| 摘要 |
<p>COMBINED ENCODER-DECODER APPARATUS HAVING A SINGLE ACTIVE FILTER A combined encoder-decoder for use in two-way radio systems. The apparatus is operable in an encoder mode to provide tone oscillations during operation of the radio apparatus in the transmit mode and also in a decoder mode to provide a control signal in response to a received tone signal of a particular frequency during operation of the radio apparatus in the receive mode. The encoder-decoder includes a single active filter and a feedback circuit selectively coupled to the filter to form an oscillator for producing the tone oscillations in the encoder mode of operation. An input amplifier/limiter is enabled to apply received tone signals to the active filter during decoder operation. A control circuit responsive to operation of the radio apparatus is coupled to the feedback circuit and to the input circuit so as to selectively enable the feedback circuit during encoder operation and the input circuit during decoder operation. The control circuit causes the feedback circuit to provide increased gain upon initiation of encoder operation so that oscillations build up rapidly. The output of the active filter is applied to a detector to which two integration circuits are connected, the first being enabled during encoder operation and the second during decoder operation. The first integration circuit responds when the signal builds up to control the feedback circuit to reduce the gain to the desired level. The second integration circuit is enabled during decoder operation so that when the signal of a particular frequency is applied to the active filter a control signal is produced which is utilized to enable the audio circuit of the receiver. The control circuit also responds to the switching of the circuit between encoder and decoder modes of operation to increase the bandwidth of the active filter to prevent encoder tone overshoot during the receive-transmit transition and to prevent decoder falsing during the transmitreceive transition.</p> |
