All about QT files

QT stands for QuickTime and refers to Apple's classic container format for multimedia files. The file extension .QT is technically identical to the now more familiar .MOV. Currently, it is mostly older QuickTime files that are still saved with the .qt file extension. These files mostly date from the 1990s and 2000s. During my research, I discovered that Apple never communicated .qt as an official file extension. The first references to it can be found at Microsoft from the late 1990s onwards. The official Apple standard recommends .mov as the file extension.

History of QuickTime

Apple released QuickTime on 2 December 1991. QuickTime enabled the playback of digital video on personal computers without special hardware. At the time, this was a real technical sensation. The first videos ran at a very low resolution of just 160×120 pixels and about 10 frames per second, which is nothing compared to today's 4K videos with 25 fps and more.

While QuickTime was initially only available for Macintosh, the Windows version followed in 1994. At that time, the file extension .qt also became established, even though, as mentioned above, it was never communicated as such by Apple. Background: for Mac OS (Classic), the file extension was not relevant; here, the file type was recognised via internal type and creator codes.

Technical structure

QuickTime files consist of hierarchically organised data units called "atoms". The central atom is the moov atom, which contains all metadata such as track definitions, time information and codec specifications. The actual media data (audio, video) is stored in the mdat atom.

A QuickTime container can contain multiple tracks:

• Video tracks (with codecs such as H.264, ProRes, CinePak, Sorenson)
• Audio tracks (such as AAC, MP3, PCM, ALAC, APAC)
• Text tracks for subtitles
• Metadata for chapter markers

This flexibility made QuickTime the standard format in professional film and video production.

QTA – the modern successor for audio

In 2024, Apple introduced a new format with iOS 18.2: QTA (QuickTime Audio). It also uses the QuickTime container, but is specifically designed for spatial audio with the new Apple Positional Audio Codec (APAC). QTA is used for voice memos on the iPhone 16 Pro/MAX. Further information on the QTA format

I find it interesting that .qta is the official file extension for QTA files, while .qt has not been standardised for (mostly) video containers. But it feels a bit like retroactive confirmation. Who knows, maybe there will be more QuickTime files with the .qt file extension in the future. Or maybe Apple will soon differentiate between .qtv?

Sources

QTFF Standardisation 2001
Microsoft KB178291 via betaarchive.com
QUICKTIME 1.0: "YOU OUGHTA BE IN PICTURES"