Buyers' Guide to Per-Title Encoding 2018
While we’re here, let’s talk about the Jump column. Briefly, this measures the data rate differential between a rung and the next-higher rung. Best practice says that this number should be between 1.0 and 2.0. If streams were closer, stream switching wouldn’t deliver any noticeable quality improvement. If they were further apart, you might unnecessarily strand some viewers at lower-quality, lower-bitrate streams. When testing per-title technologies, you should verify that they maintain this spacing.
SET MINIMUM/MAXIMUM BITRATE
When you build an encoding ladder, one fundamental decision is choosing the lowest data rate, which dictates how slow a connection can be and still watch your videos. If a per-title technology decides that the data rate must increase to maintain good quality, it should also allow you to continue to provide a stream at this minimum bitrate. At the other end of the spectrum, you’ll also want to control the maximum data rate, as this stream has the most impact on storage and particularly streaming cost.
CHOOSE H.264 PROFILE
Many producers still build their encoding ladders using the baseline profile on the bottom rungs to maintain compatibility with older iOS and Android devices. If you’re one of these, check for the ability to choose the H.264 profile in all ladder rungs.
Not all producers want to implement their per-title technologies the same way. Some will chase the lowest possible data rate with acceptable video quality, while others will demand pristine quality irrespective of the bandwidth costs. All per-title technologies should allow some control over this quality vs. bitrate decision.
VERIFY ENCODING DECISIONS WITH A QUALITY METRIC
Some per-title technologies allow you to verify encoding decisions with a quality metric to avoid ugly video. It also might prevent deploying a higher data rate that produced minimal visible quality improvements.
Other controls to check for include the ability to set the minimum and maximum number of rungs, and the minimum and maximum resolutions for your videos.
Whenever you analyze new technologies, you must create a theoretical structure for doing so. The one we’ve used is to create a fixed bitrate ladder as the baseline, encode a number of files using that ladder, and then measure the data rate and VMAF quality for each rung. Then we encode using the per-title technology, measure the data rate and VMAF, and compare the two. From a storage perspective, this is easy enough—just compare the sizes of the files created using the baseline and per-title encodes. Computing the effect on viewing quality and streaming bandwidth is a bit more complicated.
For example, our baseline ladder has seven rungs, while the per-title ladder for some clips, like the screencam clip shown in Table 1, has five or fewer. How do you compare two ladders with different numbers of rungs? How do you measure the impact on streaming bandwidths and QoE?
When the per-title technology decreased the file data rate, we gauged the per-title experience by assuming that the viewer watched the per-title stream encoded at a data rate that was at or below the target bitrate in the baseline ladder. In Table 2, the number between the two ladders was the target for the baseline encodes (4,500, 2,700, etc.). Since the per-title technology produced the 1080p stream at 897Kbps, the theoretical per-title viewers could watch that stream all the way down to 900Kbps, instead of the 360p, 480p, 540p, and 720p videos in the baseline ladder. Then we compared the data rates and VMAF scores for the original and per-title clips to compute data rate savings and quality improvements, both shown on the right.
Table 2. Baseline and per-title encodes, along with the data rate savings and quality improvement
To provide context to the VMAF numbers, note that a differential of six VMAF points equals a just-noticeable difference (JND), so the per-title clip produced what would be a very noticeable difference in all rungs except the top. There, the decrease of 3.02 would be noticeable to very few viewers. In terms of streaming bandwidth, you would just compare the total data rates of the files in each ladder to estimate the savings (11,510–5,009).
Less Is More
When a technology increased the data rates, our analysis focused on whether the additional bandwidth costs were worthwhile. For example, in the top rung of Table 3, the per-title technology boosted the data rate from the 4,500Kbps target to 5,432Kbps, increasing the cost of distributing that stream by just over 20 percent. However, this raised the overall VMAF score by an irrelevant 1.02 points, basically boosting costs by 20 percent for no visible benefit. Further down the ladder, however, the additional bandwidth delivered much higher gains and were likely worthwhile.
Table 3. Was the 20 percent increase in 1080p delivery cost worth it to boost VMAF by 1.02?
Note that this technology didn’t offer the option to create a rung at the 250Kbps minimum target bitrate, which is why that box is shaded in red. As a result, a viewer with a connection bandwidth of 250Kbps couldn’t watch your video, which likely wasn’t one of the goals of implementing per-title encoding. That’s why the ability to force the 250Kbps stream is so important.
Note that in our first reviews of per-title technologies, when the bitrate increased, we didn’t apply the “what would the viewer see if they connected at the bandwidth of the baseline encode?” analysis, which certainly would have been fair. Interestingly, if you apply such an analysis, you get a completely different picture of the benefits of increasing the data rate when delivering at lower bandwidths, as you can see in Table 4. In addition to stranding the viewer who could only connect at 250Kbps, boosting the data rate forced the viewer connecting at 500Kbps to watch the 320x180 resolution stream, and so on up the ladder.
Table 4. The experiential impact of increasing the data rate of the stream
Which analysis is correct? If you’re delivering a lot of lower-rung streams to bandwidth-restricted viewers, increasing the file data rate might actually degrade the quality of the streams that they actually do watch. On the other hand, if you’re predominantly distributing the highest quality file to all viewers, you’ll achieve the results shown in Table 3. Before analyzing any per-title technology, check your log files to understand the streams that you’re actually distributing to viewers, which will provide invaluable context for assessing these technologies.
[This article appears in the Spring 2018 issue of Streaming Media European Edition.]
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