Netflix gave us an in-depth look at its progress on a new per-title approach to compression and encoding on its tech blog earlier this week. This development is big news for any Netflix watcher who wants to maximize their viewership experience on a bandwidth-constrained connection, and it also offers perks to those looking for the ultimate experience on a high-bandwidth Internet pipe.
In 2010, Netflix rolled out the H.264 compression scheme across its content library, using a set of bitrate-resolution pairs it refers to as a bitrate ladder. To build this ladder, Netflix engineers settled on ten such pairs, ranging from 235 kbps and a 320x240 resolution to 5800 kbps at 1080p. Those pairs, individually called recipes, let a streaming client seamlessly switch from one quality level to another depending on the available bandwidth. Netflix says it performs this switching to maximize streaming quality while avoiding buffering pauses.
The issue that Netflix has been working for years to address is that not all content requires the same quality of encoding to look good. For example, an action movie with complex chase scenes and explosions can handle only minimal compression before artifacts become visible. At the other end of the complexity spectrum, cartoons often feature solid colors and contain few moving elements in a scene, making them highly compressible without visibly degrading the quality.
With Netflix's present bitrate ladder, though, cartoons and all other content are locked into a rigid encoding scheme that's best-suited to the most complex content like action flicks. As a result, those titles are often encoded with higher bitrates than are needed to produce noticeable improvements in quality for a given resolution. Worse yet, those same action flicks are capped at the Netflix-wide maximum bitrate of 5800 kbps, even though that content's quality could benefit from higher bitrates at HD resolutions.
Switching to a per-title bitrate ladder means simple content like cartoons can stream at a higher perceived quality even at modest bitrates, medium-complexity titles like dramas can be encoded more efficiently while offering the same high quality we’ve come to expect, and the busiest content like action movies can be encoded with higher bitrates than the ones that are currently available.
Netflix didn’t mention in its blog post when it plans to roll out per-title bitrate ladders, but we hope that we'll all see its benefits soon.