Abstract: Automatic switching circuitry for processing input signals of more than one frequency, incorporating information for the derivation of one or more audio channels. The circuitry, including a selectively variable bandpass device, may be one of several values corresponding to the frequency of a input signal. A demultiplexing device is provided to apply a demultiplexing operation on signals output from the bandpass device, with a control logic monitoring the demultiplexing device to determine if a demultiplexing operation has been achieved, and instructing the selection of another value for the bandpass device if no demultiplexing operation was achieved for a given value of the bandpass device.

Abstract: In a circuit for indicating reception of stereo signals (for use in e.g. a radio tuner or television) audio signals prevailing in the left (L) and right (R) output channels are applied to respective limiting circuit and the output signals derived are fed to OR gate. When a monoaural signal is detected, the signals in the two output channels will be in coincidence and so the output from the exclusive OR gate remains low. When stereo signals are detected, however, a finite phase difference exists between the two signals and the output for the OR gate is correspondingly high. A Schmitt trigger drives an LED display whenever the output from the OR gate exceeds a preset threshold level to provide an indication that a stereo signal is being reproduced. A capacitor/resistor circuit imposes a time delay on level changes at gate so that the LED display remains illuminated, even during momentary interruptions of the audio signal.