Tonsignalerkennungsschaltung and method for detection of tone signals

Tonsignalerkennungsschaltung for a receiver circuit for detecting at least one sound signal in each case a specific tone signal (f (a) a reference signal generator (41) for generating an analog reference signal (V (b) an analog / digital converter (11) which multiplies the analog input signal with the inverse reference signal (V and (c) a digital control circuit (20) that adjusts the fundamental frequency (f