CIRCUIT FOR FREQUENCY-SELECTIVE DETECTION OF AUDIO SIGNALS

摘要

A circuit for detecting at least one audio signal in a mixed signal has a number of bandpasses equal to the number of possible signal frequencies. The mean frequencies of the bandpasses each correspond to one of the signal frequencies. Each bandpass emits a bandpass signal corresponding to its filter characteristic. To detect audio signals with single bandpasses, with minimal interference and short response time, the level of each bandpass signal is compared with the levels of the output signals of two comparison bandpasses which also receive the mixed signal and whose mean frequencies are higher and lower respectively than the mean frequencies of the corresponding bandpass. Depending on the comparison, a detection signal is produced which indicates the arrival of an audio signal at the bandpass input if the comparison level is less than the level of the corresponding bandpass signal.