In order to decide upon the format, encoding, quality, bitrate, keyframes and the method by which the video clip will be viewed in any online media presentation, we must first consider the target audience’s requirements. These requirements are screen resolution, browser requirements, compatibility (e.g. lack of support for Flash on target devices) and bandwidth limitations. Once the target audience’s requirements are decided the file format, the encoding method and the media player can be established.
The major available file formats (sometimes referred to as container formats) for video clips for online multimedia presentations are, according to Bluejay (2010): .mp4, .flv, or .avi. Any of these formats are suitable for streaming video over the web in a presentation and, unfortunately, none of them are suitable for all purposes. Since October 2010, Apple’s browser, Safari, has not had the ability to play any Adobe Flash media pre-installed and all their increasingly popular mobile devices, specifically the iPad which is more suited to using as a computer replacement for online multimedia presentations, do not support flash in any way. Therefore depending on the target use environment, it may be necessary to allow the user to choose from the most suitable (i.e. compatible) format.
Within the .mp4 file format there are two video compression formats that can be used H.264 or MPEG-4. The former, H.264, is the preferred method as it gives a better picture and quality against file wsize when compared to MPEG-4 with the only disadvantage being that, as it takes longer to encode than MPEG-4 on some devices however as Bluejay (2010) states “in reality the higher processor use is combined with a smaller file size, so these likely cancel each other out”.
The .flv file format, Blueberry Software (2011) state that the plug-in required is “installed in virtually all internet browsers, so this format is widely compatible with PC and Macs”, is usually encoded with the H.264 compression format, however there are others such as VP6, Sorenson Spark or, more interestingly and relevant to the evaluation below ‘ScreenVideo’ compression, which was specifically developed for screen recordings to produce very high quality, but larger, video files.
The .avi file format may use many different types of compression technique which needs to be carefully evaluated as the codec chosen may also be necessary to play back the video. The target system may be unable to play back the .avi file if it does not have the correct codec installed. This format does not compete with the others for screen recording video presentations in terms of quality and file size.
No matter which file format and video compression is used, quality (i.e. resolution) also affects video clips with a simple rule: the higher the quality, the larger the file size. Therefore acceptable viewing quality is an important consideration when considering usability versus download speed. Bitrate is a similar quality factor as a higher bitrate means higher quality but larger file sizes. The encoder will try to attain the quality set at the bitrate specified, but if the quality is higher than can be achieved at the specified bitrate, the encoder will increase the bitrate to reach the quality (i.e. quality takes priority over bitrate). The number of keyframes also affects quality so this should be minimised for speed or maximised for quality.
In conclusion, the video clip should choose the correct balance of the above formats and compression techniques for the target audience. This is often a compromise given limited resources of one kind or another to produce acceptable, rather than perfect results.
Blueberry Software (2011) Choosing a movie format [Online]. Available at http://www.bbsoftware.co.uk/BBFlashBack/Support/UserGuides/MovieFormats.aspx (Accessed 16 Mar 2011).
Bluejay, M (2010) Getting video onto your website: Web video demystified [Online]. Available at http://websitehelpers.com/video/ (Accessed 16 Mar 2011).
W3 Schools (2011) Browser Display Statistics [Online]. Available at http://www.w3schools.com/browsers/browsers_display.asp (Accessed 16 Mar 2011).
W3 Schools (2011) Multimedia Video Formats [Online]. Available at http://www.w3schools.com/media/media_videoformats.asp (Accessed 16 Mar 2011).