Editing Subcarrier (section)

==FM stereo==
Stereo broadcasting is made possible by using a subcarrier on [[FM radio]] [[radio station|station]]s, which takes the left channel and "subtracts" the right channel from it &mdash; essentially by hooking up the right-channel wires backward (reversing [[Electrical polarity|polarity]]) and then joining left and reversed-right.  The result is modulated with [[suppressed carrier]] [[amplitude modulation|AM]], more correctly called sum and difference modulation or SDM, at 38&nbsp;[[Hertz|kHz]] in the FM signal, which is joined at 2% modulation with the mono left+right audio (which [[bandpass|range]]s 50&nbsp;Hz ~ 15&nbsp;kHz). A 19&nbsp;kHz [[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 [[demodulate]]s the L+R and L&minus;R signals, it adds the two signals ([L+R] + [L&minus;R] = 2L) to get the left channel and subtracts ([L+R] &minus; [L&minus;R] = 2R) to get the right channel. Rather than having a [[local oscillator]], the 19&nbsp;kHz pilot tone provides an in-phase [[reference signal]] used to reconstruct the missing [[carrier wave]] from the 38&nbsp;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&nbsp;kHz to 10&nbsp;kHz allocated bandwidth of the AM signal may be used for audio.