VIDEO COMPRESSION (MPEG)
MPEG (Moving Pictures Experts Group) is a compression standard for digital video sequences, such as used in computer video and digital television network. In addition, MPEG also provides for the compression of the sound track associated with the video. It derives its name from its originating organization, the Moving Pictures Experts Group. If you think JPEG is complicated, MPEG is a nightmare! MPEG is something you buy, not try to write yourself. The future of this technology is to encode the compression and uncompression algorithms directly into integrated circuits. The potential of MPEG is vast. Think of thousands of video channels being carried on a single optical fibre running into your home. This is a key technology of the 21st century.
Besides reducing the data rate, MPEG have several important features. The movie can be played forward or in reverse, and at either normal or fast speed. The encoded information is random access that is, any individual frame in the sequence can be easily displayed as a still picture. This goes along with making the movie editable, meaning that short segments from the movie can be encoded only with references to themselves, not the entire sequence. MPEG is designed to be robust to errors. The last thing you want is for a single bit error to cause a disruption of the movie.
The approach used by MPEG can be divided into two types of compression: within the frame and between frame. Within-the-frame compression means that individual frames making up the video sequence are encoded as if they were ordinary still images. This compression is performed using the JPEG standard, with just a few variations. In MPEG terminology, a frame that has been encoded in this way is called an intra-coded or I-picture (intrapicture).
Most of the pixels in a video sequence change very little from one frame to the next. Unless the camera is moving, most of the image is composed of a background that remains constant over dozens of frames. MPEG tapes advantage of this with a sophisticated form of delta encoding to compress the redundant information between frames. After compressing one of the frames as an I-picture, MPEG encodes successive frames as predictive-coded or P-pictures. That is, only the pixels that have changed since the I-pictures are included in the P-picture.
While these two compression schemes form the backbone of MPEG, the actual implementation is immensely sophisticated than described here. For example, a P-picture can be referenced to a Z-picture that has been shifted accounting for motion of objectives in the image sequence. There are also bidirectional predictive-coded or B-pictures. There are referenced to both a previous and a future I-picture. This handles regions in the image that gradually change over many of frames. The individual frames can also be stored out-of-order in the compressed data to facilitate the proper sequencing of the, P, and B-pictures. The addition of colour and sound makes this all the more complicated.
The main distortion associated with MPEG occurs when large sections of the image change quickly. In effect, a burst of information is needed to keep up with the rapidly changing scenes. If the data rate is fixed, the viewer notices "blocky" patterns when changing from one scene to the next. This can be minimized in the networks that transmit multiple video channels simultaneously such as cable television. The sudden burst of information needed to support a rapidly changing scene in one video channel, is averaged with the modest requirements of the relatively static scenes in the other channels.
MPEG Standards
Work on MPEG began in 1988. Since that time, a number of different MPEG standards have evolved. The current suite of MPEG standards includes the following:
•    MPEG-1: This was the first integrated audio and visual coding standard ever produced. It enables the coding of moving pictures and audio on digital storage media at up to about 1.5 Mbps. This standard is widely used. It includes Audio Layer 3, also known as MP3.
•      MPEG-2: MPEG-1 lacked the techniques necessary to handle standard broadcast interlaced video with good compression. These techniques were added in MPEG­2. MPEG-2 is widely used in digital television set top boxes and DVD's.
• MPEG-4: The original goal of MPEG-4 was to provide an audio-visual coding standard for very-low-bit-rate applications, such as mobile applications and nonbroadband Internet access. It has evolved to a powerful and flexible set of capabilities for a variety of application areas. A key capability of MPEG-4 is the coding of individual objects. This allows users to interact with objects within a scene, whether they derive from "real" sources such as moving video, or from synthetic sources, such as computer-aided design output or computer­generated cartoons.
•     MPEG-7: This standard is entitled Multimedia Content Description Interface. Unlike the three previous standards, MPEG-7 is not concerned with the techniques for encoding audio-visual data. Rather it is concerned with providing information about the content of MPEG-encoded audio-visual data. MPEG-7 allows fast and flexible searching for material that a user is interested in.
• MPEG-21: This is a multimedia framework standard that seeks to describe an environment capable of supporting the delivery and use of all content types by different categories of users in multiple application domains.

AUDIO COMPRESSION (MP3)
MPEG not only defines how video is compressed, but it also defines a standard for compressing audio. This standard can be used to compress the audio portion of a movie, or it can be used to compress stand-alone audio.
To understand audio compression, we need to begin with the data. CD-quality audio, which is the de facto digital representation for high-quality audio, is sampled at the rate of 44.1 kHz (i.e. a sample is collected approximately once every 23 ~ts). Each sample is 16 bits, which means that a stereo (2-channel) audio stream results in a bit rate of 2x44.1x 1000x16=1.41Mbps
By comparison, telephone-quality voice is sampled at a rate of 8 kHz, with 8-bit samples, resulting in a bit rate of 64 Kbps, which is not coincidentally the speed of an ISDN link.
Clearly, some amount of compression is going to be required to transmit CD­quality audio over, say, the 128-Kbps capacity of an ISDN data/voice line pair. To make matters worse, synchronization and error correction overhead require that 49 bits be used to encode each 16-bit sample, resulting in an actual bit rate of
49/16 x 1.41 Mbps = 4.32 Mbps
MPEG addresses this need by defining three levels of compression, as shown in Table 4.1.

Layer III, widely known as MP3, is the most commonly used level. To achieve these compression ratios, MP3 uses techniques that are similar to those used by MPEG to compress video.
MP3 stands for MPEG layer 3 audio compression. The layer 3 compression option can reduce the bit rate of an audio signal by a factor of 12 with a very lowless in sound quality. MP3 software for compression and playback can be downloaded from various websites. High-quality audio MP3 files can be created from music CD's. So MP3 has become very popular for "recording" selection from CD's in personal computers. The MP3 phenomenon has led to the problem of illegal distribution of copyrighted music materials over the Internet. The recording industry is working on the development of secure digital recording methods.

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