AVI, an acronym for Audio Video Interleave,
is a multimedia container format introduced by Microsoft in November 1992,
as part of the Video for Windows technology. AVI files contain both audio
and video data in a standard container that allows simultaneous playback.
Like DVDs, AVI files support multiple audio and video streams, although
these features are rarely used. Most AVI files also use the file format
extensions developed by the Matrox OpenDML group in February 1996. These
files are supported by Microsoft, and are known unofficially as "AVI 2.0".It is a special case of the Resource Interchange File
Format (RIFF), which divides the file's data up into data blocks called
"chunks". Each "chunk" is identified by a FourCC tag. An AVI file takes
the form of a single chunk in an RIFF formatted file, which is then subdivided
into two mandatory "chunks" and one optional "chunk". The whole structure
of RIFF file is actually copied from earlier IFF format devised by Electronic
Arts in mid-eighties, the only difference being the endianness of integers
inside it, and initial FourCC. In fact, properly written IFF parser for
AmigaOS, after correcting for endianness should parse RIFF files just fine.
The first sub-chunk is identified by the "hdrl" tag.
This chunk is the file header and contains metadata about the video such
as the width, height and the number of frames. The second sub-chunk is
identified by the "movi" tag. This chunk contains the actual audio/visual
data that make up the AVI movie. The third optional sub-chunk is identified
by the "idx1" tag and indexes the location of the data chunks within the
file.
By way of the RIFF format, the audio/visual data contained
in the "movi" chunk can be encoded or decoded by a software module called
a codec. The codec translates between raw data and the data format inside
the chunk. An AVI file may therefore carry audio/visual data inside the
chunks in almost any compression scheme, including: Full Frames (Uncompressed),
Intel Real Time Video, Indeo, Cinepak, Motion JPEG, Editable MPEG, VDOWave,
ClearVideo / RealVideo, QPEG, MPEG-4, XviD, DivX and others.
Continued use despite obsolescence
AVI is considered by many to be an outdated container
format. There is significant overhead when used with popular MPEG-4 codecs
(XviD and DivX, for example), increasing file size more than necessary.
The container has no native support for those codecs' modern features like
B-Frames; to circumvent this problem, cumbersome hacks are used, causing
incompatibilities in some players. Hacks are also used to implement subtitles.
The highly efficient H.264 codecs add even more compression tricks to the
mix, and thus are even more ill-suited to the format, particularly Main
and High Profile.
Despite its limitations and the availability of more
modern container formats (see MKV and MP4), AVI remains popular among file-sharing
communities. This is probably due to it still being treated as a first
class citizen in Windows Media Player, the main front-end for DirectShow.
The tendency has been to install codec packs like ffdshow to augment DirectShow,
using AVI as a common-ground, lowest-common-denominator format. Ironically,
while the codec/container incompatibilities mentioned in the above paragraph
have undermined AVI's near-ubiquity, the obscurity and tech-savviness of
those involved in the file-sharing groups has rendered this irrelevant.
In June 2005, DivX, Inc. has released its own container
format called DivX Media Format (.divx extension) to succeed the AVI +
DivX combo. However, this format is basically an enhanced AVI format (based
on the same RIFF structure) and so far, has gained no perceivable traction
in file-sharing communities, where DivX codec is popular.
AVI format Description
One of the oldest formats in the x86 computer world
is AVI. The abbreviation 'AVI' stands for 'Audio Video Interlaced'. This
video format was created by Microsoft, which was introduced along with
Windows 3.1. AVI, the proprietary format of Microsoft's "Video for Windows"
application, merely provides a framework for various compression algorithms
such as Cinepak, Intel Indeo, Microsoft Video 1, Clear Video or IVI. In
its first version, AVI supported a maximum resolution of 160 x 120 pixels
with a refresh rate of 15 frames per second. The format attained widespread
popularity, as the first video editing systems and software appeared that
used AVI by default. Examples of such editing boards included Fast's AV
Master and Miro/Pinnacle's DC10 to DC50. However, there were a number of
restrictions: for example, an AVI video that had been processed using an
AV Master could not be directly processed using an interface board from
Miro/Pinnacle. The manufacturers adapted the open AVI format according
to their own requirements.
AVI is subject to additional restrictions under Windows
95, which make professional work at higher resolutions more difficult.
For example, the maximum file size under the FAT16 file system is 2 GB.
The FAT32 file system (came with OSR2 and Windows 98) brought an improvement:
in connection with the latest DirectX6 module 'DirectShow', files with
a size of 8 GB can (at least in theory) be created. In practice however,
many interface cards lack the corresponding driver support so that Windows
NT 4.0 and NTFS are strongly recommended. Despite its age and numerous
problems, the AVI format is still used in semi-professional video editing
cards. Many TV cards and graphic boards with a video input also use the
AVI format. These are able to grab video clips at low resolutions (mostly
320 x 240 pixels).
