Flash Video

Flash Video

Filename extension	.flv, .f4v, .f4p, .f4a, .f4b
Internet media type	video/x-flv, video/mp4, video/x-m4v, audio/mp4a-latm, video/3gpp, video/quicktime, audio/mp4
Developed by	Adobe Systems (originally developed by Macromedia)
Type of format	Media container
Container for	Audio, video, text, data
Extended from	FLV: SWF F4V: MPEG-4 Part 12

Flash Video is a container file format used to deliver video over the Internet using Adobe Flash Player versions 6–10. Flash Video content may also be embedded within SWF files. There are two different video file formats known as Flash Video: FLV and F4V. The audio and video data within FLV files are encoded in the same way as they are within SWF files. The latter F4V file format is based on the ISO base media file format and is supported starting with Flash Player 9 update 3.^[1][2] Both formats are supported in Adobe Flash Player and currently developed by Adobe Systems. FLV was originally developed by Macromedia.

The format has quickly established itself as the format of choice for embedded video on the web. Notable users of the Flash Video format include YouTube, Hulu, Google Video, Yahoo! Video, metacafe, Reuters.com, and many other news providers.

The standards documentation for BBC Online states that the BBC now preferentially accepts Flash videos for submission, to the disadvantage of other formats previously in use on its sites^[3] such as RealVideo or WMV.

Flash Video FLV files usually contain material encoded with codecs following the Sorenson Spark or VP6 video compression formats. The most recent public releases of Flash Player also support H.264 video and HE-AAC audio. All of these codecs are currently restricted by patents.

Flash Video is viewable on most operating systems, via the Adobe Flash Player and web browser plugin or one of several third-party programs.

Flash Player

The Adobe Flash Player is a multimedia and application player originally developed by Macromedia and acquired by Adobe Systems. It plays SWF files which can be created by the Adobe Flash authoring tool, Adobe Flex, or a number of other Adobe Systems and third party tools. It has support for a scripting language called ActionScript, which can be used to display Flash Video from an SWF file. Because the Flash Player runs as a browser plug-in, it is possible to embed Flash Video in web pages and view the video within a web browser.

Format details

Commonly, Flash Video FLV files contain video bit streams which are a proprietary variant of the H.263 video standard,^[4] under the name of Sorenson Spark (FourCC FLV1).^[5][6] Sorenson Spark is an older codec for FLV files but it is also a widely available and compatible one, because it was the first video codec supported in Flash Player.^[7] It is the required video compression format for Flash Player 6 and 7.^[8] Flash Player 8 and newer revisions also support the playback of On2 TrueMotion VP6 video bit streams (FourCC VP6F or FLV4). On2 VP6 is the preferred video compression format for use with Flash Player 8 and higher.^[5][9] On2 VP6 can provide a higher visual quality than Sorenson Spark, especially when using lower bit rates. On the other hand it is computationally more complex and therefore will not run as well on certain older system configurations.^[8][10]

Flash Player 9 Update 3, released on December 3, 2007,^[11] also includes support for the new Flash Video file format F4V and H.264 video standard (also known as MPEG-4 part 10, or AVC) which is even more computationally demanding, but offers significantly better quality/bitrate ratio.^[12] Specifically, Flash Player now supports video compressed in H.264 (MPEG-4 Part 10), audio compressed using AAC (MPEG-4 Part 3), the F4V, MP4 (MPEG-4 Part 14), M4V, M4A, 3GP and MOV multimedia container formats, 3GPP Timed Text specification (MPEG-4 Part 17) which is a standardized subtitle format and partial parsing support for the 'ilst' atom which is the ID3 equivalent iTunes uses to store metadata. MPEG-4 Part 2 video (e.g. created with DivX or Xvid) is not supported.^[13] In an interview with BBC News, the main programmer of Flash Jonathan Gay said that the company had wanted to use H.264 when video support was originally added to Flash, but had been deterred by the patent licensing fees of around $5million (£3.5million) per year.^[14]

The Flash Video FLV file format supports two versions of a so called 'screenshare' (Screen video) codec which is an encoding format designed for screencasts. Both these formats are bitmap tile based, can be lossy by reducing color depths and are compressed using zlib. The second version is only playable in Flash Player 8 and newer.

Audio in Flash Video files is usually encoded as MP3. However, audio in Flash Video FLV files recorded from the user's microphone use the proprietary Nellymoser Asao Codec.^[15] (Flash Player 10 released in 2008 also supports the open source Speex codec.^[16]) FLV files also support uncompressed audio or ADPCM format audio. Recent versions of Flash Player 9 support AAC (HE-AAC/AAC SBR, AAC Main Profile, and AAC-LC).^[17]

Support for encoding Flash Video files is provided by an encoding tool included with Adobe's Flash Professional and Creative Suite products, On2's Flix encoding tools, Sorenson Squeeze, FFmpeg and other third party tools.

