A block diagram of a typical frequency stabilizing system used is shown in Figure. It uses a basic frequency, Standard, a crystal oscillator, and the carrier frequency of  the FM signal is compared with it. We know that reactance modulator works across the tank circuit of a LC oscillator, whose output is isolated by a buffer stage. The output of buffer is fed to an amplitude limiter and subsequently to the class C power amplifiers (not shown).

A small portion of the signal is taken from the limiter output and fed to mixer in which this signal is mixed with a signal from the crystal oscillator. The difference signal which is usually about one- tenth- to one-twentieth of the master oscillator frequency is amplified and applied to a phase discriminator. The output of the phase discriminator is a DC signal which is applied to the reactance modulator as a correcting voltage to counteract any drift in the average frequency of the master oscillator.

The discriminator is so designed that it reacts to slow changes in the incoming frequency, but not to the normal frequency variation due to frequency modulation. Also the discriminator provides positive 0/p voltage if the input frequency exceeds the discriminator tuned frequency and 0  negative 0/p voltage, if it is lower.

A stereo signal consists of two channels which can be labeled L and R (left and right), once channel for each speaker. The ordinary mono signal consists of the summation of the two channels, i.e. L + R, and this can be transmitted in the normal way. If a signal containing the difference between the left and right channels, L - -R, is transmitted then it is possible to reconstitute the left and right only signals. By adding the stun and difference signals i.e. (L + R) + (L – R), gives 2L, i.e. the left signal.

and subtracting the two signals, i.e.(L + R) – (L – R) gives 2R, the right signal. This can he achieved relatively simply by adding and subtracting signals electronically. It only remains to find a method of transmitting the stereo difference signal in a way that does not affect any mono receivers.