Even with the advent of powerful processors, bigger hard-drives and previously unimaginable Internet speeds, the need for the compression of data can never be overstated. While text files and documents take hardly any space to store and to transmit; Image, Audio and Video content is a different ball game all together. Uncompressed data, especially Video and Audio content, simply takes up a lot of space. Compression reduces the consumption of these important resources.     
   
Compression can be broadly classified into two types- Lossless and Lossy. In the lossless type of compression, no data is lost during the compression process. When the compressed form is decompressed, all the contents are faithfully reproduced without any changes to the original. Computer programs, Software applications, Databases etc. are compressed using these techniques, since every bit of data is critical. Due to these limitations, compressions of only about 50-60% can be managed. Lossy compression reduces file size by permanently eliminating redundant information, so that only a part of the original data is retained and reproduced. Most Video and Audio compressors use Lossy compression, to take advantage of the limitations of the human sensory system, to compress data in ways that are lossy, but nearly indistinguishable from the original to the human eye. This is used on CD and DVD media. In Audio compression, methods of psychoacoustics are used to remove non-audible components of the audio-signal to make compression more efficient. Lossy compression techniques are known to produce high Compression.   
   
Uncompressed AVI files have huge file sizes. AVI codecs (Compressor ? Decompressor) greatly reduces AVI file sizes while maintaining optimum quality. AVI?s popularity and extensibility largely rests on the fact that you can apply numerous codecs with different capabilities to them. Other video formats like MPEG or WMV are different. Unlike AVI, which is more of a specification, MPEG is a standard compression technique. With MPEG, you do not have the extensibility of applying your own codec. Thus, it is difficult to adopt MPEG to use newer and better compression technologies. While, with AVI, you just use a better codec that takes advantage of the new compression technology. The Windows Media format (WMV) also uses codecs. You just have to select an appropriate profile and the profile will internally select between 3 - 4 codecs. This makes the entire affair of creating a WMV quite simple. As AVI lets you select from scores of different codecs, you face a challenge of selecting the right codec.    
   
Many AVI compressors, tailored for different needs, have been designed. These can be characterized and judged by the quality of reproduction and the Compression Ratios they achieve. Which Compressor to choose, largely depends on the intended use, and the desired quality of output. Many codecs can be downloaded directly from the internet.    
 
When selecting the right codec, think about these points:

1. Availability of the codec: Codecs like CINEPAK and MICROSOFT VIDEO 1 come bundled with Windows and therefore all Windows users can view them. On the other hand, the DivX codec provides good compression but users must separately install the DivX package before viewing the videos.
   
2. Compression Ratio and Quality: Different codecs differ in their capability to compress videos at the highest possible ratio while keeping optimum quality. For videos intended for web, you can choose to compromise more on quality.  
 
3. Color Reproduction: A codec can also reduce the color depth of the video to increase compression considerably. Some codecs can also alter other video properties like saturation or brightness. 
   
4. Specialized Codec: Some codecs specialize in compressing a specific type of video ? like screen-recordings while others are more general. Specialized codecs provide significantly better compression than general-purpose codecs when you use them with the correct type of video.  
   
Popular Codecs 

CINEPAK: This is a popular Codec distributed by Super Mac Inc and by Radius. It supports all Windows formats, Quicktime and most game machine files. Playback size and frame-rate largely depend on the computer you are using. It handles Videos which contain a lot of motion quiet well. It is a good choice for distributing AVI files.    
    
MICROSOFT MPEG-4 VIDEO: MPEG is a standard defined by the Moving Pictures Experts Group. There are many codecs complying with the MPEG 4 standard, including ones from leading manufacturers like Apple, Ligos, DivX, etc. The Microsoft MPEG-4 Codec gives high quality compressions, is easy to use and enjoys good compatibility.   
 
MICROSOFT VIDEO 1: This is a popular codec bundled with all versions of Windows. Microsoft Video 1 provides excellent video quality and reasonable compression ratio.   
    
MJPEG: This is a Video adaptation of the JPEG standard for images. It is ideal for editing and Video Capture, with many editing systems built around MJPEG. One major advantage is that compression can be achieved in Real Time. However, there are many implementations doing the rounds, raising compatibility issues amongst Videos compressed by different systems.    
    
DivX: This provides high quality compression. The codec boasts support for different languages, good visual quality, speed and compression. However, users have reported synchronization issues. Users have also reported compatibility problems amongst files compressed using different versions of DivX.   
    
To achieve low file size and optimum quality, it is best to use Video/Audio Codecs when saving to AVI. A Codec reduces the file size, but it also affects the quality. While different Codecs provide different levels of efficiency, a lot actually depends on the specific settings of each Codec for every compression operation. The output file size also depends on the original dimensions, the frame rate, the editing carried out on the source, the quality of output desired, and other parameters. Overall choose Microsoft Video 1 when distributing to a wide audience, since it has the best compatibility. The Microsoft MPEG-4 codec is a good choice for high quality compressions. If you are looking for high speed performance and have a large number of videos, MJPEG gets the compression done very quickly. 

Last update: 07:57 PM Friday, October 21, 2005