適用於影像電話之緩衝器控制及實作

Abstract

CCITT H.261 訂定了影音服務中影像傳送的解碼標準，其所使用的位元率 為 p ＊ 64 kbits/s ，其中 p 的範圍是 1 到 30。在非常低的位元率， 例如 64 或 128 kbits/s 時，這有限的頻寬必須有效的利用，每張傳送 的影像才能達到可接受的品質。因此，在此情況下，適當的選擇編碼參數 及緩衝器控制是非常的重要。我們的研究，是從電信研究所提供的影像電 話原型開始；目標是希望改進該系統所產生的影像品質。畫質的提昇可分 為兩個方向：第一是加上圖框間編碼以改進編碼效益，其次是改良現有緩 衝器的控制方式。為此目地，我們曾探討在空間及時間上解析度的選擇問 題；在低位元率時，一般認為一個好的系統，至少應保持一最低限度的清 晰度，亦即，圖框率將會低。在實用的硬體限制考量下，我們提出的編碼 方式適於即時影像壓縮，並在影像電話系統上得到驗證。 CCITT H.261 describes the video decoding standard for trans- mitting the video signals of audiovisual services. At very low bit-rate, such as 64 and 128 kbits/s, the limited bandwidth must be effectively utilized to achieve an acceptable overall picture quality. Therefore, the proper selection of encoding parameters and/or the buffer control strategy becomes very critical. Our study starts from an H.261 videophone system originally designed by TL, Ministry of Communication. Our goal is to enhan- ce the compressed video quality produced by this system. There are two directions of improvement. One is adding additional ope- rations such as interframe coding to increase coding efficiency. The other is improving the quantizer/buffer control scheme of the current system. For this purpose, we investigate the trade- off between temporal and spatial resolutions. When bit-rate is extremely low, it is generally believed that a high performance codec should keep a minimum picture quality for every single picture. Hence, the frame rate should be lowered. Taking also into account of the hardware limitation, this proposed scheme is suitable for real-time video encoding, and has been proved on our experimental videophone system.