語音封包遺失之錯誤傳播長度測量與即時通訊效應研究

Measurement of Error Propagation Length from Speech Packet Loss and Studies of Its Real-time Communications Effects

池勇輝(Yong-Huei Chih)

指導教授 ： 賴文彬

元智大學/電機通訊學院/通訊工程學系/碩士(2009年)

摘要

語音封包遺失是損傷VoIP即時通訊品質的重要因素。語音封包遺失損傷中，包含「不完美的封包遺失補償(PLC)」與後續的「錯誤傳播(EP)」效應。其中，EP會影響判斷封包重要性的準確度，必須延遲發送時間，才能考量錯誤傳播效應，以提昇即時封包重要性的準確度。
本研究旨在探討VoIP封包重要性的準確度與通訊的即時性之間取捨的問題。在通話的當下，為了提昇封包重要性的準確度，可刻意延遲發送時間，藉以參考後續音框之錯誤傳播效應；然而，此延遲將受到VoIP即時性的限制，謂之兩難。本研究以離線的方式探討中低位元率語音編碼器之錯誤傳播效應，提出一個能解決「最小化額外編碼延遲」與「最大化即時封包重要性的準確度」此兩難問題之最佳化策略。對於VoIP即時通訊而言，語音封包如能正確地被分類，再透過差異性服務網路傳輸，對較重要的封包給予較多的網路資源，將可提昇VoIP之整體通話品質。

關鍵字

語音編碼 ； 錯誤傳播 ； 封包遺失補償 ； 網路電話

並列摘要

Packet loss is the most important quality degradation factor of real-time Voice over IP (VoIP) communications. This factor consists of two effects: Imperfect Packet Loss Concealment (IPLC) and IPLC associated Error Propagation (EP), where ignoring the latter will affect the accuracy of real-time VoIP packet importance. Thus, it is necessary to include the EP effect to enhance the accuracy of real-time packet importance with the cost of an extra encoding delay.
This research studies the dilemma between the accuracy of packet importance and the realtime of communications. At the moment of conversation, an intentional delay for a speech frame to account for its EP effect can help to enhance the accuracy of packet importance; however, such a delay will be limited by the real-time requirement of VoIP, thus it becomes the dilemma mentioned above. To solve this problem, we first studied the EP effects of median and low-bit rate speech codecs using an off-line method, and then proposed an optimal strategy to balance between the demands of minimizing the extra encoding delay and maximizing the accuracy of real-time packet importance. For real-time VoIP communications, the integral VoIP quality can be greatly enhanced on a differentiated service network if proper network resources are reserved for VoIP packets accurately according to their real-time packet importance.

並列關鍵字

Speech Codec ； Error Propagation ； Packet Loss Concealment ； VoIP

參考文獻

[2] K. Maheswari and M. Punithavalli, “A Survey of Packet Loss in VoIP,” International Journal of Computational Intelligence Research, vol 5, pp. 57-66, 2009.

[3] IETF RFC 3393, IP Packet Delay Variation Metric for IP Performance Metrics (IPPM), Nov. 2002.

[5] K. Clüver and P. Noll, “Reconstruction of missing speech frames using sub-band excitation,” in IEEE-SP International Symposium on Time-Frequency and Time-Scale Analysis, Jun. 1996.

[13] M. Marchese, QoS Over Heterogeneous Networks, John Wiley, June 2007.

[15] ITU-T Recommendation P.862, Perceptual evaluation of speech quality (PESQ): An objective method for end-to-end speech quality assessment of narrow-band telephone networks and speech codecs, Feb. 2001.