM broadcasting, subcarriers are impossible, with current technology, due to those stations being so narrowband. In this case, other methods are used to create AM stereo.
[edit] Television
Likewise, TV 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 only every other pixel 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 systems use different subcarrier frequencies, in addition to differences in encoding.
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 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.
Note that in composite video, subcarriers stay with the baseband signal even after demodulation of the main carrier, and are not separated out except right in the TV set. The exception is the monophonic audio, which travels on its own separate carrier or wire and is not part of the video at all. S-Video and component video retain the subcarriers but on separate wires, to avoid electromagnetic interference between them and the high frequencies (and therefore picture sharpness) of the luminance signal.
[edit] Private audio
Before satellite, Muzak was transmitted to department stores on FM subcarriers. The FCC also allowed betting parlors 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 67kHz or 92kHz), 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 USA, and as Subsidiary Communications Multiplex Operations (SCMO) by the CRTC in Canada.
[edit] Datacasting
The RDS/RBDS subcarrier (57kHz) 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, European stations rely on it quite a bit. An upgraded version is built into digital radio.
MSN Direct uses subcarriers to transmit weather and other information to wristwatches. Most 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.
[edit] Telemetry and foldback
Many stations use subcarriers for internal purposes, such as getting telemetry 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 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.
[edit] MCPC satellites
Analog 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-S2 or another MPEG-2-based system), where audio and video data are packaged together in a single transport stream.
