| 摘要 |
<p>An apparatus (10) for detecting one or more predetermined tones transmitted over a communication network, each predetermined tone having a predetermined frequency, comprises a data memory (14) for storing data including the predetermined frequency of each of the one or more predetermined tones, an input (16) for receiving a signal transmitted over the communication network, and a frequency divider (404) for dividing the received signal into at least two frequency sub bands so as to provide at least two components of the received signal in different frequency sub bands. The different frequency sub bands are selected based on the predetermined frequencies of the one or more predetermined tones. A frequency discriminator (410) is arranged to determine a frequency of each tone in the at least two components and a decision logic block (416) is arranged to provide an indication that a predetermined tone has been detected when the determined frequency of a tone in a component corresponds to the predetermined frequency of one of the one or more predetermined tones.</p> |
