What Is H.264?

这是我们正在进行的“What Is...?" articles, 旨在提供定义, history, 以及围绕在线视频行业的重要术语和问题的背景.

Executive Summary

H.264是最广泛使用的 codec on the planet, with significant penetration in optical disc, broadcast, and streaming video markets. However, many uses of H.264 are subject to royalties, something that should be considered prior to its adaption. Other factors to consider include comparative quality against other available technologies, like Google’s WebM, as well as the general availability of decoding capabilities on target platforms and devices. 本文讨论H。.264和竞争技术从这些角度.

The H.264 Spec

H.264 is a video compression technology, or codec, that was jointly developed by the 国际电信联盟 (as H.264) and International Organization for Standardization/International Electrotechnical Commission Moving Picture Experts Group (作为MPEG-4第10部分，高级视频编码，或AVC). Thus, the terms H.264和AVC的意思是一样的，可以互换.

As a video codec, H.264可以合并到多个容器格式中, 并且通常以MPEG-4容器格式生成, which uses the .MP4扩展，以及QuickTime (.MOV), Flash (.F4V)、移动电话3GP (.3GP)、MPEG传输流(.ts). 大多数时候，但不是所有时候，H.264 video is encoded with audio compressed with the AAC (Advanced Audio Coding) codec, 这是一个ISO/IEC标准(MPEG4 Part 3).

The Nuts and Bolts

作为一个基于标准的编解码器，H.264已经被多个厂商实现, 每个版本都提供不同级别的质量和可配置性. The most widely used H.264编解码器现在包括苹果编解码器, 在Apple Compressor和QuickTime Pro中使用的是什么, the MainConcept codec, 已获得Adobe许可的, Microsoft, Rhozet, Sorenson Media和Telestream的编码产品, and the x264 codec, 在大多数共享软件编码器中使用的免费软件库, 并由许多UGC供应商创建定制的大批量H.264 encoding systems.

Of the three, the 苹果编解码器产生的质量最低 by a significant margin. Otherwise, x264在质量上比MainConcept略胜一筹, though the 差异可能不明显大多数流媒体生产者使用的编码参数.

H.264 Profiles and Levels

At a high level, there is some uniformity in the compression parameters available for each codec. For example, all H.264编解码器使用不同的配置文件来编码视频. To explain, there are multiple encoding techniques and algorithms available in H.264 to compress the file. The basic tradeoff for most of them is enhanced quality but a more complex bitstream that’s harder to decode.

Profiles specify which of those techniques and algorithms can be used to create a bitstream, and constitute a convenient meeting point for device manufacturers and video producers. For example, the Baseline profile, 低功耗设备主要使用的最简单的轮廓是哪一种, 不允许使用b帧或CABAC熵编码. Quality is still good, but not as high as streams produced using the Main or High Profiles that incorporate both techniques.

However, 因为这些技术并没有被使用, 基线编码流的解码要求是适度的, 因此，像最初的具有视频功能的iPod这样的低功耗设备可以播放视频. iPod文档明确规定这些设备只能播放H.264 video encoded to the Baseline profile, so video producers seeking to produce content for playback on the iPod encoded using the Baseline profile.

H.264 levels specify the maximum data rate and video resolution that a device can play back. For example, Apple’s iPad 2 specifications indicate that the device can play video encoded using the Main Profile, Level 3.1. This means a maximum video resolution of 1280x720 @ 30 frames per second at a maximum data rate of 14Mbps. 维基百科有一个详细的图表 specifics of each H.264 level.

In general, 在为设备生产时，配置文件和级别是最重要的, 因为视频编码使用了错误的配置文件, 或超出级别中指定的参数, 不会在这些设备上玩. 相比之下，当为电脑制作时，玩家启用H.264 playback, whether QuickTime, Flash, Silverlight or HTML5, can play back video produced at the most advanced Profile supported by most streaming encoding tools (the High Profile), 在超过全分辨率1080p及更高的配置下.

当生产一般的电脑播放, it’s more important to consider the practical limitations regarding delivery to the target viewer, 以及目标计算机的回放能力. Though the Flash Player can technically play 1080p video encoded at 15Mbps on a low-power netbook, 很少有连接能够实时传输这样的数据速率, 运送的费用会让人望而却步, and the frame rate produced by that low power CPU would probably not be pleasing to viewers.

Other H.264 Encoding Parameters

除了概况和水平，还有很大的差距在H.264编码参数可供视频制作者编码成H.264 format. For example, Figure 1 is the H.264压缩接口从Adobe媒体编码器. 如您所见，您可以选择Profile和Level，但不能选择其他H.264 parameters.

Apple Compressor H.264 Settings

Figure 1. Adobe媒体编码器的简单H.264 encoding interface.

在复杂性光谱的另一端是 Figure 2的四个屏幕中的两个屏幕显示可用的编码选项 x264Encoder，一个基于x264的QuickTime编码器. As you can see, 除此之外，它的质量非常高, one of the reasons that x264 is popular among serious compressionists and high-volume UGC sites is because it exposes a wide-variety of H.264编码参数，支持广泛优化.

x264Encoder H.264 Settings

Figure 2. 使用x264Encoder的一些编码选项.

