| 摘要 |
A decoder for receiving and decoding a coded signal includes a particular frequency fo with a first and second amplitude. The first amplitude of the frequency fo is associated with the end of a first coded message while the second amplitude of the frequency fo is associated with the beginning of a second coded message. The decoder includes a receiver for receiving the coded signal, a tone detector circuit for detecting the frequency fo, a level sensor responsive to the receiver output, a function block for performing a command and a control unit responsive to the tone detector circuit and the sensor means. The tone detector and control unit operate in one of at least two possible states. In the first state the control unit causes the tone detector to respond to the first amplitude of the frequency fo. When the level sensor senses the second amplitude of the frequency fo the control unit responds by placing the tone detector circuit into its second state. In its second state, the tone detector circuit detects whether the level sensor was tripped by the frequency fo or by some other frequency. If the tone detector circuit detects the frequency fo, the control unit causes the decoder to end its decoding of the first message and begin the decoding of a second message. If the tone detector circuit in its second state fails to detect the frequency fo, the control unit returns the tone detector circuit to its first state so it may continue to decode the first amplitude of the frequency fo associated with the first coded message. |
