ebml parser

  • ebml

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103123.0.05 years ago10 years agoMinified + gzip package size for ebml in KB


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EBMLEBML stands for Extensible Binary Meta-Language and is somewhat of a binary version of XML. It's used for container formats like WebMwebm or MKVmkv.


This is for version 3.0.0 and up, which has undergone a massive rewrite and now builds with RollupJSrollup.
Version 2.2.4 is the last version to have guaranteed legacy semantics.
Install via NPM or Yarn:
npm install ebml --save
# or
yarn add ebml
The Decoder() class is implemented as a Node Transform streamnode-stream-transform. As input it takes EBML. As output it emits a sequence of chunks: two-element arrays looking like this example.
[ "tag",
    name: "TimecodeScale",
    type: "u",
    value: 1000000

The first element of the array is a short text string. For tags containing values, like this example, the string is 'tag'. ebml also has nesting tags. The opening of those tags has the string 'start' and the closing has the string 'end'. Integers stored in 6 bytes or less are represented as numbers, and longer integers are represented as hexadecimal text strings.
The second element of the array is an object with these members, among others:
  • name is the Matroskamkv Element Name.
  • type is the data type.
u: unsigned integer. Some of these are UIDs, coded as 128-bit numbers. i: signed integer. f: IEEE-754 floating point number. s: printable ASCII text string. 8: printable utf-8 Unicode text string. d: a 64-bit signed timestamp, in nanoseconds after (or before) 2001-01-01T00:00UTC. b binary data, otherwise uninterpreted.
  • value is the value of the data in the element, represented as a number or a string.
  • data is the binary data of the entire element stored in a Uint8ArrayMDN-Uint8Array.

Elements with the Block
mkv-block and SimpleBlockmkv-sblock types get special treatment. They have these additional members:
  • payload is the coded information in the element, stored in a Uint8ArrayMDN-Uint8Array.
  • track is an unsigned integer indicating the payload's track.
  • keyframe is a Boolean value set to true if the payload starts an I frame (SimpleBlocks only).
  • discardable is a Boolean value showing the value of the element's Discardable flag. (SimpleBlocks only).

And the value member shows the block's Timecode value.
This example reads a media file into memory and decodes it. The decoder invokes its data event for each Element.
const fs = require('fs');
const { Decoder } = require('./lib/ebml.js');

const decoder = new Decoder();

decoder.on('data', chunk => console.log(chunk));

fs.readFile('media/test.webm', (err, data) => {
    if (err) {
        throw err;

This example does the same thing, but by piping the file stream into the decoder (a Transform stream).
const { Decoder } = require('./lib/ebml.js');

const ebmlDecoder = new Decoder();
const counts = {};

    .on('data', chunk => {
        const { name } = chunk[1];
        if (!counts[name]) {
            counts[name] = 0;
        counts[name] += 1;
    .on('finish', () => console.log(counts));
State of this project
Parsing should work. If it doesn't, please create an issuenew-issue.
d-type elements (timestamps) are not yet decoded to Javascript timestamp values.
Thanks to @chrisprice we got an encoder!
(in alphabetical order)