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Compression Formats

The most commonly used compression formats are AVI, QuickTime, and the MPEG family of standards:

• AVI (Audio Video Interleave) is Microsoft's Video for Windows standard for storing video and audio information. Video for Windows does not require any special hardware, making it the lowest common denominator for multimedia applications. AVI and Video for Windows were developed for playback of audio and video from hard disks and CD-ROMs on personal computers. They are adequate for downloading a video file from the internet for subsequent playback from the computer's hard drive, but they are not well suited for real-time or streaming video playback over networks.
• QuickTime 4 is Apple's video format standard. One of the strengths of QuickTime 4 is that it supports a broad range of formats for video, audio, text, and its own brand of MIDI music. It also supports VR imaging, MP3 and Flash. Since QuickTime 4 is compatible with so many digital media formats, many Macintosh and Windows content creation packages include the QuickTime 4 format as a standard feature.
• MPEG (Moving Picture Experts Group) refers to a family of digital video compression standards and file formats. MPEG generally produces better-quality video than competing formats, such as Video for Windows, Indeo and QuickTime. MPEG files can be decoded by special hardware or by software. MPEG achieves high compression rate by storing only the changes from one frame to another, instead of all the data on each frame. MPEG uses lossy compression, but the loss is generally undetectable. MPEG currently has two major standards in use:
  • MPEG-1, which provides a video resolution of 352 x 240 at 30 frames per second (fps). This produces video quality slightly below the quality of conventional VCR videos. MPEG-1 is designed primarily for CD-ROM-based applications, and is optimized for approximately 1.5 Mbits/sec combined data rates.
  • MPEG-2, offers resolutions of 720 x 480 and 1280 x 720 at 60 fps, with full CD-quality audio. This is sufficient for all the major TV standards, including NTSC, and even HDTV. MPEG-2 is used by DVD-ROMs. While decompressing an MPEG-2 data stream requires only modest computing power, encoding video in MPEG-2 format requires significantly more processing power.

A new version of MPEG called MPEG-4 (there is no MPEG-3) is still being defined at the time of writing (early 2001). Tools based on the MPEG-4 standard may become the future platform for computer music, audio for gaming, streaming Internet radio, and other multimedia applications. However, this standard is still under discussion.

Compression Types

Compression types are divided into lossless and lossy.

• Lossless Compression means either that no information is lost during the compression process or that so little data is lost that the viewer cannot detect it. The GIF format is a lossless image and video (GIF89a or animated GIF) compression format.
• Lossy Compression is where information is lost during the compression process. JPEGs and MPEGs are lossy compression formats. The degree of loss ranges from undetectable to highly objectionable depending on how aggressively you need to compress the file. Sharp edges and straight lines become increasingly blurry the more you compress the video.