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STEREO_INDICATOR

2017-01-03 13:34  
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STEREO_INDICATOR
On most FM tuners, the stereo indicator lights upon detection of the 19-kHz pilot tone. However, this doesn't mean that the program is actually stereophonic, since the pilot tone is often transmitted with mono programs also. A similar situation exists on stereo amplifiers, where the stereo LED is simply con-trolled from the mono/stereo switch.The LED-based stereo indicator described here lights only when a true stereo signal is fed to the inputs. Differential amplifier A1 raises the difference between the L and R input signals. When these are equal, the output of A1 remains at the same potential as the output of A2, which forms a virtual ground rail at half the supply voltage. When A1 detects a difference between the L and R input signals, it supplies a positive or negative voltage with respect to the virtual ground rail, and so causes C3 to be charged via D1 or C4 via D2. Comparator A3/A4 switches on the LED driver via OR circuit D3/D4. The input signal level should not be less than 100 mV to compensate for the drop across Dl or D2. The sensitivity of the stereo indicator is adjustable with P1.