by Steven G. Estrella, Ph.D.
Moving images are an illusion. When we view an animation or video we are viewing a series of still pictures presented in rapid succession. The success of the illusion is dependent on the quality of the individual images and the rate at which they are presented.
Animations are generally a series of 8-bit graphic images created in a graphics program. The images can then be compiled into an animated GIF file or QuickTime movie file for use in multimedia presentations and on the world wide web. Frame rates in animations will vary according to the content. For example, an animation that features type will frequently pause one or more seconds between frames to allow the user to read the type. Animations of moving characters can be successful with frame rates as low as 4 frames per second.
Video can be captured using a standard video recorder and then digitized using video digitizing hardware within a multimedia computer. If you have a digital video recorder, the process is even faster because the conversion to digital takes place within the camera.
For video to be effective, the frame rate must
be at least 10 frames per second and the bit depth of the individual frames
must be 16 or greater. The dimensions of the digitized video can be as small
as 160 by 120 or as large as 640 by 480 pixels.
Full-screen, full-motion video ("broadcast quality") is said to exist when each
frame is 640 pixels wide by 480 pixels tall, each pixel has a color range of
24-bits, and the frames are presented at a rate of 30 per second. The amount of
disk space needed to store broadcast quality video is truly enormous. A grid of
640 by 480 pixels contains 307,200 pixels. If each pixel uses 24-bits to
represent color then a single frame uses 7,372,800 bits or 921,600 bytes (one
byte = eight bits). 30 frames would then use 27,648,000 bytes or nearly 28
megabytes. At this rate, one minute of broadcast quality video would consume
1,658,880,000 bytes or 1.65888 gigabytes of hard disk storage. Most computers
today ship with between 4 and 12 gigabytes of storage making their hard drives
inadequately small for storing broadcast quality video.
Fortunately, multimedia video doesn't have to be broadcast quality to be
usable. By reducing the grid to 320 by 240 or even 160 by 120, reducing the
frame rate to 20 or 15 frames per second, and lowering the color depth (bit
depth) to 16 bits for standard video and 8 bits for computer animations, the
storage requirements become much more manageable. By using various compression
schemes the storage requirements can be even further reduced. For example, a
typical "talking head" video features a person on a stable background.
Compression/decompression schemes (codecs) use a technique called "frame
differencing" to store only those elements of each frame which are different
from the previous frame. Compression can greatly reduce storage requirements
often with little loss of picture quality. Using a digital video recorder and
a computer with video editing software, you could record a movie demonstrating
instrumental techniques and then add a voiceover narrative.
QuickTime comes with several software CODECs
(compressor/decompressor) to reduce file size while retaining quality. For
music, the QDesign Music Compressor is excellent. For speech, the QualComm
PureVoice Compressor is a good choice. For video, the Sorenson compressor does
an impressive job of reducing file size for the visual portion of the video.
When used in combination with the QDesign or QualComm audio compressors, file
size can be made manageable for transmission over the internet. A "Fast Start" feature
is also available to allow the movie to begin playing while still downloading
to the user's computer. QuickTime can also be used to stream content using
a special streaming media server.
QuickTime movies can be loaded onto any web server and included in web pages
by using the appropriate EMBED tag contained within an OBJECT tag. This method
of embedding QuickTime movies is compatible with all major browsers.
<object classid="clsid:02BF25D5-8C17-4B23-BC80-D3488ABDDC6B" width="320"height="260"
codebase="http://www.apple.com/qtactivex/qtplugin.cab">
<param name="SRC" value="sample.mov">
<param name="AUTOPLAY" value="false">
<param name="CONTROLLER" value="false">
<embed src="sample.mov" width="320" height="260" autoplay="false" controller="false" pluginspage="http://www.apple.com/quicktime/download/">
</embed>
</object>
For more detailed and advanced editing of video and audio, of course, you
might purchase professional software like Final Cut Express and Pro Tools LE.
Using free and shareware software available from Apple and others, however,
you can create multimedia presentations to inspire and educate your students.
Figure 1 - QuickTime
Apple Computer's QuickTime software can be used to create movies with any
combination of video, audio, MIDI data, text, and animations.