Video Compression

"Video compression is like making concentrated orange juice: water is removed from the juice to more easily transport it, and added back later by the consumer."

By Elizabeth Cheesbrough

In Video Compression for Multimedia , Jan Ozer describes Video Compression as "a process where a collection of algorithms and techniques replaces the original pixel-related information with more compact mathematical descriptions." Decompression, then, is "the reverse process of decoding the mathematical descriptions back to pixels for ultimate display." (p. 35) Raw images require so much storage that some type of data compression is used for storage. For example, a 1024 x 1024 24 bit image require 3 Mbytes of space. A 640 x 480 24 bit image requires 1 Mbyte of space so a video that runs at 640 x 480 resolution with 24 bits of color and 30 frames per second (fps) would require 30 Mbytes of storage per second. Some nice overview of Video Compression can be found at the following sites:

Where audio is concerned, the market quickly realized that digital storage media are qualitatively better than analog. Audio CDs are now standard. For video, however, the differences in quality have yet to be defined. Which quality (and thus which method) is right for a specific application? It will take a while before digital video is as widely accepted as digital audio, specifically since an affordable solution has yet to be found. One of the keys to the technology is the compression of digital video.

Video compression is almost always a "lossy" procedure. This means that the decompressed file is not an exact duplicate of the initial uncompressed file. Some data is lost in the compression/decompression process.

There are two types of data compression algorithms, one that maintains data integrity and one that does not. Lossless compression maintains data integrity, i.e., after compression and decompression, the resulting image is exactly the same as the initial uncompressed image. These techniques are also used for compressing data files and executable program files. Lossy compression does not maintain data integrity, i.e., the uncompressed image is similar to but not exactly the same as the initial image. There is a tradeoff between amount of data lost and the amount of compression. Lossy compression methods provide a higher compression ratio than lossless methods.

According to Ozer, "the true measure of a compression technology is how little you notice its presence, or how effectively it can reduce video data rates without adversely affecting video quality." (p. 14)

Both software and hardware tools may be used to accomplish video compression. The software tools are called "codecs" an abbreviation of coder/decoder or compressor/decompressor.

For video, there are two types of compression - intraframe and interframe . Intraframe compression occurs within a single frame, while interframe compression involves the elimination of redundant information across multiple frames. More explaination of this types of compression with examples can be found at the following site.

Another characteristic of data compression methods is symmetric versus assymetric. A symmetric method compresses and decompresses images in the same amount of time. This type of technique would be used for data transmission as in interactive video. An assymetric technique might take a long time to compress an image but the decompression would be rapid. Assymetric techniques can achieve much higher data compression ratios than symmetric techniques. Video compression is a key element required for establishing a point-to-point digitized video connection. Compression is required for reducing the vast amount of digitized video data (about 0.5 GB/min. of video) into a more manageable size, allowing the storage and transmission of video within the limitations of today's technology storage devices and communication channels.

Some Popularly known Compression Techniques are:

However, currently the standardized compression algorithms for the video are:

One of the most challenging job is to select a proper compression/decompression technology. Here are some of the guidelines and other helpful information on how to evaluate a compression / decompression technique . A much more detailed comparision of Compression Techniques can be found here.

Video conferencing imposes an additional, and very demanding, requirement on video compression. Video compression and decompression must be performed on-the-fly in order to maintain the synchronization with audio and to provide a live point-to-point connection.

Resources on the Web.