The QuickTime Movie File Format

The QuickTime movie file format, often abbreviated to QuickTime file, is a special file format that contains multiple tracks for storing different kinds of media. Don’t confuse this special movie file format with other files that are simply QuickTime-compatible formats. Examples of QuickTime-compatible file formats include AIFF, MP3, MPEG, WAVE, JPEG, and TIFF, just to name a few. A QuickTime movie file uses a .mov file extension.

Important: Because QuickTime recognizes so many media file formats, you may notice that a lot of media files in the Finder have a QuickTime icon, or the QuickTime Player application may open when you double-click the icons. To be accurate, these files are called QuickTime-compatible files, but they are not necessarily QuickTime movie files. For example, an AIFF file is a QuickTime-compatible audio file format; it is not a QuickTime movie file.

How Is Information Stored in a QuickTime Movie?

QuickTime movie files store media data in separate tracks. You create a QuickTime movie file by adding tracks that point to the media you want to use. The media may be embedded in the file itself or in a reference movie in another file. This track architecture is powerful and flexible, allowing you to store and synchronize multiple pieces of video and audio media in a single file.

There are many types of tracks allowed in a QuickTime movie file. Here are a few examples of what might be included in a QuickTime movie file used by Final Cut Pro:

  • An audio track: This track contains audio media data of a certain duration, encoded with a particular audio codec, sample rate and bit depth. The track may be mono or contain two (stereo) or more interleaved channels of audio samples.
  • A video track: This track contains video media data (a number of video frames) of a certain duration (determined by the track’s frame rate), with particular horizontal and vertical dimensions, and encoded with a particular video codec.
  • A single still image: This track contains media for a single still image. The image data has particular horizontal and vertical dimensions and is compressed with a particular codec.
  • A timecode track: A track that contains a number count and frame rate that corresponds to the video frames in a video track. This track can be captured from a videotape or created later in Final Cut Pro.
  • A text track: This track contains text information that changes at specified times. A text track could contain subtitles, or marker notes about a video or audio track.

Codecs Supported in QuickTime

Because the QuickTime file format is so flexible, practically any kind of media can be stored in a track of its kind. However, in order to play back media stored in a track, the QuickTime framework installed on your computer must be able to recognize the type of compression (codec) used to encode the data.

Compression is necessary for video and audio storage on computers because the data rates would otherwise be prohibitively high. And no matter how large computer storage devices become, compression is still desirable because it means faster transfer time and storing more information with less data.

The QuickTime framework libraries support a remarkable number of video and audio codec (compressor/decompressor) algorithms. The QuickTime frameworks are extensible, so if a company invents a codec, the company can provide a QuickTime codec to support it. Thus, if the media in a QuickTime file is not playing back because the format or codec of the media is not recognized, you may be able to download and install it.

The QuickTime framework supports codecs that are commonly used today as well as codecs that were once popular. When you go to export a QuickTime movie file, the long list of available codecs demonstrates how extensive QuickTime codec support is. At the same time, this list can be potentially daunting. Look for the codec you need and ignore the rest.

Distinguishing Between File Formats and Codecs

A file’s format specifies the unique way data is stored and organized in a file, regardless of what the content of that data represents. A codec is an algorithm that transforms image or sound data into a more compact, albeit temporarily unintelligible form for the purpose of compression (reducing data size for transmission or storage). The codec must be reversed in order to see the original content of the media data.

A file format determines consistent guidelines for where information is stored and found in a file. For example, a Microsoft Word file will always store the name of the creator in a particular location in the file’s structure. On the other hand, a codec is specific to media-intensive data, such as video or audio, and is used simply to reduce the data size.

Understanding Codec and File Format Naming Conventions

The distinction between file formats and codecs is often confused by shared naming conventions. For example, MPEG-2 defines both a file format (a structure for organizing video and audio data within media tracks) and a codec (an algorithm for encoding and decoding video and audio data for the purposes of compression).

The following codec and file format examples may help to clarify the distinction.

  • TIFF: This refers to a graphics file format. TIFF files may or may not use a codec, or type of compression called LZW compression.
  • JPEG: This is a type of compression that can be used on any still images or individual video frames. Images encoded with JPEG compression can be stored in the JPEG file format. QuickTime can open files in the JPEG file format as well as decode images compressed with the JPEG codec.
  • QuickTime: This refers to the QuickTime movie file format, which can contain multiple media tracks, each containing data encoding with a number of possible codecs. QuickTime is not a codec, but rather has the ability to present images and sound stored with a number of codecs.
  • AIFF and WAVE: These are audio file formats that contain uncompressed audio data.
  • DV: There are several DV codecs available for NTSC, PAL, and other varieties such as DVCPRO HD. A DV camcorder uses a DV codec to turn full-resolution image data into compressed media, which is then stored on tape. The raw data stream from tape can be captured to your hard disk in a file format called a DV stream. Applications such as iMovie can capture and edit DV stream files, while applications like Final Cut Pro capture media into tracks within a QuickTime media file, allowing for more flexibility such as adding and manipulating timecode tracks.

Time in QuickTime Movie File Tracks

Each track in a QuickTime movie file has its own playback rate and duration definition. Usually, the rate and duration of each track are the same, or related, because the various tracks (such as audio and video) are intended to play back in sync.

Because each track has its own independent definitions of time, you can assign a video track a frame rate of 29.97 fps and the timecode track 23.98 fps. One example of how this is useful is when you are editing film (24 fps) transferred to NTSC video (29.97 fps).