.MKV

Introduction

Matroska is an open, royalty-free container format, first released in 2002. A Matroska file can hold any number of video, audio and subtitle tracks in almost any codec, together with chapters, tags and attached files such as fonts. That flexibility makes it a common choice for films with several languages and subtitle tracks.

The same format uses .mkv for video, .mka for audio only, .mks for subtitles only and .mk3d for stereoscopic video. WebM, used for video on the web, is a restricted form of Matroska.

Structure

Matroska is built on EBML, the Extensible Binary Meta Language, a binary form of XML. Every element is an ID, a size and the data. IDs and sizes are variable-length integers: the number of leading zero bits in the first byte says how many bytes follow, so small values take little space. Elements nest, so the file is a tree.

A file starts with an EBML header, ID 1A 45 DF A3, whose DocType is matroska or webm. Then comes one Segment holding everything else: a SeekHead index of the top-level elements, the Info element with the timescale and duration, the Tracks element describing each track, and Clusters of encoded frames, each cluster starting with a timestamp. Cues near the end let a player seek quickly.

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