2016年4月7日
By 1月时特约编辑
专题文章

如何选择直播的最佳数据速率

When the only tool you have is a hammer, every problem looks like a nail. 当你有客观的视频质量测量工具, you can use them to check all the assumptions that you’ve made in the past. One of those assumptions is that copious outbound bandwidth is necessary to achieve high-quality live streaming. 虽然这在某种程度上是正确的, 正如我为本文进行的测试所证明的那样, 你很快就会达到收益递减的水平.

演习目的

The object of the exercise was to show how output quality was affected by the data rate of the incoming video in a cloud transcoding scenario. 具体地说, Ustream等网站, YouTube生活, and Brightcove/Zencoder input a single stream and transcode in the cloud into an adaptive group of streams. 而更高的数据速率总是更好, outbound bandwidth from many sites is limited and can be expensive in an offsite conference or similar scenario. The question is this: How much difference will a typical user see between an affordable 3Mbps 720p stream and a possibly extravagant 6Mbps 720p stream?

事实证明，并非如此.

我是如何测试的

我用两个夹子进行了测试. 一个是我说话的头剪辑, the other a song from a recent concert appearance of the Loose Strings in Galax, Va. The Loose Strings clip is a single-camera concert shoot with lots of pans and zooms. 这比会说话的头夹更有挑战性, but not exactly a chase scene from Mission: Impossible when it comes to encoding complexity.

这两个片段最初都是1080p AVCHD, 我复制到磁盘上了吗, 导入Premiere Pro, 添加时间码, 输出为ProRes. I then played the clips out using a Blackmagic Design DeckLink HD Extreme 3D card via HDMI and captured with a Matrox Monarch HDX.

原因我稍后会解释, 我首先捕获了一个20Mbps的720p流, 然后是10Mbps的流, 然后我捕获的数据速率降低到1Mbps. 这些作为源测试剪辑.

在转码场景中, 你不关心输入片段的质量, you care about the quality of the clip encoded from that source clip. To measure this, I created two presets in Sorenson Squeeze, one for 640x360 at 1.2 mbps的输出, 另一个为1280x720, 2Mbps输出, both using x264 with the Fast Encoding preset that many live producers use during transcoding. Then I input all the source clips into Squeeze, and encoded with those presets.

让我们都保持清醒, I’ll call the original Monarch-encoded clips the “source” clips, 都是720p制作的, and the 360p and 720p Squeeze-encoded clips the “transcoded” clips.)

下一个, I measured the quality of the source and transcoded clips using the Moscow University 视频质量 Measurement Tool (VQMT), 以及SSIMWave视频质量体验监视器(SQM), 你可以在里面读到更多《百家乐app下载》.”

These tests raised some interesting test-related technical issues. 顺便说一下背景, 在测试编码的VOD片段的质量时, 从源剪辑开始, 对它进行编码, 并使用所选工具将输出与源进行比较. 容易,peasy.

在现场场景中, the source file isn’t as easy to identify because it’s changed slightly so many times. 例如, 在我的捕捉工作流程中, the clip was processed by the Blackmagic card to transmit via HDMI, 然后在捕获过程中被帝王蝶缩放. 第一个, I tried comparing the Monarch-encoded source clips with a 720p clip that was scaled and output in Premiere Pro, but the small scaling and other differences were so substantial that they essentially masked the true compression-related quality differences.

So, rather than comparing the source clips produced by the Monarch-to-Premiere Pro output, 我将它们与前面提到的20Mbps源剪辑进行了比较. 分析在Squeeze中产生的转码片段, I encoded the 20Mbps clip using the same 360p and 720p presets, 并将所有其他编码片段与这些输出进行比较.

I can’t say for sure that this is the best approach—and I’m open to suggestions for future 文章—but this is the procedure I used for this exercise. 现在，让我们看看结果.

结果,请

Let’s start with a brief description of each test and what it purports to measure. The Moscow University test measures the comparative quality of the clips, 分数越低越好. Scores are not tied to anticipated subjective ratings in any way.

SSIM波测试为每个片段分配一个数字分数, and those numerical scores are designed to align with anticipated subjective evaluations. 例如, all clips that score in the 80–100 range should be considered excellent quality by real-world viewers, while clips that measure between 60 and 80 should be rated as good quality by real viewers.

以此为背景，让我们深入研究结果.

的头部特写

表1 shows the talking head results as measured by the MSU VQMT tool. 简单说一句, the Moscow University VQMT offers a range of different analyses, 包括峰值信噪比(PSNR), 结构相似指数(SSIM), 视频质量度量(VQM), 这是我用这个工具进行的所有测试中使用的度量. 在VQM测试中，分数越低越好.

Talking head source and transcoded clips as measured by the VQMT. 分数越低越好.

蓝线显示源剪辑的分数, which starts high at 1Mbps and drops down quite nicely with the increased data rate, with the quality improvement continuing to persist even at the higher data rates. The red line shows the VQM values for the 360p transcoded clips, while the yellow line shows the values for the 1080p transcoded clips. 正如你在图表中看到的, while the values continue to improve with the increased data rate of the source clip, 在4Mbps左右，收益递减.

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