Pilot signal

In AM stereo, the bandwidth is too narrow to accommodate subcarriers, so the modulation itself is changed, and the pilot tone is infrasonic (below the normal hearing range, instead of above it) at a frequency of 25 Hz.

However, SECAM features continuous subcarriers which don't need their phase tracked due to being frequency-modulated as compared to the QAM approach of the other systems, thus making it unnecessary.

In cable service plant infrastructures, two or more pilot frequencies are used to allow network amplifiers to automatically adjust their gain over temperature swings.

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.

Spectrum of an FM broadcast signal. The pilot tone is the orange vertical line on the right of the spectrogram.
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.