Thursday, January 20, 2011

Digital Video, MPEG and Associated Dgital Artifacts


Compression and Digital Video

Figure 2.1 - Compression Techniques
Simple Compression Techniques Various techniques exist, including :
Interpolative Techniques
This technique aims to send a subset of the pixels and use interpolation to reconstruct the intervening pixels. This technique is particularly useful for motion sequences, as certain frames are compressed by still compression ; the frames between these are compressed by doing an interpolation between the other frames and sending only the data needed to correct the interpolation.
Predictive Techniques
This relies on the fact that there is nearly always some redundancy between frames in a sequence. There are two common methods :
Transform Coding Techniques
A transform is a process that converts data into an alternate form which is more convenient for some particular purpose. Transforms are ordinarily designed to be reversible. Useful transforms typically operate on large blocks of data and perform some complex calculations. In general transform coding becomes more useful with larger blocks. The Discrete Cosine Transform (DCT) is especially important for video compression.

The DCT The DCT is performed on a block of horizontally and vertically adjacent pixels (typically an 8 by 8 block of pixels). The outputs represent amplitudes of two dimensional spatial frequency components. These are called DCT coefficients. The coefficient for zero spatial frequency is called the DC coefficient and it is the average value of all the pixels in the block. The rest of the coefficients represent progressively higher horizontal and vertical spatial frequencies in the block.
Since adjacent pixel values tend to be similar or vary slowly from one to another, the DCT processing provides opportunity for compression by forcing most of the energy into lower spatial frequency components. In most cases, many of the higher frequency coefficients will have zero or new-zero values and therefore can be ignored.
The decoder performs the reverse process, but due to the transcendental nature of the DCT the reverse process can only be approximated and hence some loss takes place. The trick is to use some cunning methods of keeping coefficients so that the loss is minimally visible.
Statistical Coding (or Entropy Coding) This takes advantage of the statistical distribution of the pixel values. Some data values can occur more frequently then others and therefore we can set up a coding technique that use less bits for these values. One widely used form of this coding is Huffman encoding. This technique has the overhead that a syntax has to be pre-defined or sent for the decoder to work.

Need for Compression

HDTV is defined as having twice the horizontal and vertical resolution of conventional television, a 16:9 picture ratio and at least 24 frames per second. Using this definition, HDTV has approximately double the number of lines of current broadcast television. This combined with the resolution increase means that 6 times more bandwidth is needed for transmission. This is an ideal place for compression, as this will reduce the data rate and hence the bandwidth.
This is the number one application for digital video. This application includes video kiosks, training, corporate presentations and video libraries. The advantages of using digital video (and particularly MPEG) are :
Multimedia used in student training has also been shown to improve achievement by an average of 38 percent.
Since digital video clips are stored in files, they can be easily integrated into many databases just like text or numeric fields. For example, a travel agency can keep video clips of their holiday locations as well as more mundane information and really show what it is like to go for a holiday in a particular resort.


Akash Mondal

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