A stereo indicator gives a visual indication when a stereo transmission is in progress. It is also essential for the alignment of the decoder if only a limited amount of test equipment is available. A stereo indicator circuit using a single transistor is shown in Fig. 22.6.

The stereo indicator circuit operates by detecting the 38 kHz signal generated when the 19 kHz pilot tone is passed through the frequency doubling circuit in the decoder. This 38 kHz signal is fed through a limiting resistor and blocking capacitor to a shunt rectifier. The voltage derived from this shunt rectifier is applied to the base of a transistor. The transistor remains cut off when the signal is absent and is turned on when it is present. Therefore, ...

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