A story of lowering quality for improving accessibility

Vittorio Giovara
May 3, 2019 · 4 min read

Hello, I’m Vittorio Giovara. You may remember me from such blog posts as Vimeo joins the Alliance for Open Media and Luminous colors, stunning high quality: HDR has arrived. But today I’m covering an unusual topic: while I usually advocate for high-end video quality, next-generation video codecs, and accurate color representation, this time I’ll take a turn towards the exact opposite end of that spectrum.

Vimeo is introducing a low-quality profile at 426 ✕ 240.

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But first a bit of introduction. Vimeo is able to stream videos at the best available quality thanks to adaptive streaming in the form of DASH or HLS, depending on the viewing conditions. If the network allows it, the best video rendition will be played; otherwise, the video quality scales down to meet available bandwidth. However, if there’s insufficient bandwidth for even the lowest video quality, you’re forced to wait until enough of the video has buffered, and that still doesn’t guarantee a smooth video experience. Playback can be interrupted at any moment.

If you live anywhere in the world with good landline internet, you may rarely notice this (outside of complete outages). But if you watch Vimeo on a mobile connection, you may have a different story about that. In this day and age, that is simply unacceptable. So my team, the Transcoding team, took up this task and embarked on what turned out to be a two-year odyssey: we needed to design a low-quality profile providing the highest quality possible for a very constrained environment.

What are these constraints, you might ask? Well, with the help of the Data team and the Player team, we identified a global average bitrate that took into account mobile internet and bandwidth from emerging markets. When the results were in, there were only 380 kb/s available, and that’s being pretty conservative. There is very little space and a lot to cram there: by downscaling the (former) lowest profile by one third, we were able to find a good compromise that enabled us to stream something more than a blocky mess. With some careful tuning of our x264 encoder, we could somehow fit a 240p video stream into just 300 kb/s.

Discounting some bandwidth overshoot, can we fit audio in the remaining 64 kb/s? It looks like we can, by employing a little trick that I’ve never seen widely adopted: instead of using the normal Advanced Audio Coding profile, the audio stream is encoded with the High Efficiency profile, or HE-AAC. This profile was introduced roughly 10 years ago, and it offers several additional coding tools that work especially well in low-bitrate scenarios. Essentially, any device in the wild supports this profile, which allows for an acceptable audio stream, perfect for such constrained conditions.

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This is exciting, isn’t it? Well, it already happened. We silently introduced this additional profile two weeks ago, and the response has been outstandingly positive. Over this time, we served about 10 TB per day of 240p content, for a total of 12.5 years of consumption, again only from 240p videos. Even for Vimeo, these numbers are pretty high.

In the meantime, buffering has been slashed by half. The following graph shows how 240p is consuming the higher buffer ratio from 360p, meaning that if you’re on a constrained network, you’re half as likely to be able to watch videos at 240p rather than staring at a blank screen.

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Towards the end of the project, something in particular really struck me: by offering this low-quality profile, we actually had a more profound impact on people than I could ever imagine. Not only does this improve the Vimeo experience, but it also makes videos accessible for hundreds of millions more internet users. While this consideration isn’t exactly new — as Chris Zacharias and others have noted — it’s something that’s easy to overlook.

Most of the messages I received made me realize how important accessibility is, and while I’ll keep on advocating for 10-bit HDR 8K, I’ll never forget how important this lesson about the web is. High-quality video is great, but so is making a video accessible to millions of people, and Vimeo will always stand by both ideals.

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