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Introduction
Video has been an important medium for communications and entertainment for many decades. Video technology describes the means for electronically reconstructing a sequence of images. The creation and composition of the sequence of still images involve basic functions such as capturing, recording, and processing. Video technology has evolved rapidly over the last 30 years. Initially video was captured and transmitted in analog form. The success of the analog videotape format left its mark on the entire 1980s decade. However, video technology, from the 1980s and on, followed the general trend toward “everything digital” due to the advent of digital integrated circuits and computers. Nowadays video can be delivered via communication networks and appropriate storage discs (e.g., digital video discs). The major advantages of digital video format are that it is not susceptible to degradation of quality over the years, it is editable, and it can be easily integrated with audio, text, and graphics. In addition, it can be transmitted without loss in quality, depending on the physical medium (cable, air, etc.). The quality of the digital video is based on three key factors: frame rate, color depth, and frame resolution. Frame rate is the number of images displayed per second that provokes the perception of motion, for example, the National Television Standards Committee (NTSC) standard for full motion is 30 frames per second. Color depth is the number of bits per pixel for color information representation, for example, 24 bits can represent 16,777,216 colors (256 x 256 x 256). Frame resolution is the display dimension measured in pixels, represented by the number of horizontal pixels times the number of vertical pixels, for example, 800 x 600. Typical digital video formats,analogous to the component analog video, are RGB and YCbCr, where Y represents a pixel brightness and CbCr pure color. Because the eye is less sensitive to color than brightness, chrominance is usually subsampled, and thus represented by fewer bits (e.g., 4:2:2) to reduce the size of the digital output. However, subsampling is not enough. Further techniques that cut down the size of digital output are scaling of the aforementioned key factors (e.g., reduce frame rate to 15 frames per second) and compression. Once digital video has been produced, it can be either presented asynchronously through the progressive (or not) download-store-present method or streamed across a communication network. Video streaming takes advantage of advances in video scaling, compression techniques, and protocols that help restore the temporal relationships within each stream containing the media information, or synchronize various streams of multimedia information. The major advantage of video streaming is that the video is reproduced at the media client while it is being downloaded; therefore, it is not stored at the client. In addition, because the file is not downloaded, copyright is protected.
Technical Overview
The MPEG-2 video compression algorithm achieves high rates of compression by exploiting the redundancy in video information. MPEG-2 takes advantage of both the temporal redundancy and spatial redundancy present in motion video. Temporal redundancy appears when successive frames of the video display images of the same scene. The temporal redundancy is based on the fact that the content of the scene usually remains fixed or changes only slightly between successive frames. Spatial redundancy occurs because small pieces of the picture, which are called pels, are often replicated (with minor changes) within a single frame of video. MPEG-2 includes a wide range of compression mechanisms. The MPEG-2 encoders must therefore have the ability to choose the proper compression mechanism best suited to a particular scene/sequence of scenes. In general, the more sophisticated the encoder, the more efficient it is at selecting the most appropriate compression mechanism, and therefore the higher the picture quality for a given transmission bit rate. On the other hand, the MPEG-2 decoders also come in various types and have varying capabilities, such as ability to handle high-quality video and cope with errors, jitters, and delays.
Interaction
An extremely important feature that led to the adoption of MPEG-2 in digital TV is the ability to use a return channel to allow the user to control the content or scheduling of the transmitted video/audio/data. This feature is known as interaction and has become the key discriminator between traditional video and MPEG-2. MPEG-2 defines an interaction channel using the digital storage media command and control (DSM-CC) toolkit. Interaction channels may be used for diverse services, including:
Video has been an important medium for communications and entertainment for many decades. Video technology describes the means for electronically reconstructing a sequence of images. The creation and composition of the sequence of still images involve basic functions such as capturing, recording, and processing. Video technology has evolved rapidly over the last 30 years. Initially video was captured and transmitted in analog form. The success of the analog videotape format left its mark on the entire 1980s decade. However, video technology, from the 1980s and on, followed the general trend toward “everything digital” due to the advent of digital integrated circuits and computers. Nowadays video can be delivered via communication networks and appropriate storage discs (e.g., digital video discs). The major advantages of digital video format are that it is not susceptible to degradation of quality over the years, it is editable, and it can be easily integrated with audio, text, and graphics. In addition, it can be transmitted without loss in quality, depending on the physical medium (cable, air, etc.). The quality of the digital video is based on three key factors: frame rate, color depth, and frame resolution. Frame rate is the number of images displayed per second that provokes the perception of motion, for example, the National Television Standards Committee (NTSC) standard for full motion is 30 frames per second. Color depth is the number of bits per pixel for color information representation, for example, 24 bits can represent 16,777,216 colors (256 x 256 x 256). Frame resolution is the display dimension measured in pixels, represented by the number of horizontal pixels times the number of vertical pixels, for example, 800 x 600. Typical digital video formats,analogous to the component analog video, are RGB and YCbCr, where Y represents a pixel brightness and CbCr pure color. Because the eye is less sensitive to color than brightness, chrominance is usually subsampled, and thus represented by fewer bits (e.g., 4:2:2) to reduce the size of the digital output. However, subsampling is not enough. Further techniques that cut down the size of digital output are scaling of the aforementioned key factors (e.g., reduce frame rate to 15 frames per second) and compression. Once digital video has been produced, it can be either presented asynchronously through the progressive (or not) download-store-present method or streamed across a communication network. Video streaming takes advantage of advances in video scaling, compression techniques, and protocols that help restore the temporal relationships within each stream containing the media information, or synchronize various streams of multimedia information. The major advantage of video streaming is that the video is reproduced at the media client while it is being downloaded; therefore, it is not stored at the client. In addition, because the file is not downloaded, copyright is protected.
Technical Overview
The MPEG-2 video compression algorithm achieves high rates of compression by exploiting the redundancy in video information. MPEG-2 takes advantage of both the temporal redundancy and spatial redundancy present in motion video. Temporal redundancy appears when successive frames of the video display images of the same scene. The temporal redundancy is based on the fact that the content of the scene usually remains fixed or changes only slightly between successive frames. Spatial redundancy occurs because small pieces of the picture, which are called pels, are often replicated (with minor changes) within a single frame of video. MPEG-2 includes a wide range of compression mechanisms. The MPEG-2 encoders must therefore have the ability to choose the proper compression mechanism best suited to a particular scene/sequence of scenes. In general, the more sophisticated the encoder, the more efficient it is at selecting the most appropriate compression mechanism, and therefore the higher the picture quality for a given transmission bit rate. On the other hand, the MPEG-2 decoders also come in various types and have varying capabilities, such as ability to handle high-quality video and cope with errors, jitters, and delays.
Interaction
An extremely important feature that led to the adoption of MPEG-2 in digital TV is the ability to use a return channel to allow the user to control the content or scheduling of the transmitted video/audio/data. This feature is known as interaction and has become the key discriminator between traditional video and MPEG-2. MPEG-2 defines an interaction channel using the digital storage media command and control (DSM-CC) toolkit. Interaction channels may be used for diverse services, including:
- Display and control of small video clips
- Ability to select and pay for video on demand
- Access to remote information servers
- Access to remote databases, therefore having access to systems providing online shopping, banking, and so on
- Internet access