In this unit we are discussing about the data compression which is very important for multimedia applications because uncompressed graphics, audio, and video data require considerable storage capacity. Compressing a data file reduces its size, and hence it can be easily transferred over the digital network. The amount of compression depends on the compression method and compression rate. The terms compression and coding are treated as synonyms. The different compression techniques available and their classification is also given in this unit.
4.2 Data compression
As already we know the multimedia applications makes use of various components like text, graphics art, audio, animation and video. All these are stored in the form of files and it contains binary numbers 0 and 1. Uncompressed graphics , audio and video data files require considerable storage capacity, in case of uncompressed video the requirement is even more, and given the today's technology it is not even possible. The same is true for multimedia communications, where different files are to be transferred to various locations on the network. Data transfer uncompressed video on the digital network requires very high bandwidth to be provided for a single point - to - point communication. To provide feasible and cost effective solutions, most multimedia systems handle compressed digital video and audio data streams. Compressing a data file reduces its size, and hence it can be easily transferred over the digital network. The amount of compression depends on the compression method and compression rate. The terms compression and coding are treated as synonyms.
There are many compression techniques for multimedia information compression. They can be classified in many ways according to different criteria. Some classification is based on the compression algorithms used in the compression techniques. Some classify them based on the results of the compression techniques. The second type of classification is more useful to end users and developers who design and develop systems to support compressed multimedia data. Two classifications in this category are whether the original data can be reconstructed exactly after using a compression technique, and whether the output of a compression system is of constant bit rate.
If the original data can be reconstructed exactly after using compression technique , we call this compression technique lossless, otherwise, it is lossy.
Lossless compression techniques are normally used for compressing computer programs and legal and medical documents where no error or loss is allowed . Lossless compression techniques exploit only data statistics ( Data redundancy). The compression ratio achievable is normally low.
Lossy compression techniques are normally used for compressing digital audio, image, and video in most multimedia applications where some errors or loss can be tolerated. They use both data statistics and human perception properties. Thus , they can produce very high compression ratio.
4.2.1 Storage space and Compression requirements
We use text, graphics, audio, and video in multimedia projects. Images have considerably higher storage requirements than text, audio and video have even more higher storage space than the images. Not only the huge amount of storage required , but the data rates for the communication of continuous media are also important. Here are the some specifications which gives us an idea about how much storage space is required.
•For the presentation of text, two bytes are used for each character, each...