2008年8月26日
By 1月时特约编辑
专题文章

那么多H.264编解码器，这么少的时间

This article appears in the August/September issue of 流媒体 杂志. 点击在这里免费订阅.

If you produce Windows Media files, your encoder is working with code supplied by Microsoft. When you’re producing VP6 videos, your encoder is licensing some code from On2. 但是如果你产生H.264 video, you could be using a codec from a number of different companies, 包括苹果, Dicas, MainConcept, 索伦森, 还有其他几个. Unlike the quality of either WMV or VP6-based Flash files, which are relatively homogenous due to their common origin, H的质量.264 video varies depending upon who developed the codec and, to a lesser degree, the encoding tool.

在这篇综述中，我将分析H.264 files that use the Apple, Dicas, and MainConcept codecs. I’ll look at three usage cases: standard definition, high definition, and a screencam tutorial. And I’ll detail the encoding parameters that I used in each section below.

评估质量, I looked at three factors: still-image quality, 运动的质量, 以及播放的流畅性. 评估静止图像的质量, I captured frames from each compressed file and compared them side by side, and you’ll see a lot of those comparisons in this article. 测试运动质量, 我实时回放了文件, looking for artifacts like mosquitoes, 条带, 抖动, and other artifacts that only appear during real-time playback. 最后, 测试平滑度, I loaded each compressed file into Inlet Technology’s Semaphore to detect whether the encoder dropped any frames to meet the target data rate.

您将在每个测试中看到, I rated all three codecs in each of these categories and then picked a winner. Let’s start with a brief overview of the H.264 codec, followed by a description of the codecs and encoding tools.

H.264年概述
如果你产生H.264 files with different encoding tools, you know that the encoding parameters exposed by the various tools differ in both nature and extent. To a large degree, that’s because the H.264 standard doesn’t define a standard encoder; it defines a standard decoder, and codec vendors are free to employ any number of encoding techniques so long as the standard decoder can play the compressed file.

Even when a codec vendor decides to implement a specific encoding technique, it may decide not to expose those controls to the user, either in its own encoding tools (like Apple with Compressor) or to licensees of the codec (like 索伦森 with the Apple codec). Or the codec vendor may expose the options to its application developers but the licensee may decide not to expose them to its end users, preferring to make the encoding tool as simple as possible to use.

Even the absence of specific controls (such as search pattern or 熵编码) doesn’t mean that these techniques are not being used since the codec or application vendor can tie them to higher-level controls such as a quality slider. This is why different encoding tools have very different interfaces, which makes it very challenging to understand exactly what’s going on under the hood. 尽管H.264 analysis tools like Inlet Semaphore provide some insight into the encoding techniques used by an encoding tool, 在很大程度上, 每一个小时.264 codec is a black box of techniques—you input a file, 调整编码参数, and assess the quality of the result. Essentially, that’s what I did in this comparison.

苹果编解码器
The Apple codec is used in Apple Compressor, 草谷/Canopus ProCoder, and several versions of 索伦森 Squeeze prior to the new release, 挤压5.0. The Apple codec is very simple to use; Apple exposes very few encoding parameters in any encoding tool. 例如, w在这里 the other codecs let you select the profile (Baseline, Main, 或高), Compressor includes a check box for Frame Reordering that encodes in the Main profile and can’t currently produce files conforming to the High profile.

I produced all Apple-codec test files in both Squeeze and Compressor to identify which encoding tool produced the best results. 最终, I rated the SD and tutorial tests produced by Apple Compressor rather than Squeeze, because t在这里 was very little noticeable difference. I used the Squeeze files for the high-definition comparison because Compressor couldn’t hit the data rate target.

Dicas
The Dicas codec was developed by Dicas Digital Image Coding GmbH and is available in Telestream’s Episode Pro. Dicas提供了H的范围.264编码控制, 包括配置文件, 熵编码, number of B-frames and reference frames, 以及启用/禁用阻塞过滤器, along with the usual bitstream-related controls.

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