隨著網路的發達，網際網路除可傳送數據資料外，人們也可以使用行動通信系統透過網際網路與IP電話做連結。由於行動通信與VoIP所使用之語音編碼技術不盡相同，因此語音轉碼（speech transcoding）是網路語音系統中不可缺少的機制，此技術尚可應用在網路連線遊戲及語音聊天室等娛樂用途。
傳統上最佳的語音轉碼方法是使用完全解碼（full decoding）的方式，在過程上必需進行語音的壓縮及解壓縮處理，造成運算複雜度過高與時間延遲長的缺點。為此，本論文利用脈衝代換之快速碼簿搜尋法，提出一套部份解碼（partial decoding）方式的語音轉碼方法，利用語音訊號的特性，以碼框（frame）為單位，分析代表各語音所需的語音參數，藉由參數的轉換以達到語音轉碼的效果。該組目標音訊參數亦符合原壓縮方法之壓縮格式。可運用在AMR與G.729A語音壓縮標準上，並可有效地降低運算複雜度，就每一音框所需的時脈刻劃時間（clockticks），約為完全解碼法的7.2%，且可得到與完全解碼法接近之語音品質。

As the development of the internet technique, we not only can transmit the data but also connect 3GPP with VoIP over internet . Because of the coding schemes of 3GPP are not the same as VoIP, speech transcoding scheme is needed in the voice system over internet. Speech transcoding scheme can make the connection between users successful, and furthermore, it can be used in entertainment applications, such as audio chat rooms and online games.
Full decoding technique is an intuitive and traditional speech transcoding method, but it requires high computational complexity and long processing time. In this work, we propose a partial decoding technique with fast codebook search, which utilizes the pulse replacement method, on ACELP coding architecture. There is no need to redo all the decoding and encoding processes. Partial decoding method can be directly applied to ACELP based speech coding, such as AMR and G.729A speech standards. It achieves excellent voice quality as the full decoding method does while it only requires 7.2% computation loading on clockticks per frame.

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