AVI is a file format, like MP3 or JPG. But unlike these
formats, AVI is a container format, meaning it can contain video audio
compressed using many different combinations of codecs. So while MP3 and
JPG can only contain a certain kind of compression (MPEG Audio Layer 3
and JPEG), AVI can contain many different kinds of compression (eg. DivX
video + WMA audio or Indeo video + PCM audio), as long as a codec is available
for encoding/decoding. AVI all look the same on the "outside", but on the
"inside", they may be completely different. Almost all tools on this site
are not just DivX tools, but also AVI tools, so will probably work with
other codecs. There is no such thing as a "normal" AVI file, but the closest
you can get is probably an AVI file that contains no compression. AVI files
has been around since the time of Windows 3.1, so by no means is it a new
thing, and is probably the most common video format around (although its
popularity wavered a few years ago, but has since come back with a vengeance
due to the emergence of DivX). AVI files may also have limits under Windows
95/98, and for more information, please read this article. Note that AVI
files without file limits (other than the Windows Fat32 file limit) are
usually referred to as OpenDML AVI files.
QuickTime is a multimedia technology developed
by Apple Computer, capable of handling various formats of digital video,
sound, text, animation, music, and immersive panoramic (and sphere panoramic)
images.The most recent version is 7.1.1 and is available for
Mac OS X. O
nly version 7.1 is currently available for Microsoft
Windows.
Overview
The QuickTime technology has three major components:
1. the QuickTime file format itself — openly documented
and available for anyone to use royalty-free
2. a media player which Apple makes available for free
download on its website and bundles with each of its computers
3. software development kits available for the Macintosh
and Windows platforms. These kits allow people to develop their own software
to manipulate QuickTime and other media files
History
Apple released the first version of QuickTime on December
2, 1991 as a multimedia add-on for System Software 6 and later. The lead
developer of QuickTime, Bruce Leak, ran the first public demonstration
at the May 1991 Worldwide Developers Conference, where he played Apple's
famous 1984 TV commercial on a Mac, at the time an astounding technological
breakthrough. Microsoft's competing technology — Video for Windows — did
not appear until November 1992. [edit]
QuickTime 1.x
That first version of QuickTime laid down the basic
architecture which survives essentially unchanged today, including multiple
movie tracks, extensible media type support, an open-ended file format,
and a full complement of editing functions. The original video codecs included:
* the Apple Video codec (also known as "Road Pizza"),
suited to normal live-action
* the Animation codec, which used simple run-length
encoding and better suited cartoon-type images with large areas of flat
color
* the Graphics codec, optimized for 8-bit images, including
ones that had undergone dithering
Apple released QuickTime 1.5 for Mac OS in the latter
part of 1992. This added the SuperMac-developed Cinepak vector-quantization
video codec (initially known as Compact Video), which managed the unheard-of
feat of playing back video at 320¥240 resolution at 30 frames per second
on a 25 MHz 68040 CPU. It also added text tracks, which allowed for things
like captioning, lyrics, etc., at very little addition to the size of a
movie.
In an effort to increase the adoption of QuickTime,
Apple contracted an outside company, San Francisco Canyon Company, to port
QuickTime to the Windows platform. Version 1.0 of QuickTime for Windows
provided only a subset of the full QuickTime API, including only movie
playback functions driven through the standard movie controller.
QuickTime 1.6.x came out the following year.
Version 1.6.2 first incorporated the "QuickTime PowerPlug"
which replaced some components with PowerPC-native
code
when running on PowerPC Macs.
QuickTime 2.x
Apple released QuickTime 2.0 for Mac OS in February
1994 — the only version never released for free. It added support for music
tracks, which contained the equivalent of MIDI data and which could drive
a sound-synthesis engine built into QuickTime itself (using sounds licensed
from Roland), or any external MIDI-compatible hardware, thereby producing
sounds using only small amounts of movie data.
Following Bruce Leak's departure
to Web TV the leadership of the QuickTime team was
taken over by Peter Hoddie.
QuickTime 2.0 for Windows appeared in November 1994.
The next versions, 2.1 and 2.5, reverted to the previous
model of giving QuickTime away for free.
They improved the music support and added sprite tracks
which allowed the creation of complex animations with the addition of little
more than the static sprite images to the size of the movie.
QuickTime 3.x
The release of QuickTime 3.0 for Mac OS on March 30,
1998
introduced the now-standard revenue model of releasing
the software for free,
but with additional features of the Apple-provided
QuickTime Player and Picture Viewer applications
that end-users could only unlock by buying a QuickTime
Pro license code.
QuickTime 3.0
added support for graphics importer components that
could read images from GIF, JPEG, TIFF and other file formats, and video
output components which served primarily to export movie data via FireWire.
