AC (Alternating Current) Current in a
conductor that changes direction over time.
(audio coding 3) Dolby's digital audio data compression
algorithm adopted for HDTV transmission
and used in laserdiscs and CDs for 5.1 multichannel home theater
use. See: Dolby Digital. Competes
with DTS Zeta Digital. The
terms AC-1 and AC-2 are other versions developed by
Dolby for different applications.
acquisition time The time required for a sample-and-hold
(S/H) circuit to capture an input analog value; specifically, the
time for the S/H output to approximately equal its input.
A word formed from the first letters of a name, such as laser
for light amplification by stimulated emission
of radiation, or by combining initial letters or parts of
a series of words, such as radar for radio detecting
and ranging. The requirement of forming a word
is what distinguishes an acronym from an abbreviation (or
initialism as the Canadian academicians say). Thus modem
[modulator-demodulator] is an acronym, and AES
[Audio Engineering Society] is an abbreviation
or initialism. [Unsubstantiated rumor has it that the word "acronym"
itself is an acronym, created from the phrase "abbreviating
by cropping remainders off names to
yield meaning" -- but it has never been confirmed.]
active crossover A loudspeaker
crossover requiring power to operate.
Usually rack-mounted as a separate unit, active crossovers require
individual power amplifiers for each output frequency band. Available
in configurations known as stereo 2-way, mono 3-way,
and so on. A stereo 2-way crossover is a two channel unit
that divides the incoming signal into two segments, labeled Low
and High outputs. A mono 3-way unit is a single channel
device with three outputs, labeled Low, Mid and High.
In this case, the user sets two frequencies: the Low-to-Mid, and
the Mid-to-High crossover points. Up to stereo 5-way configurations
exist for very elaborate systems. See: passive
active equalizer A variable
equalizer requiring power to operate. Available in many different
configurations and designs. Favored for low cost, small size, light
weight, loading indifference, good isolation (high input and low
output impedances), gain availability (signal boosting possible),
and line-driving ability. Disadvantages include, increased noise
performance, limited dynamic range, reduced reliability, and RFI
susceptibility; however, various manufactures continue to develop
technologies improve on these limitations.
adaptive delta modulation (ADM) A variation of delta
modulation in which the step size may vary from sample to sample.
ADAT (Alesis Digital
Audio Tape) Digital tape recording system developed by Alesis,
and since licensed to Fostex & Panasonic, putting 8-tracks of
16-bit, 44.1kHz digital audio on S-VHS tape.
ADAT ODI (optical digital interface) See ADAT
ADAT Optical Alesis's
proprietary multichannel optical (fiber optic) digital interface
specification for their family of ADAT modular digital multitrack
recorders. This standard describes transmission of 8-channels of
digital audio data through a single fiber optic cable.
A/D (analog-to-digital converter)
The electronic component which converts the instantaneous value
of an analog input signal to a digital word (represented as a binary
number) for digital signal processing. The A/D is the first link
in the digital chain of signal processing.
ADPCM (adaptive differential pulse code
modulation) A very fast data compression algorithm based
on the differences occurring between two samples.
ADSL (Asymmetrical digital subscriber Line) A
communication technology used to transmit digital data over
telephone wires. It is expected to transmit up to six megabits per
second and be used for on demand services such as video conferences.
AES (Audio Engineering
Society) Founded in 1949, the largest professional organization
for electronic engineers and all others actively involved in audio
engineering. Primarily concerned with education and standardization.
AES/EBU interface The serial transmission
format standardized for professional digital audio signals (AES3-1992
AES Recommended Practice for Digital Audio Engineering - Serial
Transmission Format for Two-Channel Linearly Represented Digital
Audio Data) A specification using time division multiplex for
data, and balanced line drivers
to transmit two channels of digital audio data on a single twisted-pair
cable using 3-pin (XLR) connectors. Issued as ANSI S4.40-1985 by
the American National Standards Institute. In addition, information
documents are being prepared describing the transmission of AES3
formatted data by unbalanced coaxial cable and by fiber optic cable.
