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alternating current
(AC) a periodic current the average value of
which over a period is zero.
aliasing
(1) in
signal processing, distortion introduced in a digital signal when it is
undersampled. (2) in computer graphics, distortion due to the
discrete nature of digital images that causes straight lines to appear jagged.
(3) in computer software, a single object having two different
identities, such as names in memory space.
active RC filter an
electronic circuit made up of resistors, capacitors, and operational amplifiers
that provide well-controlled linear frequency-dependent functions, e.g., low-,
high-, and bandpass filters.
active device a device that
can convert energy from a DC bias source to a signal at an RF frequency. Active
devices are required in oscillators and amplifiers.
active filter (1) a filter
that has an energy gain greater than one, that is, a filter that outputs more
energy than it absorbs. (2) a form of power electronic converter designed to
effectively cancel harmonic currents by injecting currents that are equal and
opposite to, or180◦outofphasewith, the target harmonics. Active filters allow the
output current to be controlled and provide stable operation against AC source
impedance variations without interfering with the system impedance.
Active mixer a mixer that
uses three terminal devices such as FET rather than diodes as nonlinear element.
One advantage of active mixers is that they can provide conversion gain.
adaptive differential pulse code modulation (ADPCM)
a modulation scheme in which only the difference between successive signal
samples is encoded for transmission, and the quantization of the coding is
adapted to the characteristics of the signal source.
Additive white Gaussian noise(AWGN)
the simplest form of channel degradation in a communication system in which the
source of errors in the channel can be modelled as the addition of random noise
with a Gaussian distribution and a constant(white)power spectrum.
aerial cable any
fully-insulated electric power cable which is carried overhead upon poles, as
opposed to the use of the more usual overhead bare conductors.
algorithm (1) a systematic
and precise, step-by-step procedure (such as a recipe, a program, or set of
programs) for solving a certain kind of problem or accomplishing a task, for
instance converting a particular kind of input data to a particular kind of
output data, or controlling a machine tool. An algorithm can be executed by a
machine.
alphanumeric mode relates
to alphabetic characters, digits, and other characters such as punctuation
marks. Alphanumeric is a mode of operation of a graphic terminal or other
input/output device. The graphics terminal should toggle between graphic and
alphanumeric data.
alternating current (AC) a
periodic current the average value of which over a period is zero.
amplifier a circuit element
that has a linear input-output signal relationship, with gain in voltage,
current, and/or power.
amplitude descriptor of the
strength of a wave disturbance such as an electromagnetic or acoustic wave.
Amplitude response the
magnitude of the steady-state response of a fixed, linear system to a
unit-amplitude input sinusoid.
amplitude spectrum the
magnitude of the Fourier transform |F(ω)|,−∞ <ω< ∞of a signal f(t).
AM to PM conversion phase
variations of an output signal, due to passing through an active device, where
the phase of the output signal varies in response with the amplitude of the
input signal.
AM video the amplitude
modulated video carrier wave is produced by an amplitude modulated video
transmitter where the amplitude of the wave form varies in step with the video
signal similar to that shown in the figure.
analog data data
represented in a continuous form with respect to continuous time, as contrasted
with digital data represented in a discrete (discontinuous) form in a sequence
of time instant
analog multiplier a device
or a circuit that generates an analog output signal that is proportional to the
product or multiplication of two analog input signals,
analog signal a signal
represented in a continuous form with respect to continuous time, as contrasted
with digital signal represented in a discrete (discontinuous) form in a sequence
of time instant.
analog-to-digital (A/D) converter
a device that changes an analog signal to a digital signal of corresponding
magnitude. This device is also called an encoder, ADC, or A/C converter.
Analytic signal refers to a
signal that has a Fourier transform that is zero valued for negative
frequencies; i.e., the signal has a one sided spectrum.
angle modulation a type of
modulation where either the frequency (FM) or the phase (PM) of a carrier are
varied.
angular frequency the rate
of change of the phase of a wave in radians per second.
antenna a device used to
couple energy from a guiding structure (transmission line, waveguide, etc.) into
a propagation medium, such as free space, and vice versa. It provides
directivity and gain for the transmission and reception of electromagnetic
waves.
ASCII American standard
code for information interchange.
asynchronous system a
(computer, circuit, device) system in which events are not executed in a regular
time relationship, that is, they are timing-independent. Each event or operation
is performed upon receipt of a signal generated by the completion of a previous
event or operation, or upon availability of the system resources required by the
event or operation.
atmospheric attenuation
decrease in the amplitude of a signal propagating through the atmosphere, due
primarily to absorption and scatter.
attenuation the exponential
decrease with distance, in the amplitude of an electric signal traveling along a
very long transmission line due to losses in the supporting medium. In
electromagnetic systems attenuation is due to conductor and dielectric losses.
In fiber optic systems attenuation arises from intrinsic material properties
(absorption and Rayleigh scattering) and from waveguide properties such as
bending, microbending, splices, and connectors.
attenuator a device or
network that absorbs part of a signal while passing the remainder with minimal
distortion.
audio science of processing
signals that are within the frequency range of hearing, that is, roughly between
20 hertz and 20 kilohertz. Also, name for this kind of signal.
audio channels the portion
of the circuit containing frequencies that correspond to the audible sound
waves. Audio frequencies range from approximately 15 hertz to 20,000 hertz.
audio coding the process of
coding and compressing an audio signal for storage on a digital computer or
transmission over a digital communication channel
aural subcarrier in a
composite television signal, the frequency division multiplexed carrier placed
outside the visual passband that carries the audio modulation. In the NTSC
(United States) system, it is placed 4.5 Mhz higher than the visual carrier.
autocorrelation a measure
of the statistical dependence between two samples of the same random process.
For a random process X(t), the auto-correlation is the expectation Rxx (t1,t2) =
E [X (t1)X (t2)].
autocorrelator a circuit
that computes the autocorrelation function.
automatic gain control (AGC)
a method to control the power of the received signal in order to be able to use
the full dynamic range of the receiver and to prevent receiver saturation.
average power the average
value, taken over an interval in time, of the instantaneous power. The time
interval is usually one period of the signal.
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