It also added video effects which programmers could apply in real-time
to video tracks. Some of these effects would even respond to mouse clicks
by the user, as part of the new movie interaction support (known as wired
movies). [edit]
QuickTime interactive
During the development cycle for QuickTime 3.0
part of the engineering team was working on a more
advanced version of QuickTime to be known as QuickTime interactive or QTi.
Although similar in concept to the wired movies feature released as part
of QT 3.0, QTi was much more ambitious. It allowed any QuickTime movie
to be a fully interactive and programmable container for media. A special
track type was added that contained an interpreter for a custom programming
language based on 68000 assembly language. This supported a comprehensive
user interaction model for mouse and keyboard event handling based in part
on the AML language from the Apple Media Tool.
The QTi movie was to have been the playback format
for the next generation of HyperCard authoring tool. Unfortunately both
the QTi and the HyperCard 3.0 projects were canceled in order to concentrate
engineering resources on streaming support for QuickTime 4.0, and the projects
were never released to the public. [edit]
QuickTime 4.x
Apple released QuickTime 4.0 for Mac OS on June 8,
1999.
This added graphics exporter components which could
write some of the same formats that the previously-introduced importers
could read, though interestingly not GIF (possibly because of the LZW patent).
It added the second version of the Sorenson video codec, and support for
streaming.
QuickTime 4.1,
released at the beginning of 2000, added support for
movie files larger than 2 gigabytes on Mac OS 9 and later, and dropped
support for 68K Macs. Users gained the ability to control the QuickTime
Player via AppleScript. [edit]
QuickTime 5.x
QuickTime 5.0 for Mac OS appeared on April 23, 2001.
It added "skins" to the QuickTime Player and multiprocessor
image compression support. The other notable addition was the controversial
move of making full screen video modes only available to QuickTime Pro
license holders, a state of affairs that remains to this day. Four years
later, on May 9, 2005, Apple released iTunes 4.8 with support for viewing
full screen QuickTime video, through iTunes. [edit]
QuickTime 6.x
QuickTime 6.0 for Mac OS,
released on July 15, 2002, first included a version
for Mac OS X.
QuickTime 7.x
QuickTime 7 was released on April 29, 2005
with Mac OS X v10.4 featuring complete MPEG-4 compliance,
H.264/MPEG-4 AVC codec, live resizing, multi-channel audio, full-screen
overlay, and full support for interactive animations created with Apple's
new tool Quartz Composer. Version 7 was also released for 10.3.9.
QuickTime file format
A QuickTime file (*.mov)
functions as a multimedia container file that contains
one or more tracks, each of which store a particular type of data, such
as audio, video, effects, or text (for subtitles, for example).
Each track in turn contains track media, either the
digitally-encoded media stream
(using a specific codec such as
Cinepak,
Sorenson codec,
MP3,
JPEG,
DivX, or
PNG)
or a data reference to the media stored in another
file or elsewhere on a network. It also has an "edit list" that indicates
what parts of the media to use.
Internally, QuickTime files maintain this format as
a tree-structure of "atoms,"
each of which uses a 4-byte OSType identifier to determine
its structure.
An atom can be a parent to other atoms or it can contain
data, but it cannot do both.
Apple's plans for HyperCard 3.0
illustrate the versatility of QuickTime's file format.
The designers of Hypercard 3.0
originally intended to store the equivalent of an
entire HyperCard stack
(similar in structure to a complete web site, with
graphics, buttons and scripts)
as a QuickTime interactive file.
The ability to contain abstract data references for
the media data, and the separation of the media data from the media offsets
and the track edit lists means that QuickTime is particularly suited for
editing, as it is capable of importing and editing in place (without data
copying) other formats such as
AIFF,
DV,
MP3,
MPEG-1, and
AVI.
Other later-developed media container formats such
as
Microsoft's Advanced Streaming Format
or the open source
Ogg and Matroska
containers lack this abstraction, and require all media
data to be rewritten after editing.
QuickTime and MPEG-4
To create an MP4 file, choose MPEG-4 in the Export
dialog.
On February 11, 1998
the ISO approved the QuickTime file format as the
basis of the MPEG-4 Part 14 (.mp4) container standard.
Supporters of the move noted that QuickTime provided
a good "life-cycle" format, well suited to capture, editing, archiving,
distribution, and playback (as opposed to the simple file-as-stream approach
of MPEG-1 and MPEG-2, which does not mesh well with editing).
Developers added MPEG-4 compatibility to QuickTime
6 in 2002. However, Apple delayed the release of this version for months
in a dispute with the MPEG-4 licensing body, claiming that proposed license
fees would constrain many users and content providers. Following a compromise,
Apple released QuickTime 6 on 15 July 2002. [edit]
Profile Support
QuickTime 6 had limited support for MPEG-4
in that it could only encode and decode Simple Profile
(SP).
