<!--
{
  "availability" : [

  ],
  "documentType" : "symbol",
  "framework" : "QuickTime File Format",
  "identifier" : "/documentation/quicktime-file-format/Base_media_information_header_atom",
  "metadataVersion" : "0.1.0",
  "role" : "Atom",
  "symbol" : {
    "kind" : "Atom",
    "modules" : [
      "QuickTime File Format"
    ],
    "preciseIdentifier" : "__docc_universal_symbol_reference_$Base_media_information_header_atom"
  },
  "title" : "Base media information header atom ('gmhd')"
}
-->

# Base media information header atom ('gmhd')

An atom that indicates that this media information atom pertains to a base media.

## Overview

The layout of a base media information header atom is as follows.

|Data                                                                                                                       |Type     |
|---------------------------------------------------------------------------------------------------------------------------|---------|
|``doc://com.apple.qtff/documentation/quicktime-file-format/Base_media_information_header_atom/Size``                       |4 bytes  |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Base_media_information_header_atom/Type`` = `'gmhd'`            |4 bytes  |
|``doc://com.apple.qtff/documentation/quicktime-file-format/Base_media_information_header_atom/Base_media_info_atom``       |`'gmin'`‡|
|``doc://com.apple.qtff/documentation/quicktime-file-format/Base_media_information_header_atom/Text_media_information_atom``|`'text'` |

‡ denotes required atom.

## Topics

### Data fields

[`Size`](/documentation/quicktime-file-format/Base_media_information_header_atom/Size)

A 32-bit integer that specifies the number of bytes in this base media information header atom.

[`Type`](/documentation/quicktime-file-format/Base_media_information_header_atom/Type)

A 32-bit integer that identifies the atom type.

[`Base media info atom`](/documentation/quicktime-file-format/Base_media_information_header_atom/Base_media_info_atom)

An atom that defines the media’s control information, including graphics mode and balance information.

[`Text media information atom`](/documentation/quicktime-file-format/Base_media_information_header_atom/Text_media_information_atom)

An atom that contains information about text media.


---

Copyright &copy; 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)