评估云转码选项的4个关键

Many high-quality live productions involve cloud transcoding, since sending a single stream into the cloud for transcoding is often the most bandwidth- and cost-efficient approach. I've been benchmarking hardware and software transcoders for the last 12 months or so; this column shares the key differentiating points among trans­cod­ers as I see them. 

密度/成本

用于大容量操作, the first comparison factor to nail down is the cost per encoding ladder per hour. 来计算这个, you create a representative encoding ladder for all transcoders, making sure to align parameters like bitrate control method (always constant bitrate), 关键帧间隔, 封闭的共和党, 预设, 以及尽可能接近的侧写. This is often complicated by conflicting recommendations from different vendors (like one recommending two B-frames and another recommending three B-frames) that you have to reconcile before starting testing. 

一旦设置了命令, I typically encode on an AWS or other cloud instance running Ubuntu with a version of FFmpeg modified for the transcoders that I'm testing or the stock version of FFmpeg for x264 and x265. 计算每小时每个编码阶梯的成本, you start to encode a single ladder with the encoder, 如果你达到了全帧速率, you open another terminal window and start another stream. 最近，在c5d上运行x264.大号的大约1美元.75 hour, I produced six full encoding ladders at 55 fps or higher using 60 fps source files. This yields a cost per ladder per hour of about $0.30. As long as you're testing on a platform with a known hourly price, this one is simple. 

质量

Not so simple is quality, which has three aspects: objective, subjective, and transient. It's easy to create the four encodes necessary to produce rate-distortion curves and BD-Rate comparisons with Video Multimethod Assessment Fusion (VMAF), SSIMWAVE, 峰值信噪比(PSNR), or other metrics; just remember to follow the tuning recommendations for each codec. For example, x264 has tuning settings for PSNR (which I also use for VMAF) and SSIM. The problem with objective metrics when comparing different trans­coding engines is that they often don't accurately reflect how human eyes will actually rate the videos. So I almost always include subjective comparisons via a service like Subjectify.我们还是GBTech.

Assessing transient quality is easy: I load different files into Moscow State University's 视频质量 Measurement Tool (VQMT) and run each metric. VQMT produces a results plot that shows the metric score over the duration of the file. 明显的质量下降, and you can click a button to actually view the frames at that location and confirm the degraded quality. If the quality drop lasts more than a couple of frames, 它可能会被一个实际的观众注意到. 

Transient quality is typically where software transcoders tend to degrade, particularly when running lower-quality 预设 like veryfast or ultrafast (for x264 and x265). So you can't just look at overall score; you have to look for transient quality issues. 

延迟

在编码器之间你不会看到巨大的延迟增量, but since a second or two of latency can increase overall quality and decrease the likelihood of transient issues, 你也要追踪这个. 虽然我使用的是云中的Ubuntu实例, I'm actually running the tests from a Windows-based computer in my office. 测量延迟, I capture the tests with a screen capture program like Camtasia, which provides both an accurate measurement and a record I can share with the client. 

数据速率的一致性

Data-rate consistency is how tightly the data rate produced by the transcoder tracks the average data rate, measured by the standard deviation or by eyebal­ling data-rate plots of the streams. 有视频点播, data-rate consistency isn't a concern because all viewers are watching at different times. 有直播, if you spike the data rate by 50% during the GOOOAAALLL celebration for hundreds of thousands of viewers, you could exceed the resources of your delivery ecosystem and crash the event. If you're distributing even a few thousand streams, this isn't important; if you're in the hundreds of thousands, 这是一个你需要考虑的比较. 

结束语:首先，H.基于264的转码在云中运行良好, HEVC requires many more resources and may not; hardware is often a better option. 第二个, 虽然这个测试听起来很简单, even after benchmarking codecs for more than 25 years, I've found it remarkably hard to get it right the first time. So build time for retesting and re-retesting into your planning.

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