JPEG, MPEG

JPEG and MPEG are the abbreviated names of two standards groups that fall under both the International Organization for Standardization and the International Electrotechnical Commission. JPEG is the Joint Photographic Expert Group, which works on the standards for compression of still digital images. MPEG is the Moving Picture Expert Group, which works on the standards for compression of digital movies. JPEG and MPEG are also the names for the standards published by the respective groups.

Digital images or video are compressed primarily for two reasons: to save storage space for archival, and to save bandwidth in communication, which saves time required to send the data. A compressed digital image or video can be drastically smaller than its original size and saves a great deal of storage space. Upon retrieval, the data can be decompressed to get back the original image or video. When one needs to transmit the data through a channel of a certain bandwidth, the smaller data size also saves transmission time.

There are many different compression algorithms , each with its own characteristics and performance. The compressed image or video may be a disk file, or a signal in a transmission channel. People need compression standards for interoperability. As long as the data meet the appropriate compression standards, one can display the image or play the video on any system that would observe the same standards.

JPEG

JPEG was proposed in 1991 as a compression standard for digital still images. JPEG compression is lossy , which means that some details may be lost when the image is restored from the compressed data. JPEG compression takes advantage of the way the human eyes work, so that people usually do not notice the lost details in the image. With JPEG, one can adjust the amount of loss at compression time by trading image quality for a smaller size of the compressed image.

JPEG is designed for full-color or grayscale images of natural scenes. It works very well with photographic images. JPEG does not work as well on images with sharp edges or artificial scenes such as graphical drawings, text documents, or cartoon pictures.

A few different file formats are used to exchange files with JPEG images. The JPEG File Interchange Format (JFIF) defines a minimum standard necessary to support JPEG, is widely accepted, and is used most often on the personal computer and on the Internet. The conventional file name for JPEG images in this format usually has the extension ~.JPG or ~.JPEG (where ~. represents a file name).

A recent development in the JPEG standard is the JPEG 2000 initiative. JPEG 2000 aims to provide a new image coding system using the latest compression techniques, which are based on the mathematics of wavelets. The effort is expected to find a wide range of applications, from digital cameras to medical imaging.

MPEG

MPEG was first proposed in 1991. It is actually a family of standards for compressed digital movies, and is still evolving. One may think of a digital movie as a sequence of still images displayed one after another at video rate. However, this approach does not take into consideration the extensive redundancy from frame to frame. MPEG takes advantage of that redundancy to achieve even better compression. A movie also has sound channels to play synchronously with the sequence of images.

The MPEG standards actually consist of three main parts: video, audio, and systems. The MPEG systems part coordinates between the video and audio parts, as well as coordinating external synchronization for playing.

The MPEG-1 standard was completed in 1993, and was adopted in video CDs. MP3 audio, which is popular on the Internet, is actually an MPEG-1 standard adopted to handle music in digital audio. It is called MP3 because it is an arrangement for MPEG Audio Layer 3. The MPEG-2 standard was proposed in 1996. It is used as the format for many digital television broadcasting applications, and is the basis for digital versatile disks (DVDs), a much more compact version of the video CD with the capacity for full-length movies. The MPEG-4 standard was completed in 1998, and was adopted in 1999. MPEG-4 is designed for transmission over channels with a low bit rate . It is now the popular format for video scripts on the Internet.

These MPEG standards were adopted in a common MPEG format for exchange in disk files. The conventional file name would have the extension ~.MPG or ~.MPEG (where ~ represents a file name). MPEG-7, completed in 2001, is called the Multimedia Content Description Interface. It defines a standard for the textual description of the various multimedia contents, to be arranged in a way that optimizes searching. MPEG-21, called the Multimedia Framework, was started in June 2000. MPEG-21 is intended to integrate the various parts and subsystems into a platform for digital multimedia.

see also Graphic Devices; Music, Computer; World Wide Web.

Peter Y. Wu