語音變換(speech transformation)包含可以改變語音的音調以及說話的速度，語音的資料可利用語音變換技術，讓平凡單調的聲音變得豐富有趣，也可以做為保密或隱私之應用，使得語音處理的應用變得更多元化。 最理想的語音變換方法是使用語音分析合成(analysis-synthesis)的方式，獲得語音基本的重要特徵，藉由這些特徵可以很彈性的合成出高品質的語音；然而，當基頻週期或音框長度被改變時，以一個固定音框(frame)來進行語音分析合成，會造成基頻週期不同步的嚴重問題。為此，本論文提出利用線性預測編碼(linear prediction coding)的分析合成技術結合音框基頻週期同步(pitch synchronization)的機制，來解決此問題；針對有聲音框較穩定的特性，依LPC殘餘訊號(residual)及口腔頻譜(formant spectral)係數的特徵，調整基頻週期及口腔頻譜係數，或者改變音框訊號的長度，結合以交互相關法(cross-correlation)的音框基頻週期同步機制，來合成變換後語音的輸出訊號。經由模擬實驗的結果證實，語音音調特性的改變，及合成的語音品質，皆令人滿意，尤其是女聲變男聲之效果更佳。 Speech transformation that can change the tone and the speed of a speaker is useful in privacy protection and entertainment. Intuitively, the best approach for speech transformation is the analysis-synthesis method because it has the freedom for synthesizing arbitrary voice once the speech parameters are obtained. However, pitch out of synchronization is a serious problem in most frame-based speech analysis and synthesis processes when the pitch or the frame length is changed. In this paper we propose a speech transformation method based on the linear prediction coding with pitch synchronization to solve the problem. The speech transformation is only performed in voiced frames because they are relatively stable and have high energy. Cross-correlation is calculated to locate the synchronization point, i.e., the pitch mark. Arbitrary pitch scaling and time scaling can then be performed. Simulations show that the speech can be modified to different timbre and tone, or speaking speed, with high quality, especially in the case of transforming from female to male.
