Text Videoconferencing and chatting
Videoconferencing is the conduct of a videoconference (also known as videoteleconference) by a set of telecommunicationtechnologies which allow two or more locations to communicate by simultaneous two-way video and audio transmissions. It has also been called 'visual collaboration' and is a type of groupware.
Videoconferencing differs from videophone calls in that it's designed to serve a conference or multiple locations rather than individuals. With the introduction of relatively low cost, high capacity broadband telecommunication services in the late 1990s, coupled with powerful computing processors and video compression techniques, videoconferencing has made significant inroads in business, education, medicine and media. Like all long distance communications technologies (such as phone and Internet), by reducing the need to travel to bring people together the technology also contributes to reductions in carbon emissions, thereby helping to reduce global warming.
Technology
The core technology used in a videoconferencing system is digital compression of audio and video streams in real time. The hardware or software that performs compression is called a codec (coder/decoder). The other components required for a videoconferencing system include:
Video input: video camera or webcam
Video output: computer monitor, television or projector
Audio input: microphones, CD/DVD player, cassette player, or any other source of PreAmp audio outlet.
Audio output: usually loudspeakers associated with the display device or telephone
Data transfer: analog or digital telephone network, LAN or Internet
Computer: a data processing unit that ties together the other components, does the compressing and decompressing, and initiates and maintains the data linkage via the network.
There are basically two kinds of videoconferencing systems:
1. Dedicated systems have all required components packaged into a single piece of equipment, usually a console with a high quality remote controlled video camera. These cameras can be controlled at a distance to pan left and right, tilt up and down, and zoom. They became known as PTZ cameras. The console contains all electrical interfaces, the control computer, and the software or hardware-based codec. Omnidirectional microphones are connected to the console, as well as a TV monitor with loudspeakers and/or a video projector. There are several types of dedicated videoconferencing devices:
1). Large group videoconferencing are non-portable, large, more expensive devices used for large rooms and auditoriums.
2). Small group videoconferencing are non-portable or portable, smaller, less expensive devices used for small meeting rooms.
3). Individual videoconferencing are usually portable devices, meant for single users, have fixed cameras, microphones and loudspeakers integrated into the console.
2. Desktop systems are add-ons (hardware boards, usually) to normal PCs, transforming them into videoconferencing devices. A range of different cameras and microphones can be used with the board, which contains the necessary codec and transmission interfaces. Most of the desktops systems work with the H.323 standard. Videoconferences carried out via dispersed PCs are also known as e-meetings.
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Videoconferencing modes
Videoconferencing systems use several common operating modes:
1. Voice-Activated Switch (VAS);
2. Continuous Presence.
In VAS mode, the MCU switches which endpoint can be seen by the other endpoints by the levels of ones voice. If there are four people in a conference, the only one that will be seen in the conference is the site which is talking; the location with the loudest voice will be seen by the other participants.
Continuous Presence mode displays multiple participants at the same time. The MP in this mode takes the streams from the different endpoints and puts them all together into a single video image. In this mode, the MCU normally sends the same type of images to all participants. Typically these types of images are called layouts and can vary depending on the number of participants in a conference.
Echo cancellation
A fundamental feature of professional videoconferencing systems is Acoustic Echo Cancellation (AEC). Echo can be defined as the reflected source wave interference with new wave created by source. AEC is an algorithm which is able to detect when sounds or utterances reenter the audio input of the videoconferencing codec, which came from the audio output of the same system, after some time delay. If unchecked, this can lead to several problems including:
1). the remote party hearing their own voice coming back at them (usually significantly delayed)
2). strong reverberation, which makes the voice channel useless, and
3). howling created by feedback. Echo cancellation is a processor-intensive task that usually works over a narrow range of sound delays.
