How Video Streaming Adapts Quality to Your Connection
Why a video drops from sharp to blurry and back: how adaptive bitrate streaming cuts video into short segments and picks a quality level for each one.
In this article
When a streamed video gets blurry for a few seconds and then sharpens again, nothing is broken. It is adaptive bitrate streaming doing its job. The video exists in several quality levels, and the player switches between them while you watch, choosing whichever your connection can handle right now. This article explains how that works and where it came from.
The video is prepared in several versions
Before a video is streamed, it is encoded at multiple bitrates. Bitrate means how much data each second of video uses. A low bitrate version is small and fits a slow connection but looks softer. A high bitrate version looks sharper but needs a fast connection. According to Wikipedia's article on adaptive bitrate streaming, each version is then cut into short segments, each a few seconds long.
Because the segments are ordinary files, they can be served by normal web servers and delivered through content delivery networks, the networks of servers that keep copies of popular files close to viewers. Apple's page on its streaming format makes the same point: it uses the same HTTP protocol as the web, so ordinary web servers and delivery networks can handle it.
The manifest is a menu
The player first downloads a small text file that lists what is available. In Apple's HTTP Live Streaming (HLS), described in the standard RFC 8216, this is a Master Playlist. It lists several 'variant streams', which are different encodings of the same content that vary in bit rate, format or resolution. Each variant points to its own Media Playlist, which lists the segments in order and says how long each lasts.
- Master playlist: the list of quality levels on offer.
- Media playlist: the ordered segments for one quality level.
- Segment: a few seconds of audio and video, downloaded as a small file.
How the player chooses
The Wikipedia article explains that the player usually starts at the lowest bitrate so playback begins quickly. It then requests higher bitrate segments when its measured download speed allows and drops to lower ones when the network slows. The decision rule is called an ABR algorithm and lives in the player. Some algorithms look mainly at recent download speed, some at how much video is waiting in the buffer, and some at both.
The buffer is the stretch of video already downloaded but not yet played. If the buffer is shrinking, the player can fetch a lower quality segment to avoid stalling. Since quality can be adjusted segment by segment, the player does not need to hold a large buffer, which helps video start sooner.
Switching without a glitch
For switching to look smooth, all the versions must line up. RFC 8216 says the server must make each variant stream present the same content with matching timestamps, so the player can jump from one to another at a segment boundary without a visible break.
A short history
Wikipedia traces modern HTTP-based adaptive streaming to the company Move Networks, whose work in 2004 to 2006 was first called Quantum Streaming; the article says ABC chose it for its HD video site in 2007. Microsoft demonstrated adaptive streaming on NBC's Olympics website in August 2008 and made Smooth Streaming generally available in April 2009. Apple released HLS in June 2009 with iOS 3.0. The standards body MPEG developed Dynamic Adaptive Streaming over HTTP, known as DASH, which became an international standard in November 2011 and was published as ISO/IEC 23009-1:2012 in April 2012.
Apple's HLS was later published as RFC 8216 in August 2017. The Wikipedia article states that HLS and DASH are the two adaptive systems still actively developed, and that a shared container format called CMAF, proposed in 2016 and published in 2018, can be used by both.
Why it sometimes looks poor anyway
- The best version on offer may be low: the player can only choose among the versions that were prepared.
- The connection may fluctuate faster than the player reacts.
- The player may be cautious after a recent stall, staying at a lower level.
- The device's own limits, such as screen size, may influence the choice.
The key idea
Adaptive streaming trades a steady picture for a steady flow. Instead of one long file at one quality, the video is a stack of short, interchangeable pieces. The player's job is to keep picking the largest piece it can download in time, and the quality changes you notice are the visible result of that choice, made again every few seconds.
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