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Digital Media Converter 4.2
Reducing Video File Sizes

Size does matter.  These three words ring especially true for video files.  Video files are among the largest files there are.  They fill up your hard disk, slow down your PC, get too big to be copied other computers, and can be almost impossible to distribute over the Internet.  Though the Internet bandwidth available is always increasing, and storage options like large hard drives and DVDs become less expensive every year, a typical video continues to occupy 'too much space'.


Today, there are a host of video formats available.  This article discusses two file formats, and some ways in which you can significantly reduce file sizes.

1. AVI
2. Windows Media Video

AVI stands for Audio Video Interleaved.  This format has been around for some time.  The AVI format is very flexible because you have a choice of several different compressors to balance file size against quality.  It's also very easy to navigate and edit using software like Video Edit Magic.  AVI files are not a good choice for distribution because file size is simply too large, even when compressors are used.


You can get the best possible quality by using an uncompressed AVI file.  However, uncompressed AVI files are enormous, too big to practically distribute or to store.  You need to compress AVI files, and explore other ways of reducing the file size.  You may lose some quality when trying to reduce file size.  Some ways to reduce the size of AVI files is:

1. Reduce the resolution.
2. Reduce the frame rate.
3. Increase the key frame rate
4. Change the compressor.

The article Understanding Compression In AVI has an in-depth description of AVI compression.

Reducing the resolution
  You can reduce the resolution of the original AVI file to significantly decrease the file size.  Some AVI files have very high resolutions.  Resolutions that are too high cannot be displayed on a normal PC or TV, so you lose no effective quality by reducing the resolution.  For example, you might have AVI files with a resolution of 1024 X 768 or even higher.  For instance, 720 x 480 is the standard resolution for DVDs.  Since normal television sets work at lower resolutions than this, no quality is lost for TV applications, and it also looks good on a PC.  When you reduce a file from 720 X 480 to 320 X 240, both the height and width are reduced by a factor of 2, which means that the total amount of data needed to store the video is reduced by a factor of 4.  This reduces the file size significantly.  (It does not reduce the file size to 1/4 because most video files also store audio, and reducing video resolution does not change the amount of storage needed for the audio portion.) 
  Reducing the Frame Rate  
  Digital video is actually a series of images, which change rapidly and give the illusion of motion.  The frame rate is the number of frames displayed per second.  The frame rate has a significant impact on the file size.  US television has a frame rate of 30 frames per second (fps), while films in theatres are usually shown at 24 frames per second.  Lowering the frame rate of your video to 15 or 10 fps can reduce the file size by as much as 200% or 300%.  However, reducing the frame rate may affect the smoothness of the playback.  Acceptable frame rates vary depending on the specific video clip.  You may have to experiment to find the lowest acceptable value.  
  Increasing the Key Frame Rate  
  In an AVI file, the entire image in the first frame is stored in the file.  Later frames are not saved as complete images—only the differences between the frames are saved.  The key frame rate defines how often full frames ("key frames") are saved in AVI files.  For example, if you specify the key frame rate as 50, then after every 50 frames, a complete frame is saved.  A high value of key frame rate results in smaller file size because fewer full frames are stored.  However, the higher the key frame rate, the harder it is to seek within a file.  
  Changing the compressor  
  Uncompressed AVI files are huge.  AVI codecs (COmpressor-DECompressor) greatly reduce file size while maintaining optimum quality.  You can choose from the several codecs available from different sources.  However, you should keep in mind that your audience must have the same codec to uncompress and view the file.  Different compressors deliver different results.  Cinepak Codec, from Radius, is a good choice while distributing to a wide audience, since it has the widest compatibility.  (Cinepak is included with all versions of Microsoft Windows.)  DivX or Xvid are good choices for high compression and high quality.  You can also try experimenting with other codecs.  
  Changing Audio Settings  
  The audio portion of an AVI file can also be compressed.  As with video compression, audio compression will dramatically reduce file size, by sacrificing some quality.  In practice, high-quality audio compression often produces sound so close to the original that the difference is hard to detect.  Very commonly-used audio codecs for AVI files include MPEG Layer 3 (MP3) and CCITT -law.  For most codecs, you can also select the sample frequency.  This is the number of times per second (ranging from 8,000 to 48,000 times per second) that the encoder measures sound levels.  In general, the lower the number here, the more high-pitched sounds will be lost in the compression process, but also the smaller the final file size.  
  Reducing AVI file size in Digital Media Converter  
  You can change the parameters discussed in the preceding section through Digital Media Converter.  This will help to significantly cut down on file size while converting to AVI.  To convert and reduce file size using Digital Media Converter:  
1. Start Digital Media Converter.
2. On the File menu, select Add Files.
3. Select the AVI file you want to reconvert to a smaller AVI file.  You can also select files of other formats for conversion to AVI.
4. On the Tools menu, select Settings.
5. Under the General Tab, select AVI as the Convert to Format.
6. Select the Properties tab.  You can change the parameters discussed in the preceding section from this window.
  Now, you're ready to convert and prepare a small AVI file.  Select the file for conversion and click on the Convert button on the toolbar.  
  Windows Media Video 
  Windows Media Video provides good quality with extremely small file sizes.  It is fast becoming the most popular choice video email, web video, and permanent storage on a PC.  When creating Windows Media Video, you select a pre-defined profile that is configured for a specific distribution method, for instance "WMV For Desktop Viewing, Very High Quality Video, CD Quality Audio."  The Profile handles all the detailed settings like choosing a codec, frame rate, and so on.  Profiles are pre-defined, and are designed to create extremely small files.  
  For example, to share a video over the Internet, you can select a WMV profile designed to create files for streaming from a streaming media server like Video Desk.  Similarly, there are other profiles for different distribution needs including profiles for desktop viewing, video hosted on a (non-streaming) web server, and portable media center devices.  Within each profile, you can select from a variety of pre-configured sub-profiles.  If you're aiming for the smallest file size, lower the bit rate of the selected profile, to lower the final file size.  For a particular bit rate, the higher the resolution, the sharper the picture.  There is no difference in the file size for different resolutions if the bit rate is the same.  
  To convert to Windows Media Format using Digital Media Converter:  
1. Start Digital Media Converter.
2. On the File menu, select Add Files.
3. Select the file you want to reconvert to WMV.  You can also select an existing WMV file that you want to convert to a profile that gives smaller file size.
4. On the Tools menu, select Settings.
5. From the General Settings, select an appropriate WMV profile depending upon your distribution needs.
6. Select the Properties tab.
7. To produce the smallest possible file, select the lowest possible bit rate for your choice of Video Profile.
8. Click on OK.
  Now, you're ready to convert and prepare a small-sized WMV file.  Select the file for conversion and click the Convert button on the Tool bar.  
  A final word  
  With so much content available, it is a wise choice to reduce file size whenever possible.  Follow these tips, get the most out of your file formats, store more and then do more.  When it comes to file size, think small.  

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