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from "A Technical Guide to Teleconferencing and
Distance Learning," 3rd edition
Access Channels: Dedicated channels giving nondiscriminatory access to a local cable system by the public, government agencies or educational institutions. ACCUNET Switched Digital Services: High-speed dial-up digital data services offered by AT&T for full duplex digital transmission at speeds of 56, 64, 384 and 1536 kbps. Uses include data, voice and video services.
Acoustic Coupler: A device that allows a conventional telephone handset to feed its signal into a modem, as opposed to direct couplers, which feed the modulated/demodulated signal directly into the phone line.
Acoustic Echo Canceller: All speakerphones have some form of adaptive echo canceller that produces a synthetic replica of the potential echo to subtract from the transmit audio. Most units have a center clipping echo suppresser to remove the residual echo from the transmit signal. The goal of the acoustic echo canceller is to reduce the amount of direct and reverberant loudspeaker coupling to the microphone to prevent echo. To achieve this, the algorithms used in today's devices require an audio system that is feedback stable.
Acoustic Echo Return Loss - AERL: The minimum loss experienced by a sound in traveling from the loudspeaker to the microphone in a conference room. It is expressed in dB or decibels. A 0 dB loss corresponds to a perfectly reflective room or to very close coupling between loudspeaker and microphone. In practice, AERL figures can range from 0 to -30 dB, with a poor room having the former figure. Acoustic Echo Return Loss Enhancement - AERLE: The maximum echo cancellation provided by the acoustic canceller. Typical figures will vary from 6 to 18 dB. The larger the number the better. It is important to note whether the figure is quoted with the center clipper enabled or disabled. If quoted with center clipper disabled, it is a true measure of the cancellation provided by the echo canceller rather than the attenuation provided by the center clipper.
Acoustic Modem: A modulator-demodulator unit that converts data signals to telephone tones and back again.
Active Satellite: A satellite that transmits a signal, in contrast to a passive satellite that only reflects a signal. The signal received by the active satellite is usually amplified and translated to a different frequency before it is retransmitted. Ad Hoc: Teleconferencing technology and sites assembled for an event; equipment may be rented or permanently installed; sites are not always part of the network. Addressable: The ability to signal from the headend or hub site in such a way that only the desired subscriber's receiving equipment is affected. This makes it possible to send a signal to a subscriber and effect changes in the subscriber's level of service such as the ability to receive a program.
ADPCM - Adaptive differential pulse code modulation: A method of compressing audio data by recording the differences between successive digital samples rather than full value of the samples. There are many different types of ADPCM standards; this refers to the standard as defined in the CD-ROM XA and CD-I standards. ADSL - Asymmetric Digital Subscriber Loop: ADSL uses a regular phone line (twisted pair) without a dial tone (a dry pair) to allow transfer speeds of up to 7Mb downstream with slower speeds going back up (faster than a T1). The Telcos are using ADSL to maintain market share rather than allowing the cable industry to deploy cable...
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