File formats

Support for video in SWF file format was added in Flash Player 6, released in 2002. In 2003, Flash Player 7 added direct support for FLV file format. Because of restrictions in the FLV file format, Adobe Systems has created in 2007 new file formats listed below, based on the ISO base media file format (MPEG-4 Part 12). Flash Player does not check the extension of the file, but rather looks inside the file to detect which format it is.^[13][18] The new file formats are completely different from the older FLV file format. For example, F4V does not support Screen video, Sorenson Spark, VP6 video compression formats and ADPCM, Nellymoser audio compression formats.^[1][18] Authors of Flash Player strongly encourage everyone to embrace the new standard file format F4V (ISO base media file format). There are functional limits with the FLV structure when streaming H.264 or AAC which could not be overcome without a redesign of the file format. This is one reason why Adobe Systems is moving away from the traditional FLV file structure.^[18]

SWF files published for Flash Player 6 and later versions are able to exchange audio, video, and data over RTMP connections with the Adobe Flash Media Server. One way to feed data to Flash Media Server is from files in the FLV file format. Starting with SWF files created for Flash Player 7, Flash Player can play FLV file format directly (MIME type video/x-flv). Starting with SWF files created for Flash Player 9 Update 3, Flash Player can also play the new F4V file format.^[1]

Media type support

Supported media types in FLV file format:^[1]

• Video: On2 VP6, Sorenson Spark (Sorenson H.263), Screen video, H.264
• Audio: MP3, ADPCM, Linear PCM, Nellymoser, Speex, AAC, G.711 (reserved for internal use)

Supported media types in F4V file format:^[1]

• Video: H.264
• Images (still frame of video data): GIF, PNG, JPEG
• Audio: AAC, HE-AAC, MP3

• Use of the H.264 and AAC compression formats in the FLV file format has some limitations and authors of Flash Player strongly encourage everyone to embrace the new standard F4V file format.^[18]

FLV players

A FLV player is a type of media player that is used for playing Flash video from PC as well as from Internet websites. A FLV player can be used standalone, without the need of the Adobe Flash authoring or developmental tools. It can also be embedded in the website using Flash component or embeddable version of FLV player.

The following players support FLV files in their default installations:

Desktop-based

Microsoft Windows, Mac OS X, Unix-based

• Adobe Media Player (freeware)
• Media Player Classic (free software)
• MPlayer (free software)
• RealPlayer (freeware)
• VLC media player (free software)
• Winamp (freeware)

PDA-based

Windows Mobile, PalmOS-based

• The Core Pocket Media Player (open source software)

Delivery options

Flash Video files can be delivered in several different ways:

• As a standalone .FLV file. Although Flash Video files are normally delivered using a Flash player for control, the .FLV file itself is only playable with an FLV player. There are several third party players available.
• Embedded in an SWF file using the Flash authoring tool (supported in Flash Player 6 and later). The entire file must be transferred before playback can begin. Changing the video requires rebuilding the SWF file.
• Progressive download via HTTP (supported in Flash Player 7 and later). This method uses ActionScript to include an externally hosted Flash Video file client-side for playback. Progressive download has several advantages, including buffering, use of generic HTTP servers, and the ability to reuse a single SWF player for multiple Flash Video sources. Flash Player 8 includes support for random access within video files using the partial download functionality of HTTP, sometimes this is referred to as streaming. However, unlike streaming using RTMP, HTTP "streaming" does not support real-time broadcasting. Streaming via HTTP requires a custom player and the injection of specific Flash Video metadata containing the exact starting position in bytes and timecode of each keyframe. Using this specific information, a custom Flash Video player can request any part of the Flash Video file starting at a specified keyframe. For example, Google Video, Youtube, and BitGravity support progressive downloading and can seek to any part of the video before buffering is complete. The server-side part of this "HTTP pseudo-streaming" method is fairly simple to implement, for example in PHP, as an Apache module, or using lighttpd.
• Streamed via RTMP to the Flash Player using the Flash Media Server (formerly called Flash Communication Server), VCS, ElectroServer, Helix Universal Server, Wowza Pro, WebORB for .NET, WebORB for Java, erlyvideo (in erlang), or the open source Red5 server. As of April 2008, there are stream recorders available for this protocol, re-encoding screencast software excluded.

Flash video recording

It is possible to record online flash videos using stream recorders and screen video capture software. The former gives lossless results, while the latter allows recording any video even when anti-leeching measures are used.

See also

• Local Shared Object
• SWFObject
• Comparison of media players
• Adobe Flash Player

References

External links

• Flash Developer Center - Flash Video Articles
• Flash Video (FLV) File Format Specification (adobe.com)
• List of codecs supported by Adobe Flash Player versions from 6 to 9 (adobe.com)
• Macromedia Flash FLV Video File Format (Library of Congress)