Adaptive bitrate streaming

Adaptive bitrate streaming, often called ABR or adaptive streaming, is the way online video is sent so that the player can switch between several qualities as the network speeds up or slows down. When the network slows, the picture turns blurry for a moment instead of stopping. Bitrate means how much data the video uses each second. A sharp 1080p picture may need about 5 megabits a second, and a small, blurry one may need less than half a megabit.

Here is how it works in general. An encoder makes several copies of the same video, each at a different quality. This set of copies is called a ladder. Each copy is cut into short chunks, a few seconds long. A small file, called a manifest, lists every copy and every chunk. The video player in the app reads the manifest and downloads the video chunk by chunk, usually from a nearby content delivery network server. While it plays, it measures how fast each chunk arrives and how many seconds of video it has saved ahead. If chunks arrive slowly, it asks for the next chunk at a lower quality. If they arrive quickly, it steps up. The switch happens between chunks, so the video never has to stop. The two common standards for this are HLS and MPEG-DASH.

Take, for example, a passenger watching a film on a train through the Western Ghats. As the train enters a tunnel and the signal drops, the picture softens for a few seconds and then sharpens again on the other side.

Consider a cricket final watched live by tens of millions of people. A service doing live streaming at scale can lower the top quality for everyone at the busiest moment, so that its servers and networks are not overloaded.

Say a coaching company streams classes to students in small towns on slow connections. A white-label streaming platform gives each student the best quality that the student's connection can carry, and quality analytics shows how often the picture had to drop.

How is adaptive streaming different from an ordinary download? An older video player downloads one file at one quality. If the network slows, the video stops and shows a spinning circle while it waits. It is also different from a video codec. The codec compresses each copy of the video. Adaptive streaming decides which copy to play next, according to the bandwidth available at that moment.