How to Choose the Optimal Data Rate for Live Streaming
Let’s see what this looks like with the SSIMWave test, which uses the SSIMplus algorithm developed by the company to predict the subjective evaluation of actual viewers (Chart 2). As you can see, these tests follow the same pattern as the VQM. Specifically, the source clip continues to improve at higher data rates, while the 360p transcoded clip essentially stops improving at around the 3Mbps mark. At 3Mbps, the 1080p clip hits 92.5 percent and improves perhaps another 1.5 percent at higher capture bitrates. Of course, with all scores at 90 or above, all clips should be perceived as excellent by subjective viewers.
Talking head source and transcoded clips as measured by the SSIMWave SQM. Higher scores are better.
Taken together, the two tests indicate that data rates in excess of 3Mbps deliver very little quality benefit that a viewer will actually notice. This is partially verified by Figure 1, which shows the talking head clip split between two transcoded clips; on the left is a clip encoded from the 3Mbps source clip, on the right a clip encoded from the 6Mbps source. In this low-motion clip, it’s very tough to notice any difference between the two. In fact, taken literally, the SSIMWave tests suggest that you could drop the capture data rate down to 1Mbps and still produce a very high-quality transcoded clip with an SSIM score of around 90 for the 720p clip and 95 for the 360p clip.
Two transcoded clips, on the left from the 3Mbps source, on the right the 6Mbps source.
If you compare the 720p output from the 1Mbps and 3Mbps source files, you’ll notice a slight pulsing at key frames in the 1Mbps source clip, but otherwise, there’s very little to distinguish the two. The quality difference in the source clips likely doesn’t pass through proportionately to the transcoded clips because the overall degradation caused by the second encoding minimizes even moderate quality differences in the source clips.
This is not the first time I’ve seen this effect. Once I tried to ascertain the optimal data rate for uploading mezzanine files for transcoding by a movie service. Starting with a ProRes mezzanine file, I produced 1080p H.264 output at data rates ranging from 10Mbps to 50Mbps to represent the potential alternatives. Comparing the 10Mbps and 50Mbps files using the Moscow University tool produced about a 20 percent quality differential. Then I encoded both clips using the most demanding preset in the client’s adaptive group; when I analyzed the output, I found that the quality difference had shrunk to 3 percent.
In this analysis, the bottom line is that for a low-motion talking head clip with minimal detail in the foreground or background, source clip data rates in excess of 3Mbps are hard to justify, and you likely could produce very good quality at a substantially lower rate. Let’s see how all this changes with the concert clips.
Chart 3 shows the VQMT scores for the concert clips, both source and transcoded. Again, the source clip shows improving quality at the higher data rates, though surprisingly the benefits slow as compared to the talking head clip. The 360p clip levels out almost completely at around 3Mbps, while the 1080p clip reaches diminishing returns at the same data rate.
Concert source and transcoded clips as measured by the VQMT. Lower scores are better.
Not surprisingly, the SSIMWave SQM results (Chart 4) show a similar pattern. While the quality of the source clips continue to improve nicely at the higher data rates, the 360p clip stalls out almost completely at 4Mbps, at which point quality improvements in the 1080p clip also start to slow.
Concert source and transcoded clips as measured by the SSIMWave SQM. Higher scores are better.
Figure 2 shows the transcoded clip from the 3Mbps source on the left in the clip from the 6Mbps source on the right. As the charts suggest, there is very little difference between the two. Even when perusing the frames at high magnification, it is difficult to see major differences between the encoded clips.
Two transcoded clips: on the left from the 3Mbps source, on the right the 6Mbps source
All live events are different and your results will vary by content, and by the encoding presets used for your live encoder at the source and during transcoding. Undoubtedly, higher data rates are always better. However, outbound bandwidth can be quite expensive, particularly at off-site facilities, and blowing the budget on more bandwidth might not deliver noticeably better quality.
These results tend to indicate that at 720p, you should be able to produce quite good results at 2–3Mbps for talking head videos, with 3–4Mbps sufficient for concert or similar events, though these rules go out the window for really high-motion sporting events. Even for those events, however, once you reach the tipping point, upping the data rate of your source clip will deliver increasingly diminishing returns.
This article appears in the 2016 Streaming Media Industry Sourcebook.
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