How To Understand Video Compression


Video technology was first developed for cathode ray tube television systems, but several new technologies for video display devices have since been invented. Standards for television sets and computer monitors have tended to evolve independently, but advances in computer performance and digital television broadcasting and recording are resulting in a convergence of standards and use of content. Computers can now display television and film-style video clips and streaming media, encouraged by increased processor speed, storage capacity, and broadband access to the Internet.

If you’ve ever worked with still images, and have seen the difference in storage size between a comparable bits mapped image, BMP, and a JPEG Joint Photographic Experts Group picture, you’ve started to understand the reason for video compression. By discarding redundant information, compression can save storage space and the bandwidth needed to transmit an image. To store, process and transmit all that information in an uncompressed manner would take enormous resources.

Compression

Compression is the conversion of data to a format that takes fewer bits. The size of data in compressed form , C relative to the original size, O is known as the compression ratio R in the formula R=C/O. When you back up your computer data files and compress them you simply eliminate redundant information. If you can restore them to an identical state, the process is called lossless compression. When it comes to images, including video, you can only generate an approximation of the original image. This process is called lossy compression.

Interframe and Intraframe compression techniques

In video there are two additional forms of compression, intra frame, sometimes called spatial, and inter frame, sometimes called temporal. The most popular intra frame compression is called Discrete Cosine Transform (DCT). It’s used in the Digital Betacam, the DV format, D-50 and in JPEG’s used primarily in stills, but also M-JPEG, motion JPEG.

The inter frame compression is based on a Group of Pictures, a GOP which repeats every half second. A GOP is comprised of I frames which have the full amount of data after color and intra frame compression, but all the other frames in a GOP are virtual. After the I frame, the next frame to be encoded is a P or predictive frame, which anticipates changes in the video, discarding redundant information. Between the I and P frames, B or bi-directional frames fill in, looking both backwards and forwards.

Video Codec

A video codec is a device or software that enables video compression and/or decompression for digital video. The compression usually employs lossy data compression.

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About the Author: Justin Belden is a freelance web & graphic designer with over 15 years' experience. He is also an Avid member of the Design/Development community and a Serial Blogger who loves to help people by sharing interesting and informative tips and trick related to computer and technology.

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