Review: Capella Systems Cambria FTC Offers Per-Title Encoding
Netflix announced the beginning of the end for fixed encoding ladders with a December 2015 blog post called, “Per-Title Encode Optimization”. The big question was, what would replace them? For companies with the development budgets of Netflix and YouTube, which announced its own neural networkbased pertitle optimisation approach soon thereafter, DIY is the way to go. But what about smaller companies that need the efficiencies and quality-ofexperience benefits that pertitle encoding can provide, but that don’t have the budget for DIY Enter Capella Systems Cambria FTC, the first enterprise encoding package we know of with a scriptable pertitle optimization feature. In the copycat encoding market, the Capella launch likely presages the introduction of similar features from competitive vendors, but at the time of this writing, Cambria stands alone.
In this review, I provide a brief overview of the Cambria encoder and then dive into how the pertitle encoding works, how we tested this function, and how Cambria performed in those tests.
Capella is a privately held encoding company boasting several employees who worked at Rhozet, which developed the highly regarded Carbon Coder product that was acquired by Harmonic in 2007. Cambria FTC is a VOD file converter that runs on multiple flavors of Windows, including Windows 10. The base price is $8,700, to which you’ll have to add $5,950 for HEVC encoding and $1,500 for adaptive bitrate (ABR) output. The overall feature set is competitive, with the typical vast array of input formats, output formats, watch folder operation, and a REST API.
One relatively unique capability is support for scriptable workflows, which is how Cambria performs pertitle optimization. Via scripting, Cambria can also perform workflowlike operations such as changing the preset depending upon source properties or setting in and out points automatically. You create the scripts in Perl, and Capella supplies several example scripts, including one for pertitle encoding.
Cambria operates using two separate programs: Cambria File Convert (where you create presets, add input files, and start manual encoding projects) and Cambria Manager (where you configure job management and watch folder settings, create and monitor watch folders, and monitor encoding jobs). Like most encoders, Cambria works from presets, and you can create presets for singlefile and ABR outputs, including HLS, MPEGDASH, and Smooth Streaming.
Figure 1 shows the File Convert application, with tabs for Source (where you select the files to compress), Encoding (where you create and choose presets), and Conversion (where you can watch the encoding process, though I sent most jobs to the Cambria Manager for processing).
Figure 2 shows the Encoding tab. The Preset Editor is open with a preset for an MPEGDASH ABR group with seven video streams. The Video Stream Configuration for the 1080p stream is open, showing the configuration options for that stream. One frustration is that you can’t access x264 presets such as slow, medium, and fast from the Edit screen, though you can manually input most of the parameters necessary to achieve the same quality/performance balance. Note the Script tab in the upper section of the Preset Editor; that’s where we’ll enter the pertitle script.
You can select multiple presets in the Encoding tab, which will be applied to all files loaded in the Sources tab. When you’re ready to encode, click either Queue All Jobs to send the jobs to the Cambria Manager or Convert All Jobs to convert in the File Convert tab. Again, I sent all my encoding jobs to the Cambria Manager, which has a loglike function that makes it simpler to track multiple jobs, though both encoding workspaces deliver the same encoding performance.
That’s the overview. Now let’s look at the pertitle optimization feature.
The official name for Cambria’s feature is “source adaptive bitrate ladder,” or SABL. The starting point for every encode is the encoding ladder shown on the left in Figure 2—1080p at 4300Kbps, 720p at 2500Kbps, and so on. When enabled via a script, like that shown in Figure 3, Cambria runs a fast constant rate factor (CRF) encode of the file to gauge encoding complexity. Briefly, CRF is an encoding technique available with x264 and several other codecs that lets you select the desired quality level rather than a data rate. While encoding with CRF, x264 produces a file with the selected CRF quality level, adjusting the data rate as necessary to deliver that quality.
In this fashion, the data rate produced during the CRF encode is a measure of encoding complexity. For example, Table 1 shows the files used to test SABL, and the results of the complexity measurements from the 1080p CRF encode, where all figures show the kilobitspersecond output. The 30 Sec Peak value shows the highest data rate for any 30second chunk of the movie, while the 10 Sec Peak shows the highest data rate for any 10second chunk. The Average Complexity shows the average rate for the entire movie.
To reflect for a moment, Table 1 shows exactly why a fixed bitrate ladder is so suboptimal. Consider the 4300Kbps target data rate for the 1080p stream shown in Figure 2. Applied to the Zoolander movie, it would be too low, resulting in a poor quality file. Applied to almost all other files in the test, it would be too high, especially for synthetic files such as Camtasia or PowerPoint-based files. These files would be encoded at too high a data rate, wasting bandwidth and limiting their reach on slower bandwidth connections.
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