| 摘要 |
Echo cancellation in a full duplex loudspeaking telephone is usually effected by means of an adaptive filter, which by means of a combining (difference) circuit cancels the microphone-signal component picked up by the microphone via the acoustic echo path. For a correct operation of the adaptive filter the levels between the signals at the input line and at the input of the combining circuit should be in a given ratio. For automatically adjusting this ratio an attenuator circuit (26) is arranged before the combining circuit, and at the output of the combining circuit an amplifier (22) is added whose gain is inverse to the attenuation of the attenuator circuit. A control circuit (40) compares the levels of the signals at the input line (10) and at the input of the combining circuit and adjusts the attenuation of the attenuator circuit and the gain of the amplifier accordingly. In order to enable this adjustment to be carried out over a maximal range of acoustic conditions a further attenuator circuit (36) is arranged before the adaptive filter, which further attenuator circuit is also controlled by the control circuit.
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