什么是编码和转码?

这篇文章是流媒体的一部分 “是什么” 系列.

执行概要

流媒体 production starts with the infinite real world as captured by the lenses of our camcorders, and ends with the tightly compressed files necessary for streaming delivery. 一路走来, 视频是数字化的, 编码, 编码, 经常被转码, 在翻译的过程中可能会停下来, transsizing, 和transmuxing.

These are terms we use every day, but few, if any, have precise definitions. 直到现在. Read on for our attempt to bring clarity to the lexicon of streaming media workflows and processes as we follow the life of a video from capture to consumption (on multiple platforms, 当然).

视频拍摄在摄像机

我们从摄像机拍摄的视频开始，在这个例子中是a 松下AG HMC150 (图1). 我们以108030p (1920x1080)的分辨率拍摄, 进步, 每秒30帧)，视频以24Mbps的速度以AVCHD格式存储.

松下AG HMC150

图1. 松下AG HMC150摄像机

作为这个过程的一部分, 视频是数字化的, 或从模拟转换为数字, 编码(或压缩), 或以用于存储或传输的格式存储.

视频复制到硬盘驱动器

然后我们把摄像机里的视频传输到硬盘上. 当使用模拟视频时, 这种转移被称为视频捕获, which--like the process that occurred in the Panasonic camcorder--involved both digitization and (in most instances) compression. 然而, 当使用数码摄像机时, 视频已经以压缩的数字格式存储. 相应的, the process of transferring the video from camcorder to hard drive can be a simple file copy, 或者转码, 取决于您使用的编辑器.

例如, Adobe Premiere Pro works with AVCHD files natively, or in the original format, without any conversion whatsoever. The preferred workflow for Premiere Pro would be to copy the files from camcorder to computer outside of the application, 也许是Windows上的Windows百家乐软件管理器或Mac上的文件管理器, 然后将文件导入Premiere Pro. 这不涉及任何形式的转换，也不涉及代际损失.

摄取文件到Final Cut Pro

图2. Ingesting AVCHD into Final Cut Pro 7 involves transcoding from AVCHD to ProRes.

相比之下，如图2, Final Cut Pro 7 converts all AVCHD files into ProRes format as part of the ingest process. 你可以选择普罗斯的口味, 哪个决定数据速率, 但在所有情况下, the data rate will be at least double that of the AVCHD footage (45Mbps for ProRes 422 Proxy) up to over 13 times higher (330Mbps ProRes 4444), 这意味着最小的质量退化.

这种转换将是一个转码, which is a conversion from one format to another of like or similar quality, typically performed to make the content compatible with another process or application. All conversions from one format into a different lossy format technically involve a generational loss, 但如果你使用更高比特率的ProRes格式, 这代人丧失的实际影响将是无关紧要的.

创建mezz文件

Things get hairy definition-wise when you output a mezzanine file from the editor that you can then input into your encoding program to produce files for distribution (图3). 用Premiere Pro，你可能会输出 H.264 format, perhaps at the same or even a higher data rate than the original file. 仍然, 如果您调整全局参数，如亮度或颜色, 这种重新编码将涉及一些代际损失, 然而小.

在Adobe Media Encoder中创建夹层文件

图3. 在Adobe Media Encoder中创建夹层文件. 重新编码、编码、转码或翻译?

You could call this re-encode encoding, particularly if the mezzanine file was used for archiving. 你也可以称之为转码, since the mezzanine file would be of like or similar quality to the original. 你还可以进一步支持翻译, where you’re converting a file to a different data rate using the same codec, 虽然这不是翻译的经典应用, 下面将对此进行讨论. 也许, 然而, 最简单也最准确的说法是重新编码, or the process of saving a file back to its existing format after performing some kind of editing process.

回到创建我们的夹层文件. 在Final Cut Pro中, 如果输出到ProRes作为中间格式, 不会发生转码, 虽然再一次, 如果有一个全局的变化，比如亮度或颜色, 一些技术上的代际损失将会发生, 然而小. 但是，如果输出到更低的数据速率H.264格式, 这几乎肯定是一种编码, since you’re converting to a more lossy codec for the purposes of storage or transmission. 另一方面, the intermediate file will be visually indistinguishable from the ProRes, 谁支持转码指定. 我会投转码票，但这是灰色地带.

分布编码

将夹层文件输入到类似的编码器中索伦森挤压将视频编码为MPEG-2格式，以便刻录到DVD, 一个通用的高比特率720p MP4文件，用于多种用途, 和一个WebM文件(我知道, 除了YouTube，没有人真正使用WebM, 但这一次请跟我走吧). This is clearly encoding, since you’re converting the file for transmission.

编码在索伦森挤压

图4. 将我们的夹层文件编码为最终的发行格式.

你把MPEG-2文件刻录到DVD上, upload the WebM file to your HTML5 compliant website and send one file to an OVP running Wowza Media Server 2011年末的某个时候. 这就是事情变得有趣的地方.

翻译和翻译

在2011年NAB, Wowza发布了Wowza Media Server, which debuts a new plug-in architecture for add-ins developed for the Media Server. The first two plug-ins from Wowza provide network DVR functionality and a transcoder engine, 后者与本次讨论有关.

Wowza Media Server中的翻译、转码和转换

图5. Wowza Media Server可以翻译、转码和transmux.

In operation, I would send the 720p MP4 file to the server, where the transcoder would convert the H.264 streams into multiple lower resolution/data rate streams to use for adaptive streaming. 例如, 转码器可能会将720p流转换为四个文件, 在848x480@1000配置, 640x360@700, 480x270@500和320x180@200Kbps

Technically, this is called transrating, because the transcoder is converting the H.使用相同的编解码器，264流到更低的比特率, though you could also argue for transsizing because the output resolution is changing as well. 仍然, as the umbrella term, transcoding is also accurate and is much more widely used. 如果转码器然后将这些流转换为WebM格式(我知道, 我知道)的适应性交付(我知道, 我知道), 这显然是在转码.

I should say that the Wowza transcoder will only transcode live streams in its initial release, 尽管按需转换即将到来. 永远不要让现实阻碍了好的榜样.

在第一个版本中可用的是transmuxing, 服务器将在哪里更改容器格式, 但不是底层文件. 例如, 通过HTTP 在线直播分发文件, the packets for our four files must be stored as MPEG-2 transport streams (.ts文件)，而不是MP4文件. H.264 encoding within the wrapper is fine; it’s just the container format that has to be changed.

Wowza Media Server 2 can dynamically repackage the MP4 files into MPEG-2 transport streams today, as can Microsoft’s IIS Services and Akamai’s HD Network via its “in the network” repackaging. 在2011年NAB, Adobe announced plans to do the same with a future version of the Flash Media Server.