AF AF (audio frequency) (also abbreviated
af or a.f.) refers to alternating current (AC)
having a frequency such that, if applied to a transducer such as a
loudspeaker or headset, will produce acoustic waves within the range
of human hearing. The AF range is generally considered to be from 20
Hz to 20
kHz ( 20,000 Hz.)
AFL Abbreviation for after fade listen,
a term used on recording consoles and mixers,
referring to a signal taken after the main channel fader; hence
this sampling point tracks the main fader level. Also referred to
as post fade solo, but since PFL
already meant pre fade, AFL was adopted to prevent confusion.
Compare with PFL.
algorithm A structured set of
specifications that define methods and procedures tailored to accomplish a signal processing
or transmitting task.
For example, a fast Fourier transform (FFT),
or a finite impulse response (FIR) filter
are common DSP algorithms.
aliasing The problem of unwanted
frequencies created when sampling a signal of a frequency higher
than half the sampling rate. See: Nyquist
all-pass filter A filter that
provides only phase shift or phase delay without appreciably changing
the magnitude characteristic.
AM ( Amplitude Modulation ) A
method of transmitting data or audio signals using a radio frequency
(RF) carrier by varying the total
power ( amplitude ) of the carrier's radiated electromagnet (EM)
wave. The original signal is encoded in the carrier by mixing
the audio or data signal with the RF carrier in a modulator (
essentially a mixer ). The output of the modulator is
filtered, amplified and fed to the antenna. Broadcast
stations in the US "AM" band use this modulation technique. AM
is susceptible to static from electrical sources such as power
lines, thunderstorms and electrical motors.
ampere Abbr. I, also A. 1. A unit of
electric current in the International standard meter-kilogram-second
(mks) system. It is the steady current that when flowing in straight
parallel wires of infinite length and negligible cross section,
separated by a distance of one meter in free space, produces a force
between the wires of 2E-7 newtons per meter of length. 2.
A unit in the International System specified as one International
coulomb per second and equal to 0.999835 ampere. [After André
Ampère, André Marie (1775-1836) French physicist
and mathematician who formulated Ampère's law, a mathematical description
of the magnetic field produced by a current-carrying conductor.
Amplifier A device that
changes a small movement into a larger movement. However, in general
the most common and useful amplifiers actually use a small amount of
electrical force ( voltage ) to control a larger amount of
electrical force. Most useful amplifiers are said to be linear
amplifiers, that is the amplified output is always related to the
input in a linear relationship. This relationship ( or the ration of
output / input ) is known as the gain
of the amplifier. The process of amplifying a signal invariably
introduces some noise
into the signal, and the process cannot be 100% efficient -
amplifiers will always produce some waste heat.
amplifier classes Audio
power amplifiers are classified according to the relationship between
the output voltage swing and the input voltage swing, thus it is
primarily the design of the output stage that defines each class.
Classification is based on the amount of time the output devices
operate during one complete cycle of signal swing. This is also
defined in terms of output bias current [the amount of current flowing
in the output devices with no applied signal]. For discussion purposes
(with the exception of class A), assume a simple output stage consisting
of two complementary devices (one positive polarity and one negative
polarity) -- tubes (valves) or any type of transistor (bipolar,
MOSFET, JFET, IGFET, IGBT, etc.).
- Class A operation is where both devices conduct continuously
for the entire cycle of signal swing, or the bias current
flows in the output devices at all times. The key ingredient
of class A operation is that both devices are always on. There
is no condition where one or the other is turned off. Because
of this, class A amplifiers in reality are not complementary
designs. They are single-ended designs with only one type
polarity output devices. They may have "bottom side"
transistors but these are operated as fixed current sources,
not amplifying devices. Consequently class A is the most inefficient
of all power amplifier designs, averaging only around 20%
(meaning you draw about 5 times as much power from the source
as you deliver to the load!) Thus class A amplifiers are large,
heavy and run very hot. All this is due to the amplifier constantly
operating at full power. The positive effect of all this is
that class A designs are inherently the most linear, with
the least amount of distortion. [Much mystique and confusion
surrounds the term class A. Many mistakenly think it
means circuitry comprised of discrete components (as opposed
to integrated circuits). Such is not the case. A great many
integrated circuits incorporate class A designs, while just
as many discrete component circuits do not use class A designs.]
