Editing Pilot signal (section)

== Uses in different communication systems ==
=== FM Radio ===
[[Image:Frequency Spectrum FM-Radio.gif|thumb|right|Spectrum of an FM broadcast signal. The pilot tone is the orange vertical line on the right of the spectrogram.]] In [[FM stereo]] broadcasting, a '''pilot tone''' of 19 [[kHz]] indicates that there is [[stereophonic]] information at 38 kHz (the second [[harmonic]] of the pilot tone). The receiver doubles the frequency of the pilot tone and uses it as a frequency and phase reference to demodulate the stereo information.

[[Image:PilotSignal.png|thumb|right|Radio Spectrum of an FM Radio Broadcast channel as decoded by SDRConsole application. Shows the Pilot Signal at 19kHz, Mono, Stereo and RDS spectrum blocks.]]

If no 19 kHz pilot tone is present, then any signals in the 23–53 kHz range are ignored by a stereo receiver. A [[guard band]] of ±4 kHz (15–23 kHz) protects the pilot tone from [[Adjacent-channel interference|interference]] from the [[baseband]] [[Audio frequency|audio]] signal (50 Hz–15 kHz) and from the lower [[sideband]] of the [[double sideband]] stereo information (23–53 kHz). The third harmonic of the pilot (57 kHz) is used for [[Radio Data System]]. The fourth harmonic (76 kHz) is used for [[Data Radio Channel]].

=== AM Radio ===
In [[AM stereo]], the [[Bandwidth (signal processing)|bandwidth]] is too [[narrowband|narrow]] to accommodate subcarriers, so the [[modulation]] itself is changed, and the pilot tone is [[Infrasound|infrasonic]]<!-- Was "subsonic", but that means slower than the speed of sound --> (below the normal [[hearing (sense)|hearing]] range, instead of above it) at a frequency of 25 Hz.

=== Television ===
In some [[color television]] standards, the [[color burst]] placed after every sync pulse on visible lines (as done in [[PAL]] and [[NTSC]]) is the pilot signal to indicate that there are color subcarriers present and allow synchronizing the phase of the local oscillator in the demodulation circuitry. However, [[SECAM]] features continuous subcarriers which don't need their phase tracked due to being [[Frequency modulation|frequency-modulated]] as compared to the [[Quadrature amplitude modulation|QAM]] approach of the other systems, thus making it unnecessary.

In the [[NTSC]] television system, a pilot tone of {{frac|9|572}} MHz (15,734.27 Hz) is used to indicate the presence of [[Multichannel television sound|MTS]] stereo. 

=== Video Recording ===
In some analog video formats frequency modulation is the standard method for recording the luminance part of the signal, and is used to record a composite video signal in direct color systems, e.g. [[Video 2000]] and some Hi-band formats a pilot tone is added to the signal to detect and correct [[timebase correction|timebase errors]].

=== Cable ===
In cable service plant infrastructures, two or more pilot frequencies are used to allow network amplifiers to automatically adjust their gain over temperature swings. This is done by the amplifiers having special circuitry that track the frequencies in order to maintain a consistent gain. Without this capability, network amplifiers may drive the signal too strongly or weakly, thus requiring constant adjustment. Pilot frequencies can be generated by an agile modulator, taking the space of analog NTSC channels, or by dedicated equipment.  Sometimes it is necessary to employ several independent pilot frequencies. Most [[radio relay]] systems use radio or continuity pilots of their own but transmit also the pilot frequencies belonging to the [[Carrier wave|carrier frequency]] multiplex system.