Advanced Simple Profile (ASP) features,
like B-frames, were unsupported, making QuickTime-encoded
MPEG-4 files compare terribly with XviD and other full-featured encoders.
QuickTime 7 decodes both MPEG-4 SP and ASP, though the encoder is still
SP-only.
QuickTime 7's H.264
encoder is claimed to be Main Profile,
but actually Baseline Profile plus 1 B-frame support,
the decoder supports Baseline, Extended, and
most of Main Profile
High Profile features are unsupported.
Container Benefits
Use Passthrough to change to the MP4 container without
re-encoding the stream.
Because both the MOV and MP4 containers
can utilize the same MPEG-4 codecs, they are mostly
interchangeable in a QuickTime-only environment.
However, MP4, being an international standard, has
more support.
This is especially true on hardware devices, such as
the Sony PSP and
various DVD players;
on the software side,
most DirectShow / Video for Windows codec packs include
an MP4 parser, but not one for MOV.
In QuickTime Pro's MPEG-4 Export dialog,
an option called "Passthrough" allows a clean export
to MP4 without affecting the audio or video streams.
One recent discrepancy ushered in by QuickTime 7
is that the MOV file format now supports multichannel
audio
(used, for example, in the high-definition trailers
on Apple's site[4]),
while MP4 is limited to stereo.
Therefore multichannel audio must be re-encoded during
MP4 export.
QuickTime players
Apple releases official QuickTime media player software
for Mac OS and Windows for free under the brand QuickTime
Player.
(Earlier versions had been named MoviePlayer.)
The player also comes with a number of media-editing
and media-creation features, but users have to unlock these by purchasing
a key from Apple, turning the media player into QuickTime Pro.
A number of companies utilize QuickTime for their software,
for example:
* Apple's own media player, iTunes, utilizes QuickTime
for its audio and video playback features
* the Encyclopædia Britannica on DVD requires
QuickTime to play movie clips
* iScreensaver Designer
makes screensaver installers for Mac OS 8.6 through
OS X and Windows 98 through XP,
and builds from either platform to the other,
demonstrating QuickTime's cross-platform versatility
and stability
* many software installation compact discs
Independent players for QuickTime 6 (MPEG-4) exist
for many operating systems, and the FFmpeg library
even supports
the Sorenson video compression format,
as well as the QDesign audio codec often used alongside
it.
Apple, however, has licensed Sorenson technology exclusively.
QuickTime Alternative,
as the name implies, uses specific QuickTime libraries
and an alternative media player to avoid a full QuickTime
installation.
QuickTime ProQuickTime Pro is a paid version of Apple Computer's
free QuickTime media player technology.
The functions of QuickTime Pro
can be enabled by typing in a Name and a Serial Number
(which is purchased at the Apple Store) in the "Register"
Menu of the standard QuickTime Player.
QuickTime Pro
adds among other the following features to the standard
QuickTime player:
* Full-screen playback
(note: full-screen playback can be enabled in the
Mac OS X version with AppleScript)
* Exporting (Encoding) videos opened in QuickTime
to a wide variety of different video codecs
(MPEG-4,
H.264 (only in QuickTime Pro 7),
Animation, DV, mjpeg),
still graphic formats (TIFF, PICT, JPEG), and Audio
(WAV, AIFF, ACC, MP3).
The full QuickTime Pro is included with Final Cut Studio.
In Mac OS X, the "Pro-only" features
are actually available from within the QuickTime framework,
and
the limitations in the free version apply only to
the QuickTime Player application.
Other software that uses the QuickTime framework can
use the save/export features without the need for a license, e.g. some
video editing packages that rely on QuickTime for their export/import abilities
do so by using the QuickTime framework, no matter if a Pro key is present
or not, and iTunes and its audio encoders do not require the Pro license
to work. Because of this, Apple has often been criticized for its decision
to require Mac OS X users to buy a QuickTime Pro key to use certain Player
features.
QuickTime developmentDevelopers can use the QuickTime software development
kit to develop multimedia applications for Mac or Windows with the C programming
language or with the Java programming language.
QuickTime consists of two major subsystems: the Movie
Toolbox and the Image Compression Manager. The Movie Toolbox consists of
a general API for handling time-based data, while the Image Compression
Manager provides services for dealing with compressed raster data as produced
by video and photo codecs.
QuickTime 7.0 introduced the QuickTime Kit
(aka QTKit), a developer framework that is intended
to replace previous APIs for Cocoa developers.
This framework is for Mac only, and exists as Objective-C
abstractions around a subset of the C interface.
QuickTime 7 Pro
now offers support to export video files encoded
in H.264 at 320x240 pixels to be viewed on the new
fifth generation iPod.
JOHN
LONGENECKER |
