Compression

When image or sound is changed o digital data, it becomes extremely large.  To handle this problem, images and sound are compressed to increase storage space and improve transmission.  The following are the various forms of compression.

1. JPEG
 
JPEG stands for joint photographic experts group and is an ISO 10918 standard that has been enacted the C-Cub Microsystem.  JPEG is an image compression technology that is based on the discrete cosine transform (DCT) algorithm.  JPEG has the following objectives:  

• Compression rate and image technology should be the most up to date.
• Image compression transmission technology should be used extensively.
• Image compression should be simple and economical.    

2. MJPEG
 
MJPEG stands for motion joint photographic experts group.  It is the consecutive collection of JPEG images combined with time data.  It is the intermediate format between moving images and still images.

3. H.263

H.263 is a moving image standard suggested by ITU-T.  It is a compression method for remote business conferences and video telephones.  It has a low transmission rate (64kps) and thus is suitable for low speed communication networks.  The H.263 standard is nearly identical to the H.261 standard (black and white image compression transmission) but the performance and error recovery of H.263 has been improved.

Features of H.263

• Hybrid inter-picture prediction
• Coding transformation
• Motion compensation
• Half pixel precision
• Unrestricted motion vectors
• Syntax based arithmetic coding
• Advanced prediction
• Forward and backward frame prediction 

  4. MPEG

MPEG stands for moving pictures experts group and is the standard for compression of video and audio.  

MPEG-1

MPEG-1 is designed for coding of progressive video which can operate on networks of 1.5Mbps transmission.  It is used in Video CD (VCD) and CD-I.  The very popular MP3 format is an audio layer of MPEG-1 (MPEG-1 layer 3).

MPEG-2

MPEG-2 is designed for coding of interlace images which operate at more than 4Mbps of transmission speed.  It is used in digital TV broadcasting and in the DVD format. 

MPEG-3

MPEG-3 is designed for HDTV and has been integrated into the MPEG-2 standard.

Coding algorithm for MPEG-1/MPEG-2/MPEG-3

• DCT (Discrete Cosine Transform)
• Two way communication
• Huffman coding
• Advanced prediction
• Forward and backward frame prediction

MPEG-4

MPEG-4 deals with graphics and composite sounds created by computers.

MPEG-4 applications:

• Digital television
• Interactive graphics applications
• Interactive multimedia (World Wide Web, distribution of and access to content)

5. Wavelet

Wavelet compression is used where the images do not need to be reproduced exactly as that of the originals.  This technology allows a very high rate of compression while still maintaining an acceptable level of image quality.  Wavelet compression is widely used in various fields such as mathematics, electricity, physics, medicine and communication.