Intelecommunications, apilot signal is asignal, usually a singlefrequency, transmitted over acommunications system for supervisory, control,equalization, continuity,synchronization, orreference purposes.
InFM stereo broadcasting, apilot tone of 19kHz indicates that there isstereophonic information at 38 kHz (the secondharmonic 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.
If no 19 kHz pilot tone is present, then any signals in the 23–53 kHz range are ignored by a stereo receiver. Aguard band of ±4 kHz (15–23 kHz) protects the pilot tone frominterference from thebasebandaudio signal (50 Hz–15 kHz) and from the lowersideband of thedouble sideband stereo information (23–53 kHz). The third harmonic of the pilot (57 kHz) is used forRadio Data System. The fourth harmonic (76 kHz) is used forData Radio Channel.
InAM stereo, thebandwidth is toonarrow to accommodate subcarriers, so themodulation itself is changed, and the pilot tone isinfrasonic (below the normalhearing range, instead of above it) at a frequency of 25 Hz.
In somecolor television standards, thecolor burst placed after every sync pulse on visible lines (as done inPAL andNTSC) 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 beingfrequency-modulated as compared to theQAM approach of the other systems, thus making it unnecessary.
In theNTSC television system, a pilot tone of9⁄572 MHz (15,734.27 Hz) is used to indicate the presence ofMTS stereo.
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 correcttimebase errors.
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. Mostradio relay systems use radio or continuity pilots of their own but transmit also the pilot frequencies belonging to thecarrier frequency multiplex system.
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).