Overall, though H.264 is a standard, 各种编解码器的输出质量几乎不一致, 或用于将文件编码为H.264格式的流分发.

H.264 Royalty Status

A number of companies claim patent rights for intellectual property contributed to the development of H.264，所有这些都是专利池的成员 MPEG LA. Under the patent pool, different royalties apply to the different classes of products, as shown in Figure 3.

H.264 Royalties

Figure 3. The H.从AVC/H的MPEGLA摘要264版税结构.264 License Terms FAQ.

On the left are products sold or otherwise distributed with AVC encoders or decoders installed, while on the right are various content categories that include video encoded into H.264 format. Briefly, 在图的左边, 版税从前100张开始算起,每年销售1000台, and cap at $3.2005-2006年每年500万美元.2007-08年每年2500万美元，2009-10年每年500万美元，以及6美元.2011- 2015年每年500万美元.

为右侧的内容类别, 订阅服务有版税, 这与订阅者的数量成正比, 但只有在超过100后才能开始,000 subscribers. There are also charges for Title-by-Title content sold to viewers (pay-per-view), 但仅限于时长超过12分钟的内容.

继续顺时针向右，H没有版税.264编码的视频在互联网上免费传输, 虽然收费适用于H编码的免费电视.超过99,999个电视家庭的广播市场. Interestingly, the H.264’s patent group’s policy on free Internet video has varied significantly over time. Initially, 至少在1月1日之前是没有皇室的, 2011, 这一政策阻止了许多大量潜在用户采用H.264. Then, in February 2010, MPEG-LA宣布将推迟版税 until December 31, 2015. 最后在2010年8月， MPEGLA“永久”延长了免版税许可，一些专家声称这是为了回应谷歌的开源VP8编解码器 acquired from On2. 不管是什么原因，那些通过H.264将永远不必向MPEG LA支付许可费.

H.264 Support and Adoption

H.264 is one of the three codecs available for encoding content for Blu-ray discs, 在视频会议产品中普遍存在吗, 并被广泛应用于电视广播, 包括卫星和有线广播. 在流媒体市场，H.2005年，苹果公司在QuickTime 7中首次采用了264.264播放功能的iPod也于同年首次亮相. 2007年，Adobe将H.微软宣布支持H . 264到Flash.Silverlight在2008年支持264.

H.264目前被所有的new Android devices, in Windows Phone 7, in most new BlackBerry Smartphones, and in the HP webOS.

在浏览器支持方面，H.264 playback is currently incorporated into Apple’s Safari browser and Microsoft Internet Explorer version 9 via the HTML5 video tag, 以及(至少)11版的谷歌Chrome浏览器.0.696.16 beta. 然而，在2011年1月11日，谷歌宣布他们将删除H.264支持从谷歌Chrome浏览器在接下来的几个月里.” Mozilla和Opera都没有整合H.264 playback into their browers, citing the costs and restrictions of using a “patent-encumbered” format.

然而，微软已经发布了多个插件，使H.使用HTML5视频标签在Firefox和Chrome中播放264, 尽管根据微软, these H.264插件只能在Windows 7上运行.

With version 10.1、Flash Player启用gpu加速的H.在Windows和Macintosh平台上的Flash中播放264, 版本5的Safari浏览器也加速了H.264播放在两个平台. 在版本3中，Silverlight还启用了H的gpu加速.264 video on both platforms, and Internet Explorer 9 also includes GPU-acceleration.

在内容方面，YouTube和Vimeo都开始大力支持H.264在2010年初通过HTML5标签. By late 2010, multiple networks, including CBS, CNN, PBS, TNT, ABC and the BBC were encoding at least some of the videos distributed over the Internet using H.264. 几乎所有为iTunes制作的视频都是用H编码的.264 format.

H.264 Comparative Quality

在视频质量方面， H.264 is generally considered to produce higher quality than both On2’s VP6 and Microsoft’s VC1. 关于WebM，存在一些分歧. 例如，在他们尊敬的年度编解码器比较中， Moscow State University 在编码电影时发现, VP8 (the video coded in the WebM format) showed “20-30% lower quality at average.” Another early 由x264开发者Jason Garrett-Glaser对WebM的评估他的结论是，“VP8作为一个规格，应该比H.264基线配置文件和VC-1. 它甚至不能与H竞争.264主要还是高调.”

相比之下，流媒体得出的结论是:VP8和H之间的质量差异.264在大多数相关的数据速率下是没有意义的.”

Why H.264 Is Important To You

Codec selection is one of the most fundamental decisions facing streaming media producers. 才能做出正确的决定, producers should know the characteristics and costs of any technology that they choose. Today, H.264 is the only codec that can reach 98% of the installed base of computers (via Flash), is the predominant codec used in iTunes and can play on all major brands of mobile devices. 这是最高质量的编解码器, 虽然有些付费使用可能会有版税, 在互联网上分发免费视频永远不会有版税.