- Class B operation is the opposite of class A. Both
output devices are never allowed to be on at the same time,
or the bias is set so that current flow in a specific output
device is zero when not stimulated with an input signal, i.e.,
the current in a specific output flows for one half cycle.
Thus each output device is on for exactly one half of a complete
sinusoidal signal cycle. Due to this operation, class B designs
show high efficiency but poor linearity around the crossover
region. This is due to the time it takes to turn one device
off and the other device on, which translates into extreme
crossover distortion. Thus restricting class B designs to
power consumption critical applications, e.g., battery operated
equipment, such as 2-way radio and other communications audio.
- Class AB operation is the intermediate case. Here
both devices are allowed to be on at the same time (like in
class A), but just barely. The output bias is set so that
current flows in a specific output device appreciably more
than a half cycle but less than the entire cycle. That is,
only a small amount of current is allowed to flow through
both devices, unlike the complete load current of class A
designs, but enough to keep each device operating so they
respond instantly to input voltage demand s. Thus the inherent
non-linearity of class B designs is eliminated, without the
gross inefficiencies of the class A design. It is this combination
of good efficiency (around 50%) with excellent linearity that
makes class AB the most popular audio amplifier design.
- Class AB plus B design involves two pairs of output
devices: one pair operates class AB while the other (slave)
pair operates class B.
- Class C use is restricted to the broadcast industry
for radio frequency (RF) transmission. Its operation is characterized
by turning on one device at a time for less than one half
cycle. In essence, each output device is pulsed-on for some
percentage of the half cycle, instead of operating continuously
for the entire half cycle. This makes for an extremely efficient
design capable of enormous output power. It is the magic of
RF tuned circuits (flywheel effect) that overcomes the distortion
create d by class C pulsed operation.
- Class D operation is switching,
hence the term switching power amplifier. Here the
output devices are rapidly switched on and off at least twice
for each cycle (Sampling
Theorem). Theoretically since the output devices are either
completely on or completely off they do not dissipate any
power. If a device is on there is a large amount of current
flowing through it, but all the voltage is across the load,
so the power dissipated by the dev ice is zero (found by multiplying
the voltage across the device [zero] times the current
flowing through the device [big], so 0 x big = 0);
and when the device is off, the voltage is large, but the
current is zero so you get the same answer. Consequently class
D operation is theoretically 100% efficient, but this requires
zero on-impedance switches with infinitely fast switching
times -- a product we're still waiting for; meanwhile designs
do exist with true efficiencies approaching 90%. [Historical
note: the original use of the term "Class D" referred
to switching amplifiers that employed a resonant circuit at
the output to remove the harmonics of the switching frequency.
Today's use is much closer to the original "Class
- Class E operation involves amplifiers designed for
rectangular input pulses, not sinusoidal audio waveforms.
The output load is a tuned circuit, with the output voltage
resembling a damped single pulse. Normally Class E employs
a single transistor driven to act as a switch.
- The following terms, while generally agreed upon, are not considered
- Class F Also known by such terms as "biharmonic,"
"polyharmonic," "Class DC," "single-ended
Class D," "High-efficiency Class C," and "multiresonator."
Another example of a tuned power amplifier, whereby the load
is a tuned resonant circuit. One of the differences here is
the circuit is tuned for one or more harmonic frequencies
as well as the carrier frequency. See References: Krauss,
et al. for complete details.
- Class G operation involves changing the power supply
voltage from a lower level to a higher level when larger output
swings are required. There have been several ways to do this.
The simplest involves a single class AB output stage that
is connected to two power supply rails by a diode, or a transistor
switch. The design is such that for most musical program material,
the output stage is connected to the lower supply voltage,
and automatically switches to the higher rails for large signal
peaks [ thus the nickname rail-switcher]. Another approach
uses two class AB output stages, each connected to a different
power supply voltage, with the magnitude of the input signal
determining the signal path. Using two power supplies improves
efficiency enough to allow significantly more power for a
given size and weight. Class G is becoming common for pro
audio designs. [Historical note: Hitachi is credited with
pioneering class G designs with their 1977 Dynaharmony
HMA 8300 power amplifier.]
