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Post-Production

After filming you may want to edit your movie. Editing includes removing parts of the video you don't want and adding transitions and special effects. Digital editing software enables you to digitize your footage onto your computer, edit it, and then transfer it back to videotape, CD, DVD or to the Web. Some examples of digital video editing software are:

  • Apple's iMovie (supported by Mac only, very basic but adequate);
  • Adobe's Premiere (supported by Mac and PC platforms; more features and flexibility);
  • Apple's Final Cut Pro (supported by Mac only, very robust tool but steeper learning curve).
  • See Figure 1

Video files can be huge files. In order too make the movies accessible on CD or via the Web, they must be compressed. Compression is a process by which huge movie files are reduced in size with the removal of unnecessary and redundant audio and video data. This process will affect the quality of the final movie so should be applied in ratios as needed. Review the "Examples" section to see the effects of compression for the various target bandwidths.

Compression occurs when you either:

  1. apply a codec to both audio and video, or
  2. adjust the
    • video frame size
    • video frame rate
    • audio sample rate or
    • audio frequency rate.

A codec, meaning compression and decompression, determines both the elimination of data during compression as well as the decompression when it is played. It is a mathematical equation, or algorithm, that eliminates the redundant data within a frame or between frames on video. This detailed analysis of each frame of the movie will be very time-consuming. The types of codecs available will depend on your final delivery format. The "Examples" section of this Guideline provides some guidance on which codecs work best in which formats.

The video frame size is the size of the display window in which your movie plays. The best size is highly dependent on data rate, frame rate, codec, source material and personal preference. These factors are all interrelated so experimentation is the best way to find the optimal setting for your project. Use this chart as a guideline:

Frame Size Guidelines
Connection Speed Frame Size
T1/ 512k/Cable Modem 640x480
256k/ISDN 320x240
56k Modem 240x180

The video frame rate is how many frames are displayed each second. Higher frame rates produce smoother motion in your movie. Higher frame rates also creates larger files. Again, experimentation is the best way to determine the optimal frame rate for your project.

NTSC Frame Rates
30 fps - all frames are shown
15 fps - every other frame is shown
10 fps - every third frame is shown

The size of a digital audio files is further reduced by taking a sample of the original file. One way to sample is to reduce the bit rate. Like colors, an audio sample with a larger bit rate has a larger range of possible values. This give you a more accurate sample but creates a larger file. A 16-bit sample rate is good for music but 8-bit sample rate is sufficient for voice.

Another way to reduce an audio file is to lower the frequency rate. This method, measured in kilohertz, indicates how often a sample is created throughout the original file. CD audio has a frequency rate of 44.1 kHz but when reduced to 22.05 kHz still has very good quality. The type of audio will ultimately determine how low you can take your sampling rates.

NOTE: Encoding your audio from stereo to mono, however, is a typical tool used to reduce file sizes.

Audio Sampling
  Good Adequate
Music 44.1 kHz 22.05 kHz
Voice 22.05 kHz 11.025 kHz or lower

The compression process can occur in different ways, depending on your resources. It can be applied within the editing program on the final export or in a separate application like Media Cleaner 5, Real Producer or Sorenson Squeeze. Compression from an editing program may have limits in formats and codecs but all can be transferred to the above listed applications as uncompressed files in some formats.

If you don't have access to any other encoding software, the following are free: RealNetworks provides a free encoder - the newest version is Helix Producer Basic. QuickTimeBroadcaster is Apple's live encoding software (Note: for QuickTime movies to stream, they must be “hinted”). Microsoft's free encoding software is Windows Media Encoder 9 Series.

After editing and compression, you'll want to add a caption to your video.


Figure 1. Some of the options that exist for digital video.

chart listing post-production video options

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  Updated June 28 2007
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