Multimedia data typically means digital images, audio, video, animation and graphics together with text data. The acquisition, generation, storage and processing of multimedia data in computers and transmission over networks have grown tremendously in the recent past.

This astonishing growth is made possible by three factors. Firstly, personal computers usage becomes widespread and their computational power gets increased. Also technological advancements resulted in high-resolution devices, which can capture and display multimedia data (digital cameras, scanners, monitors, and printers). Also there came high-density storage devices. Secondly high-speed data communication networks are available nowadays. The Web has wildly proliferated and software for manipulating multimedia data is now available. Lastly, some specific applications (existing) and future applications need to live with multimedia data. This trend is expected to go up in the days to come.

Multimedia data are blessed with a number of exciting features. They can provide more effective dissemination of information in science, engineering , medicine, modern biology, and social sciences. It also facilitates the development of new paradigms in distance learning, and interactive personal and group entertainment.

The huge amount of data in different multimedia-related applications warranted to have databases as databases provide consistency, concurrency, integrity, security and availability of data. From an user perspective, databases provide functionalities for the easy manipulation, query and retrieval of highly relevant information from huge collections of stored data.

MultiMedia Databases (MMDBs) have to cope up with the increased usage of a large volume of multimedia data being used in various software applications. The applications include digital libraries, manufacturing and retailing, art and entertainment, journalism and so on. Some inherent qualities of multimedia data have both direct and indirect influence on the design and development of a multimedia database. MMDBs are supposed to provide almost all the functionalities, a traditional database provides. Apart from those, a MMDB has to provide some new and enhanced functionalities and features. MMDBs are required to provide unified frameworks for storing, processing, retrieving, transmitting and presenting a variety of media data types in a wide variety of formats. At the same time, they must adhere to numerical constraints that are normally not found in traditional databases.

An MMDB needs to manage several different types of information pertaining to the actual multimedia data. They are:

• Media data - This is the actual data representing images, audio, video that are captured, digitized, processes, compressed and stored.

• Media format data - This contains information pertaining to the format of the media data after it goes through the acquisition, processing, and encoding phases. For instance, this consists of information such as the sampling rate, resolution, frame rate, encoding scheme etc.

• Media keyword data - This contains the keyword descriptions, usually relating to the generation of the media data. For example, for a video, this might include the date, time, and place of recording , the person who recorded, the scene that is recorded, etc This is also called as content descriptive data.

• Media feature data - This contains the features derived from the media data. A feature characterizes the media contents. For example, this could contain information about the distribution of colors, the kinds of textures and the different shapes present in an image. This is also referred to as content dependent data.

  The last three types are called meta data as they describe several different aspects of the media data. The media keyword data and media feature data are used as indices for searching purpose. The media format data is used to present the retrieved information.

Many inherent characteristics of multimedia data have direct and indirect impacts on the design of MMDBs. These include : the huge size of MMDBs, temporal nature, richness of content, complexity of representation and subjective interpretation. The major challenges in designing multimedia databases arise from several requirements they need to satisfy such as the following:

1. Manage different types of input, output, and storage devices. Data input can be from a variety of devices such as scanners, digital camera for images, microphone, MIDI devices for audio, video cameras. Typical output devices are high-resolution monitors for images and video, and speakers for audio.
2. Handle a variety of data compression and storage formats. The data encoding has a variety of formats even within a single application. For instance, in medical applications, the MRI images of brain has lossless or very stringent quality of lossy coding technique, while the X-ray images of bones can be less stringent. Also, the radiological image data, the ECG data, other patient data, etc. have widely varying formats.
3. Support different computing platforms and operating systems. Different users operate computers and devices suited to their needs and tastes. But they need the same kind of user-level view of the database.
4. Integrate different data models. Some data such as numeric and textual data are best handled using a relational database model, while some others such as video documents are better handled using an object-oriented database model. So these two models should coexist together in MMDBs.
5. Offer a variety of user-friendly query systems suited to different kinds of media. From a user point of view, easy-to-use queries and fast and accurate retrieval of information is highly desirable. The query for the same item can be in different forms. For example, a portion of interest in a video can be queried by using either




1) a few sample video frames as an example,

2) a clip of the corresponding audio track or

3) a textual description using keywords.

6. Handle different kinds of indices. The inexact and subjective nature of multimedia data has rendered keyword-based indices and exact and range searches used in traditional databases ineffective. For example, the retrieval of records of persons based on social security number is precisely defined, but the retrieval of records of persons having certain facial features from a database of facial images requires, content-based queries and similarity-based retrievals. This requires indices that are content dependent, in addition to key-word indices.
7. Develop measures of data similarity that correspond well with perceptual similarity. Measures of similarity for different media types need to be quantified to correspond well with the perceptual similarity of objects of those data types. These need to be incorporated into the search process
8. Provide transparent view of geographically distributed data. MMDBs are likely to be a distributed nature. The media data resides in many different storage units possibly spread out geographically. This is partly due to the changing nature of computation and computing resources from centralized to networked and distributed.
9. Adhere to real-time constraints for the transmission of media data. Video and audio are inherently temporal in nature. For example, the frames of a video need to be presented at the rate of at least 30 frames/sec. for the eye to perceive continuity in the video.
10. Synchronize different media types while presenting to user. It is likely that different media types corresponding to a single multimedia object are stored in different formats, on different devices, and have different rates of transfer. Thus they need to be periodically synchronized for presentation.

The recent growth in using multimedia data in applications has been phenomenal. Multimedia databases are essential for efficient management and effective use of huge amounts of data. The diversity of applications using multimedia data, the rapidly changing technology, and the inherent complexities in the semantic representation, interpretation and comparison for similarity pose many challenges. MMDBs are still in their infancy. Today's MMDBs are closely bound to narrow application areas. The experiences acquired from developing and using novel multimedia applications will help advance the multimedia database technology.