What is an AAF File?

An Advanced Authoring Format file exchanges editable media compositions between production applications. It can preserve timeline structure, metadata, embedded media, and references to external assets.

Composition Mob: the edit

Master Mob and File Source Mob

Embedded essence or external media file

A clip backed by a media file resolves through a Master Mob and a File Source Mob to that media, which is stored inside the AAF or located outside it.

How AAF Files work

An AAF file is a set of linked metadata objects called Mobs, each identified by a globally unique MobID. A Composition Mob holds the edit: slots (tracks) containing source clips, filler, transitions, and effect operations. A clip whose media is a file points through a Master Mob to a File Source Mob that describes the media, and from there optionally to the tape or film it came from, so the file records what is known about where that material originated. A clip can also point through its Master Mob straight to a tape or film source, with no file media. The media itself, which AAF calls essence, may be stored inside the file or left in external files. An exporter should write a locator pointing to each external file, but a source identified by MobID alone is also valid, and the importing application then relies on an asset manager to find the media. The binary encoding defined by the AAF stored-format specifications is a Microsoft Structured Storage compound file. The AAF Edit Protocol narrows this general model to a constrained subset intended to interchange predictably between editing applications.

Key facts

Media, which the format calls essence, is either embedded in the AAF or left in external files, and the choice is made per source: one AAF can hold embedded audio and still link to external video. The Edit Protocol treats the locators that point to external files as hints, not authoritative references, and has an importer that cannot find a clip’s media, or another source it can use, substitute placeholder media for that clip and log the source it could not locate. Deliver the external media the recipient needs with the AAF, keeping the exported folder layout and file names because a relative locator is resolved against the location of the AAF file itself. An embedded export is self-contained only for what the exporter embeds: Premiere’s Embed Audio option embeds audio, while video is mixed down to an MXF file saved next to the AAF or stays in the original files that the AAF links to.
Embedding has limits of its own. Structured Storage files with 512-byte sectors cannot exceed 2 GB. The Edit Protocol tells exporters to write 4096-byte sectors, which raise that ceiling to just under 16 TB, but an exporter can still set a lower limit: Adobe documents that Premiere on Windows fails to export embedded AAFs larger than 2 GB and advises separate audio instead. In Premiere, the Separate Audio option writes the audio to a folder next to the AAF and preserves more metadata than Embed Audio.
The Edit Protocol defines a common subset for interchange, not a guarantee that a given product supports all of it: clip positions, source timecode and names, dissolves and wipes, speed changes, simple picture effects, and audio gain, pan, and fades. Proprietary processing falls outside the subset, and an importer may ignore any effect it does not support, provided it logs the fallback. The OpenTimelineIO AAF adapter, for example, reads clips, tracks, transitions, markers, and nesting but not audio or video effects.
Relinking works from identifiers carried in the AAF. Each clip resolves by MobID to a source description that can chain back to a tape name and source timecode, and the AAF Object Specification warns that media stored without a MobID is difficult to identify once it is moved or renamed. Exporters differ in what they write: Premiere writes the tape name only when its tape-source option is on and otherwise uses the file name, and Adobe notes that its AAFs do not relink automatically in Avid Media Composer and says to use Batch Import there. Conversion to another timeline format and back keeps an AAF relinkable only if those identifiers are carried through, and the Edit Protocol does not require an application to re-export metadata it could not interpret.

When AAF Files matter

Export AAF when a sequence must stay editable in another application, most often a picture edit handed to audio post-production or a finishing system. An AAF does not play like an ordinary media file: an application has to interpret the composition, and it plays back only the features it supports and the media it can find. A pipeline that needs viewable media should receive the sequence rendered to an ordinary video file instead. Test the exact export settings with the receiving application first, because support for effects and media links differs by product.

Working with AAF Files

Before production

Agree with the receiving facility on embedded or linked media, which media files it needs, audio sample rate and bit depth, and handle length before exporting.
If the export trims media to the used ranges, add handle frames, which are extra media before and after each clip: only the used range plus the handles reaches the other application.
Send the sequence rendered to an ordinary video file with the AAF as a sync and review reference, because the AAF plays only in an application that interprets it.
Import the AAF into the receiving application before the handoff and read its import log for substituted placeholders and ignored effects.