| 摘要 |
The digital tone decoder system includes a phase locked loop which receives an input tone signal, which may be accompanied by noise, and produces a replica of the input signal from which the noise has been filtered. The tone signal operates a gate to pass clock pulses from a reference oscillator into a counter for precisely N cycles of the tone signal. The counter provides digital period information of the tone signal which is compared with period (frequency) information of the tone or tone sequence to which the detector system is to respond provided by an address code element. A logic control circuit is activated when lock is acquired by the loop, and this control circuit initiates operation of the other parts of the system and produces the detector output. The clock pulse oscillator also provides system timing and alert tone signals. The detector is capable of decoding and identifying any N-tone signal sequence or group of sequences by use of the required address code element or elements, with the logic control circuit providing the appropriate timing.
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