- Class H operation takes the class G design one step
further and actually modulates the higher power supply voltage
by the input signal. This allows the power supply to track
the audio input and provide just enough voltage for optimum
operation of the output devices [thus the nickname rail-tracker].
The efficiency of class H is comparable to class G designs.
[Historical note: Soundcraftsmen is credited with pioneering
class H designs with their 1977 Vari-proportional MA5002
- Class S First invented in 1932,
this technique is used for both amplification and amplitude
modulation. Similar to Class D except the rectangular PWM
voltage waveform is applied to a low-pass filter that allows
only the slowly varying dc or average voltage component to
appear across the load. Essentially this is what is termed
"Class D" today. See References:
Krauss for details.
amplitude 1. Greatness of size;
magnitude. 2. Physics. The maximum absolute value
of a periodically varying quantity. 3. Mathematics.
a. The maximum absolute value of a periodic curve measured along
its vertical axis. b. The angle made with the positive horizontal
axis by the vector representation of
a complex number. 4. Electronics. The maximum absolute
value reached by a voltage or current waveform.
analog A real world physical quantity
or signal characterized by being a continuously variable representation
of the original event (rather than discrete jumps in signal level
representing binary numbers that recording the value of the event
at discrete moments).
Annex D Graphics An addendum to the H.320
videoconferencing protocol used for still image transfer between
dissimilar video conferencing systems.
(pronounced "ann-see") (American National Standards
Institute) An organization that develops and publishes
standards for voluntary use in the U.S.A.
Antenna a specialized
that converts radio-frequency (RF)
fields into alternating current (AC)
or vice-versa. There are two basic types: the receiving antenna,
which intercepts RF energy and delivers AC to electronic receiving
equipment, and the transmitting antenna, which is fed with AC from
electronic transmitting equipment and generates an RF field.
anti-aliasing filter A low-pass
filter used at the input of digital audio converters to attenuate
frequencies above the half-sampling frequency to prevent aliasing.
anti-imaging filter A low-pass
filter used at the output of digital audio converters to attenuate
frequencies above the half-sampling frequency to eliminate image
spectra present at multiples of the sampling frequency.
Application A computer program designed for a particular
use, such as a word processor or spreadsheet.
Society of America) Founded in 1929, the oldest organization
for scientist and professional acousticians and others engaged in
acoustical design, research and education.
ASCII (pronounced "ask-ee") (American Standard
Code for Information Interchange) An ANSI
standard data transmission code consisting of seven information
bits, used to code 128 letters, numbers, and special characters.
Many systems now use an 8-bit binary code, called ASCII-8, in which
256 symbols are represented (for example, IBM's "extended
ASIC (application-specific integrated circuit) A
large-scale integrated circuit whose function is determined by the
final mask layer for a particular application or group of applications;
for example, an IC that does all the functions of a modem.
ASPEC (adaptive spectral perceptual entropy coding)
A bit rate reduction standard for high quality audio. Jointly developed
by AT&T Bell Labs, Thomson, the Fraunhofer Society and CNET.
Characterized by high degrees of compression to allow audio transmission
asymmetrical (non-reciprocal) response Term used to describe
the comparative shapes of the boost/cut curves for variable equalizers.
The cut curves do not mirror the boost curves, but instead are quite
narrow, intended to act as notch filters.
asynchronous A transmission process where the signal is
transmitted without any fixed timing relationship between one word
and the next (and the timing relationship is recovered from the
ATM (asynchronous transfer mode) networking An
extremely fast networking technology. ATM specifies the
protocol (i.e., the order and
sequence) of the digital information on the network, but not the
physical means of transmission (e.g., fiber optic, twisted-pair,
Pressure caused by the weight of the atmosphere. At sea level it
has a mean value of one atmosphere but reduces with increasing altitude.
Audio 1. Of or relating to humanly audible sound, i.e.,
audio is all the sounds that humans hear.
2. a. Of or relating to the broadcasting or reception
of sound. b. Of or relating to high-fidelity sound reproduction.
[Audio traveling through air is vibrations, or cycles of alternating
pressure zones that travel through the air as waves. Each wave can
be view as a zone of Rarefaction
followed by a cycle of compression.]
audio compression See: digital
audio data compression