What Does Netflix Know About Prime Time Video Streaming, Anyway?
So, there I was, making a presentation to a Scandinavian consulting client regarding the potential benefits of deploying HEVC or VP9 streams to compatible clients instead of H.264. It’s an issue I’ve gone round and round about in my mind because of one data point: Netflix’s ISP Speed Index, which hovers around 4Mbps for the U.S. and many European countries.
According to Netflix, “The Netflix ISP Speed Index is a measure of prime time Netflix performance on particular ISPs (internet service providers) around the globe.” In other words, it’s about how much data Netflix can push through a particular ISP during prime time.
So, my thinking went, if Netflix can only push through 4Mbps, then that’s all this particular client would be able to push through. That being the case, switching from H.264 to VP9/HEVC wouldn’t reduce bandwidth costs, it would simply substitute a 4Mbps VP9/HEVC stream for a 4Mbps H.264 stream. The VP9/HEVC stream would likely have a higher resolution, and might look better, but there would be no bandwidth savings.
I asked my audience, “What’s your average distribution throughput during prime time?” One member of the group said, “about 4Mbps.” I nodded sagaciously, and concluded, “So, HEVC/ VP9 would improve QoE, but likely wouldn’t reduce bandwidth costs.” There followed a murmur of agreement, and I was about to move on when another in the group raised his hand.
“I just finished a study of which streams are viewed by our VOD clients,” he began. “We distribute a mix of SD and HD video, and 84 percent of all streams viewed by our clients are the top-quality HD stream, at 8Mbps, and the top-quality SD stream, at 2.5Mbps. The reason our average throughput is 4Mbps isn’t because we’re limited by our ISP’s delivery capacity; it’s because we’re distributing a mix of SD and HD videos, usually at the top-quality stream for both. I suspect it’s the same for Netflix, as well.”
Oops. Don’t you hate it when that happens?
Of course, these facts suggested a completely different conclusion regarding the economic benefits of deploying HEVC or VP9. Specifically, not only would it save bandwidth costs—likely cutting the 8Mbps HD stream down to 4.5Mbps or so and delivering lesser efficiencies down the encoding ladder—it would also boost QoE because lower-bitrate connections would be able to view higher-resolution, higher-quality streams.
Later, the gent with the statistics was kind enough to share the stream distribution with me. While the HD/SD numbers were mixed, of the clearly HD numbers, 35.93 percent of the streams were the top-quality HD file at 8Mbps, while only 2.6 percent of the streams were the second-tier HD file at 4.5Mbps. The vast majority of HD streams were delivered at the highest available rate. The bottom three video streams, all under 1Mbps, represented only .45 percent of all viewed streams.
At least for this producer, the Netflix ISP Speed Index had no relevance as a predictor of effective throughput. Interestingly, in its 2017 “State of the Internet” report, Akamai reported an average connection speed of more than 20Mbps in that particular country, which likely is the more relevant number to consider.
What does this mean? All encoding decisions must relate back to your own distribution patterns.
What’s interesting is how many important decisions this data can drive. For example, my Scandinavian client was considering reducing the number of streams in the encoding ladder since several were retrieved by less than .2 percent of all viewers. If bandwidth is highly constricted, I’d recommend constant bitrate (CBR) or 110 percent constrained variable bitrate (VBR) for encoding to reduce stream variability and the potential for stream switching forced by data rate spikes in the stream, rather than changing retrieval bandwidth. If not, 150 percent–200 percent constrained VBR would deliver slightly better quality with little risk of deliverability issues. I might even consider capped constant rate factor (CRF) as a poor man’s per-title encoding schema.
The bottom line is, always make decisions according to your own log file.
[This article appears in the April/May 2018 issue of Streaming Media magazine as "Pushing Streams in Prime Time."]
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