H.264 / MPEG-4 Part 10 White Paper-翻译

1. Introduction

　　Broadcast（广播） television and home entertainment（娱乐） have been revolutionised（彻底改变） by the advent（出现） of digital TV andDVD-video.（--广播电视和家庭娱乐方式被彻底改变了在数字电视和DVD-Video出现之后--） These applications and many more were made possible by the standardisation of video compression technology.（--这些应用和其他的许多东西慢慢变得可能实现在这个视频压缩技术标准之后--） The next standard in the MPEG series（--MPEG系列的下一个标准--）, MPEG4 is enabling（启用） a new generation of （代名）internet-based video applications（--MPEG4为新一代的给予因特网的视频应用提供了可能--） whilst（同时 ） the ITU-T H.263 standard for video compression is now widely used in video conferencing（会议） systems.（--ITU-T H.263视频压缩标准在现在被广泛的用于视频会议系统--）

　　MPEG4 (Visua（视频）l) and H.263 are standards that are based on video compression (“video coding”) technology from circa. 1995.（--MPEG4和H.263是视频压缩（视频编码）技术的标准大约从1995年开始。--） The groups responsible for these standards（--一下组织负责这些东西的标准--）, the Motion Picture Experts Group and the Video Coding Experts Group (MPEG and VCEG) （--运动图像专家组，和视频编码专家组--）are in the final stages of developing a new standard that promises to significantly outperform MPEG4 and H.263（--致力于开发一个比MPEG4更加--，）, providing better compression of video images together with a range of features supporting high-quality, low-bitrate streaming video.（--高性能，高品质，低比特流--） The history of the new standard, “Advanced Video Coding” (AVC), goes back at least 7 years.　　

　　After finalising（敲定） the original（原始） H.263 standard for video telephony（电话） in 1995（--在1995年为了视频电话敲定H.263原始标准--）, the ITU-T Video Coding Experts Group (VCEG) started  work on two further development areas（ITU-T视频编码专家组开始在两个深入的领域发展：）: a “short-term” effort to add extra features（特点，特性） to H.263 (resulting in Version 2  of the standard) （--一个是通过短期的努力去增加H.263更多的特性（制定出来版本2的标准）--）and a “long-term” effort to develop a new standard for low bit rate visual communications.（--另一个是通过长期的努力致力于发展一个新的版本为了适应低比特流的视频压缩--） The long-term effort led to the draft “H.26L” standard,（--长期项目组努力的领导起草了H.26L标准--） offering significantly（显著） better video compression efficiency（效率） than previous ITU-T standards（--提供了明显优于ITU-U标准的压缩效率--）. In 2001, the ISO Motion Picture Experts Group (MPEG) recognised the potential benefits（潜在的好处） of H.26L（--2001年MPEG小组发现了H.264潜在的有点--） and the Joint Video Team (JVT) was formed,（JVM团队就被提出来了。） including experts from MPEG and VCEG（包含了来自这两个小组的专家）. JVT’s main task is to develop the draft H.26L “model” into a full International Standard. （JVM小组的主要任务就是将“H.26L标准发展为国际标准”）In fact, the outcome will be two identical standards（事实上他们推出了两个标准）: ISO MPEG4 Part 10 of MPEG4 and ITU-T H.264.（。。。就这两个） The “official” title of the new standard is Advanced Video Coding (AVC)（官方声称这个才是真正的新版本）; however, it is widely known by its old working title, H.26L and by its ITU document number, H.264 [1].（然而更加广为人知的是采用旧的命名方式的。。。。。）

2. H.264 CODEC

In common with earlier standards (such as MPEG1, MPEG2 and MPEG4)（--和之前版本差不多的是--）, the H.264 draft standard does not explicitly（明确的） define a CODEC (enCOder / DECoder pair). （--H.264并没有显式的定义一个编解码机器--）Rather, the standard defines the syntax of an encoded video bitstream together with the method of decoding this bitstream.（--而，这个标准只定义了将视频编码为比特流和从比特流解码的方法。--） In practice, however, a compliant encoder and decoder are likely to include the functional elements shown in Figure 2-1 and Figure 2-2. （--然而，一个合格的编解码机器需要包含图中2-1和2-2的功能元素--）Whilst the functions shown in these Figures are likely to be necessary for compliance（同时对于一个合格的编解码机器来说能过表现出这些功能是十分重要的。）, there is scope for considerable variation in the structure of the CODEC（编解码机器还是有相当大的变异空间的）. The basic functional elements（这些基本功能元素） (prediction, transform, quantization, entropy encoding)（预测，传输，量化，熵编码） are little different from previous standards （和之前的标准差不多）(MPEG1, MPEG2, MPEG4, H.261, H.263); the important changes in H.264 occur in the details of each functional element.（H.264最主要的变化实在这些模块的实现细节上面。）

The Encoder (Figure 2-1) includes two dataflow paths（编码包括两条数据流路径）, a “forward” path (left to right, shown in blue)（一条向前的，（从左到右））and a “reconstruction” path (right to left, shown in magenta).（还有一条重构路径 从右到左，品红色的路径）The dataflow path in the Decoder (Figure 2-2) is shown from right to left to illustrate the similarities between Encoder and Decoder.（解码器的数据流以从右到左的方式表示，一次来说明，编解码机器的共同点‘。）

2.1 Encoder (forward path)

2.1编码器（向前路径）

　　An input frame F n is presented for encoding.（当一个输入帧提交编码的时候） The frame is processed in units of a macroblock (corresponding to 16x16 pixels in the original image).（该帧以宏模块为单位进行处理，（对应原图16*16的像素为一个模块）） Each macroblock is encoded in intra or inter mode.（每个宏块，被编码成帧内模式，或帧间模式） In either case, a prediction macroblock P is formed based on a reconstructed frame. In Intra mode, P is formed from samples in the current frame n that have previously encoded, decoded and reconstructed (uF’ n in the Figures; note that the unfiltered samples are used to form P). In Inter mode, P is formed by motion-compensated prediction from one or more reference frame(s). In the Figures, the reference frame is shown as the previous encoded frame F’ n-1 ; however, the predicton for each macroblock may be formed from one or two past or future frames (in time order) that have already been encoded and reconstructed.
　　The prediction P is subtracted from the current macroblock to produce a residual or difference macroblock D n . This is transformed (using a block transform) and quantized to give X, a set of quantized transform coefficients. These coefficients are re-ordered and entropy encoded. The entropy-encoded coefficients, together with side information required to decode the macroblock (such as the macroblock prediction mode, quantizer step size, motion vector information describing how the macroblock was motion-compensated, etc) form the compressed bitstream. This is passed to a Network Abstraction Layer (NAL) for transmission or storage.