关键词: 噪声消除, 信号音检测, 回铃音, 忙音, 空号音, 语音激活检测, 频谱图, 周期图,

Abstract: While the NC（Noise Cancellation） system is applied to the downlink of 3G mobile telephone，the signal tone distortion is found at the same time of canceling the noise.This is because the VAD algorithm of the NC system is sensitive to dramatic changes in the power of signal tone.In view of the time and frequency domain characteristics and the error factor of the network transfers，a new algorithm of the signal tone detection fit for the downlink NC system is proposed，which uses the frequency domain power and the time domain period change to detect signal tone.The academic evaluations and practice tests prove that the algorithm can more precisely detect signal tone，without degrading the performance of the NC system，so as to avoid the occurrence of distortion.

Key words: noise cancellation, signal tone detection, feedback ringing, busyness ringing, empty number ringing, voice activity detection, frequency spectrogram, frequency spectrogram