Subcarrier

A subcarrier is a sideband 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

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

reference signal used to reconstruct the missing carrier wave

from the 38 kHz signal.

For 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.

Television

Likewise, analog

encoding

.

For the audio part,

A2 Stereo

.)

In RF-transmitted composite video, subcarriers remain in the baseband signal after main carrier demodulation 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, while S-Video places the chrominance and luminance signals on separate wires to eliminate subcarrier crosstalk and enhance the signal bandwidth and strength (picture sharpness and brightness).

Private audio

Before

Muzak and similar services were transmitted to department stores on FM subcarriers. The fidelity of the subcarrier audio was limited compared to the primary FM radio audio channel. ^{[citation needed]} The United States Federal Communications Commission (FCC) also allowed betting parlors in New York state to get horse racing results from the state gaming commission via the same technology.^{[citation needed}

]

Many

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 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, and as Subsidiary Communications Multiplex Operations (SCMO) by the Canadian Radio-television and Telecommunications Commission (CRTC) in Canada

.

Datacasting

OFDMA

scheme

The

CD is playing. While it never really caught on in North America, European stations frequently rely on this system.^[1] An upgraded version is built into digital 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,

Clear Channel. The technology was known as DirectBand

.

COFDM subcarriers to transmit digital radio in a fully in-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-audio metadata

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

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.

remote broadcasting

, is also possible over subcarriers, though its role is limited.

MCPC satellites

multiple channel per carrier

(MCPC).

This is now mostly superseded by

digital TV (usually DVB-S, DVB-S2 or another MPEG-2-based system), where audio and video data are packaged together (multiplexed) in a single MPEG transport stream

.

References

^ 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

External links

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

[1] 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

[1]