基於藉語音再取樣萃取共振峰變化之聲調調整技術

摘要

語音聲調調整（pitch-scale modification）可以改變語音的音色以及聲調高低，讓原本平凡單調的聲音變得豐富有趣，也可以做為保密隱私之應用，或是結合其他語音分析/合成（analysis-synthesis）技術，讓語音處理的應用變得更多元化。

使用線性預測編碼（Linear Prediction Coding，LPC）的分析/合成方式，能夠獲得語音重要的基本組成，包括LPC係數及殘餘訊號；前者與語音訊號的頻譜包絡線（spectral envelope）有關，一般也稱之為共振峰（formant），而後者主要為語音頻譜上的基頻諧波（pitch harmonic）成分。藉由調整這些特徵，可以有效地改變合成語音的特性。然而一般由LPC極點調整共振峰的方式，容易破壞原本的共振峰結構，進而造成語音品質下降。因此本論文提出利用兩組不同比例之再取樣，分別萃取語音訊號的共振峰變化以及基頻週期改變後的殘餘訊號，經LPC合成濾波器得到調整過的語音，最後結合基頻週期同步（pitch synchronization）及音框邊界補償機制，確保合成語音的品質。經由模擬實驗的結果證實，藉由不同之再取樣比例能夠有效地控制語音的音色及聲調高低變化，而合成語音的品質亦令人滿意。

　

Pitch-Scale Modification Based on Formant Extraction from Resampled Speech

Abstract

Pitch-scale modification that can change the tone and the prosody of speech is useful in privacy protection and entertainment. One of the approaches for pitch-scale modification is the analysis-synthesis method. It has the freedom for synthesizing arbitrary voice once the speech parameters such as LPC coefficients and residual signal are obtained.

In this paper we propose a pitch-scale modification method based on formant extraction from resampled speech. The formant, which is the spectrum envelope of speech signal, can be extracted by LPC analysis, and this procedure, so-called de-formant, eliminates the short-term correlation incurred by vocal tract filter. The frequency response of LPC synthesis filter determines the timbre of synthesized speech. The residual signal mainly consists of long-term components, the pitch harmonic, which determines the tone of speech and can be easily modified by using the resampling technique. A dual-resampling mechanism is used to obtain the modified formant and modified pitch harmonic, respectively. The pitch-scale modification mentioned above is only performed in voiced frames because they have high energy and are relatively stable.  And the cross-correlation coefficients are calculated to locate the synchronization point, i.e., the pitch mark. Experimental results show that the speech can be successfully modified to different timbre and tone with high quality.