| 摘要 |
A tone detector comprising a circuit for detecting multi-frequency dialing (DTMF) signals applied to an input terminal and for generating a "DTMF present" indicating signal, and a circuit for detecting the presence of supervisory signals applied to the input terminal and for generating a "supervisory signal present" indicating signal. The dialing tones are converted to digital value signals and are preferably stored in a shift register. The digital value signals and the supervisory signals are applied in serial form to an output terminal. The "DTMF present" indicating signal is applied to a "DTMF present" terminal. Accordingly the presence of the "DTMF present" indicating signal can be monitored and thereby indicate whether the signal present on the output terminal is a digital value signal or a supervisory tone present indicating signal. The invention is thus realizable in a significantly reduced pin count integrated circuit chip for use in domestic controlling apparatus to be connected to telephone lines, such as telephone answering machines, utility meter readers, security devices, etc.
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