Understanding Digital Video Architecture

John Devcic
April 2007

To understand Digital Video Architecture, you need to start at the beginning. The beginning in this case is known as Video Compression. That is the start of this article; after that, we will get into the most common formats in use today.

In order to make digital video's use widespread, there had to be a way to reduce the amounts of data that needed to be stored and transmitted. This reduction in the amount of storage is a direct result of the advances made possible by video compression. The advances in video compression have single-handedly led to the widespread use of video to the desktop and to hundreds of channels in your home. To boil it down to its most simplistic level, compression is performed when an inputted video is analyzed and the information that is indiscernible to the viewer is dropped. Each event is given a code - the most commonly occurring events are given fewer bits and the rarer events will have more bits. These steps are usually known as signal analysis, quantization and variable length encoding.

There are 4 major ways to compress video. Discrete cosine transform (DCT), vector quantization (VQ), fractal compression (FC) and discrete wavelet transform (DWT).

DCT is not a very good compression algorithm. It samples images at regular intervals, analyzes the frequency components present in the sample, and discards those frequencies that do not affect the image as the human eye perceives it. DCT is the standard used for JPEG and MPEG.

Vector quantization is also a bad compression algorithm that looks at an array of data instead of individual values. It will then generalize what it sees, compressing the found redundant data, and at the same time keeping the desired object.

Fractal compression is a form of vector quantization, and this also is a bad compression algorithm. This type of compression is performed by finding self-similar sections of a particular image, then using a fractal algorithm to create the sections.

Discrete Wavelet Transform mathematically transforms an image into frequency components. This process is performed on the entire image. Obviously, this differs greatly from the other methods that work only on smaller sections of the desired data. The end result is a very effective hierarchical representation of an image, where every layer represents a frequency band.

MPEG stands for Moving Picture Experts Group. MPEG was established in 1988 to develop standards for digital audio and video formats. There are 4 MPEG standards being used right now. Each compression standard was designed with a specific application and bit rate in mind.

MPEG-1 is designated for 1.5Mbit/sec standard for the compression of moving pictures and audio. This was based on CD-ROM video applications and is a popular standard for video on the Internet transmitted as an .mpg file. Level 3 of MPEG-1 is the most popular standard for digital compression of audio - known as MP3. VideoCD uses MPEG-1 as its standard compression method. It is not very complicated to create and will yield okay results.

MPEG-2 is designed for 1.5 to 15Mbit/sec standard on which digital TV set top boxes and DVD compression are based. It is based on MPEG-1 but designed for the compression and transmission of digital broadcast television. The most significant improvement over MPEG-1 is its ability to efficiently compress interlaced video.

MPEG-4 is the standard of multimedia and Web compression. MPEG-4 is based on object-based compression, similar to Virtual Reality Modeling Language. Individual objects within a scene are tracked separately and compressed together to create an MPEG-4 file. The end result is a very good and efficient compression that is very scalable, from low bit rates to very high ones. This format is the one most used today.

It offers excellent video and audio quality. It is the most popular MPEG standard used right now.

JPEG stands for Joint photographic Experts Group. JPEG is not a very good compression technique for full-color or grayscale images, as it exploits the fact that the human eye will not notice small color changes in an image.

.AVI It stands for Audio Video Interleaved. This type of file is a sound and motion picture file that conforms to the standards set by Microsoft Windows Resource Interchange File Format (RIFF). You will be able to recognize this type of file by its extension .avi. This is a common file format. The video quality is good at smaller resolutions; the only major drawback is that the files tend to be large. The most commonly used video codecs that use .avi are MJPEG and DivX. To play an .avi, you could use Windows Media Player, RealPlayer, or the DivX player.

.MOV This file extension identifies an Apple QuickTime movie; .mov is an Apple QuickTime motion video file format. Developed by Apple Computer for viewing moving images, .mov is a method of storing sound, graphics and movie files. Originally QuickTime was developed for the Macintosh; it is now available for Windows as well. Such .mov files are created and played back on the Apple QuickTime player.

MJPEG is short for Motion JPEG. It is a video codec where each video field is separately compressed into a JPEG image. It best suited for broadcast resolution interlaced video, such as NTSC or PAL. MJPEG is not good for movies that are smaller than TV resolutions. MJPEG is ill suited for progressive scan computer monitors. It is lso used for short files such as the short movies that can be made by a digital camera.

DivX is a software application that uses MPEG-4 standard to compress digital video. The DivXNetworks and the open source community are developing DivX jointly. The DivX player is a powerful and stable player.

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