| 摘要 |
PURPOSE:To satisfy the echo preventing amount of approximately 10dB by lowering the volume gain of signals from a microphone even when an echo preventing function is available only from an echo canceller and is insufficient. CONSTITUTION:When the voice of an opposite party is received from a line, reception signals transmitted from the line are converted to four wires by a hybrid circuit 8 and converted into digital by and A/D conversion circuit 4. Then, they are amplified by a receiving speech amplifier circuit 9 and the voice is emitted by a speaker 10. The voice is fed back from the microphone 1 and is turned to echoes. The signals including the echoes passes through a transmitting speech amplifier circuit 2 and the A/D conversion circuit 3 and pseudo echoes generated in the echo canceller 12 are subtracted by a subtractor. The signals are detected in a level detection circuit 15, a digital control circuit 17 controls a volume gain adjustment circuit 16 so as to lower the gain only when the level is equal to or less than a prescribed value, the voice of the opposite party emitted from the speaker 10 is fed back to the microphone 1 and the voice to return to the opposite party is made small. |
