Chapter 21: Audio/Visual Devices^561
Slow the frame rate from 30 to 15 or less frames per second. However, the
slower the frame rate, the more jerky the video motion becomes.
Compress the file. There are compression techniques available that can shrink
the file size of a video file using a ration of 100:1. But, once again, video
compression is lossy compression, and the file may never look the same again.Codecs
Many streaming format systems are codecs, a software system that both compresses and
decompresses sound or video files. A codec compresses the file using software known as
an encoder and reproduces the image or sound with a player that performs the decoding
and decompression. The functions of a codec are similar to those of a modem and how it
works with data communications: it both modulates and demodulates the signal to con-
trol the analog to digital conversions. Some codecs support a range of file formats, but
most support only their own proprietary formats.
A codec uses one of two lossy compression techniques, temporal (image) or spatial (size).
Temporal compression looks to eliminate the part of the image (or sound) not needed for
continuity to the human eye or ear. The video images are examined frame by frame for
changes between frames. For example, in a video of a talking head (a clip of a person sitting
or standing with little motion), much of the image remains the same from frame to frame.
The background never changes and the motion is only head and li pmovements. Tem poral
compression compares the first frame (the key frame) with the next (the delta frame) to see if
anything has changed. The key frame is stored and only the information that changes in the
delta frame is kept. This eliminates a large portion of the file. Spatial compression deletes in-
formation that is common to the entire video or a sequence within the file. It looks for redun-
dant information that repeats at coordinate points from frame to frame.Teleconferencing Systems
When the Web first got started, threaded discussions and live chats were included on a Web
site more for fun that serious business. Today, online conferencing, in one form or another,
has a prominent position on many Web sites. In fact, some of the Web’s most active sites are
devoted to live chat discussions. The technologies of the Internet and Web now offer a wide
range of conferencing options. There are numerous online conferencing options, but in gen-
eral, your choices narrow down to text-based or streaming audio-visual conferencing; for
some smaller groups, there is the option of using an Internet phone system.Video Conferencing
It is only a very slight ste pu pfrom streaming audio and video systems to video
conferencing over the Internet. Online video-conferencing systems are the audio-visual
equivalentofalongdistancetelephonecallonthepublictelephonesystem.Justasavoice
conversation is so much more content-rich than a text-based chat, a live videoconference
provides much more than the voice-only conference.