| 摘要 |
PURPOSE:To attain participation in a conference telphone with a natural talking level for many conference participants by inputting a digital signal where its frequency spectrum is uniformed at A/D and D/A conversion to the input of the 1st and 2nd echo cancellers. CONSTITUTION:An emphasis circuit 2 having a frequency characteristic (b) making each voice spectrum flat and a de-emphasis circuit 3 having the inverse characteristic (c) to the (b) are inserted respectively to a 4-wire side transmission and reception line of a 2-wire/4-wire converter 1, the output of the emphasis circuit 2 is converted into a digital signal by an A/D converter 4 to be used as a transmission input to the 1st echo canceller 5. The transmission output of the echo canceller 5 is fed as the reception input to the 2nd echo canceller 6, and also converted again into an analog signal by a D/A converter 7 and subject to electroacoustic transducing by a speaker 9 through the de-emphasis circuit 8. A reception sound signal entering a 2-wire input is made flat for the spectrum at the reception input point of the 2nd echo canceller and also a crosstalk signal enters the transmission input of the said 2nd echo canceller with the averaged spectrum, then howling is suppressed sufficiently. |
