Language:

Get the inside track on all the hot news regarding DeskShare products, new releases, special promotions and discounts.

Subscribe to our DeskShare News.

Digital Media Converter 4.1

Understanding Compression in AVI

Even with the advent of powerful processors, bigger hard-drives and previously unimaginable Internet speeds, the need for data compression can't be overstated.  While text files and documents take little space to store and little bandwidth to transmit; image, audio and video files are a different ball game altogether.  Uncompressed data, especially video and audio content, simply takes up a lot of space.  Compressed files take up less space, saving you money on additional storage and bandwidth charges.

Compression can be broadly classified into two types-

  1. Lossless
  2. Lossy

Lossless

In the lossless type of compression, no data is lost during the compression process.  When the compressed file is decompressed, the result is identical to the data before compression.  Computer programs, software applications, databases etc. are compressed using lossless techniques, since a change in even one bit of data can make them useless or produce incorrect results.  For most files, lossless methods reduce the file size by only 50-60%.  Well known lossless compression methods include Zip and RAR.

Lossy

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 compress data in ways that are lossy, but produce very high compression levels.  Properly compressed video and audio files are nearly indistinguishable from the original to the human eye or ear.  In audio compression, psychoacoustic techniques are used to remove non-audible components of the audio signal.  Lossy compression techniques are known to produce high compression.

Uncompressed AVI files are huge.  AVI codecs (COmpressor – DECompressor) greatly reduce file sizes while maintaining optimum quality.  (Generally, a codec is a program that compresses or decompresses audio or video.  There can be several codecs—programs—that can compress using the same technique.  For instance, there are several codecs that implement MPEG 4 video compression.)  AVI’s popularity and extensibility depend on the fact that you can freely select different video and audio codecs.  Other file formats like MPEG 2 or WMV are not as flexible.  Unlike AVI, which is more of a general specification, MPEG 2 is a compression technique itself.  With MPEG, you do not have the option of substituting your own codec.  This makes it difficult to adopt MPEG to use newer and better compression technologies.  With AVI, you can just switch to a better codec.  The Windows Media format (WMV) also uses codecs, but fewer of them, and the selection process is hidden from the user.  You just have to select an appropriate profile and the profile will automatically select a codec.  This makes creating a WMV file very simple.  Because the process of making an AVI file lets you choose among up to dozens of different codecs, you face the challenge of selecting the right codec.

Many AVI compressors, tailored for different needs, have been designed.  For our purpose, the key things to know are the quality of the video playback and the compression ratio for each codec.  Which compressor to choose depends on the intended use of the file and the desired output quality.  Many codecs can be downloaded directly from the Internet.

Digital Media Converter enables you to control the frame rate, the video resolution and the key frame rate, and to choose the audio and video compressors when creating AVI files.  Changing these parameters is discussed in Reducing Video File Sizes.

When making your choice of compression methods, consider these key points about the following codecs:


CINEPAK:
  This is a popular codec developed by Super Mac Inc and by Radius.  It handles Videos that contain a lot of motion quite well.  It is a good choice for distributing AVI files because it is included with Microsoft Windows.

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 (Motion JPEG):  This is a video adaptation of the JPEG standard for images.  It is ideal for editing and video capture.  Many editing systems are built around MJPEG.  One major advantage is that compression is fast and can be carried out in real time.  However, there are many implementations of MJPEG, raising compatibility issues among videos compressed by different systems.

DivX:  This MPEG-4 compliant codec provides high quality compression.  The codec boasts offers support for multiple 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.

XviD:  XviD is a MPEG-4 compliant video codec designed to compress and decompress digital video.  It is designed as an Open Source alternative to DivX.

For almost any practical use, you will compress the audio and video when making AVI files.  Compression reduces the file size, but reduces the quality by an amount that varies depending the settings chosen.  While different codecs provide different compression ratios, a lot depends on the specific settings that you select when creating your files.  In addition to the codec selected, output file size depends on the video dimensions, the frame rate, the output quality desired, and other parameters.  Overall, we recommend Microsoft Video 1 when distributing to a wide audience, since it has the widest compatibility.  Xvid is a good choice for high quality compressions.  If you are looking for fast compression times, and have a large number of videos, MJPEG gets the compression done very quickly.


Copyright © DeskShare Incorporated.  All rights reserved.