近年來由於網際網路的蓬勃發展，越來越多的數位內容被廣泛使用，由於數位內容具有容易取得，容易複製，容易假造等特性，如何保障數位內容的擁有者或提供者的權利變成一個很重要的議題。數位版權管理系統用來保護數位內容的版權和商業交易的公平性，數位版權管理系統一般包括數位內容的加密，存取控制，數位複製的限制，版權證明，數位散佈的追蹤等多項機制。數位浮水印提供一個可行的技術，藉由在音樂，影片，影像等多媒體資料中嵌入一不可視的訊號，包括版權證明及數位複製的限制資訊，達到保護數位版權防止非法複製和散佈的目的。因此，數位浮水印在現今數位版權管理系統中扮演重要角色。

數位影片的浮水印技術可以嵌入在未壓縮或已壓縮的數位影片中。由於數位影片內容容量大，解碼和反量化的運算量高，所以我們考慮在已壓縮的數位影片中嵌入浮水印，透過我們所提出的方法，挑選並直接修改適當的可變長度碼，省去反離散餘弦轉換和反量化的高運算量，同時儘量減少可變長度碼修改前後的差異性，達到人眼視覺的不可視性。除此之外，我們兼顧嵌入資訊的安全性，利用私密金鑰和虛擬隨機亂數序列產生器保護嵌入資訊不被第三者所獲知。

實驗上我們以MPEG-1為例，發展出一套即時的數位影片浮水印技術，由於只需透過簡單的查表法且不需要原始影片即可析出影片內所隱藏的資訊，所以析出浮水印的運算量需求極低，因此可以播放影片同時進行浮水印的析出，不僅如此，我們所提出的方法也可以應用在MPEG-2和MPEG-4等數位影片中。

With the rapid growth of the Internet recently, more and more digital content are in widespread use. Because digital content are easily obtained, duplicated, and forged, how to protect the copyright of the digital content providers has become an important issue. Digital rights management systems are required to protect rights and business. DRM systems typically incorporate encryption, conditional access, copy control mechanisms, media identification and distribution tracing. Digital watermarking is a technology which embeds an invisible signal including owner identification and copy control information into multimedia data such as audio, video, images, for copyright protection against illegal copying and distribution. Thus, digital watermarking is an essential component of modern DRM systems.

The watermark can be either embedded in the uncompressed video sequence or the compressed video sequence. In this paper, we embed the watermark in I-pictures of the MPEG video sequence by modifying VLCs (Variable Length Codes) directly to avoid inverse DCT (Discrete Cosine Transform) and inverse quantization. The modification of VLCs is intended to minimize the perceptual degradation of video quality caused by the embedded watermark. Furthermore, our proposed watermarking scheme protects the embedded information by means of a secret key and a PRNG (Pseudo Random Number Generator) to generate a random binary sequence for watermarking.

We apply three MPEG-1 video sequences to test our proposed real-time watermarking scheme for digital video. Since the watermark can be extracted by simple table lookup without using original video sequences, the computational overhead is very small. Therefore, the decoding of the watermarked video sequence can be real time for viewing. The proposed algorithm can be easily applied to MPEG-2 and MPEG-4 videos also.

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