ampere , amp 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 Marie Ampère.]
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 d ue 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%
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 data characterized
by being continuously variable (rather
than making discrete jumps), and can be as precise as the available measuring technique.
bandpass filter A filter that
has a finite passband, neither of the cutoff frequencies being
infinite. The bandpass frequencies are normally associated with frequencies that define the half power
points, i.e. the -3 dB points.
band-limiting filters A low-pass and a high-pass filter
in series, acting together to restrict (limit) the
overall bandwidth of a system.
bandwidth Abbr. BW The numerical difference between
the upper and lower -3 dB points of a
band of audio frequencies. Used to figure the Q, or quality factor, for a filter.
bel Abbr. b, B Ten decibels. [After Alexander Graham
Bell.] The Bel was the amount a signal
dropped in level over a one-mile distance of telephone wire. See: decibel
Bell, Alexander Graham (1847-1922) Scottish-born American
inventor of the telephone. The first
demonstration of electrical transmission of speech by his apparatus took place in 1876. Bell also
invented the audiometer, an early hearing aid, and improved the phonograph.
capacitor A capacitor can store
small amounts of electrical charge. It consists of two electrodes
with a non-conducting layer in between them. If you think of the capacitor as a small reservoir of electricity that will be released later, you will have a rough idea of how it works.
compression 1. An increase in density and pressure in
a medium, such as air, caused by the
passage of a sound wave. 2. The region in which this occurs.
compression wave A wave propagated by means of the compression
of a fluid, such as a sound
wave in air.
condenser microphone A microphone design where a condenser
(the original name for capacitor)
is created by stretching a thin diaphragm in front of a metal disc (the backplate). By positioning the
two surfaces very close together an electrical capacitor is created whose capacitance varies as a
function of sound pressure. Any change in sound pressure causes the diaphragm to move, which
changes the distance between the two surfaces. If the capacitor is first given an electrical charge
(polarized) then this movement changes the capacitance, and if the charge is fixed, then the
backplate voltage varies proportionally to the sound pressure. In order to create the fixed charge,
condenser microphones require external voltage (polarizing voltage) to operate. This is normally
supplied in the form of phantom power from the microphone preamp or the mixing console.
current Symbol i, I Electricity. a. A flow of electric
charge. b. The amount of electric charge
flowing past a specified circuit point per unit time, or the rate of flow of electrons. [As electrons flow
in one direction, the spaces left behind, called holes, appear to flow in the opposite direction. Thus,
current can be visualized as electron flow (negative current flow), or in the opposite direction, hole
flow (positive current flow, sometimes called conventional current flow).]
cutoff frequency Filters. The frequency at which the
signal falls off by 3 dB (the half power point)
from its maximum value. Also referred to as the -3 dB points, or the corner frequencies.
decibel Abbr. dB Equal to one-tenth
of a bel. [After Alexander Graham Bell.] The preferred
method and term for representing the ratio of different audio levels. It is a mathematical shorthand
that uses logarithms (a shortcut using the powers of 10 to represent the actual number) to reduce
the size of the number. For example, instead of saying the dynamic range is 32,000 to 1, we say it is
90 dB [the answer in dB equals 20 log x/y, where x and y are the different signal levels]. Being
a ratio, decibels have no units. Everything is relative. Since it is relative, then it must be relative to
some 0 dB reference point. To distinguish between reference points a suffix letter is added as
diatonic 1. Music. Of or using only the eight tones
of a standard major or minor scale without
digital filter Any filter accomplished in the digital domain.
digital signal Any signal which is quantized (i.e.,
limited to a distinct set of values) into digital words
at discrete points in time. The accuracy of a digital value is dependent on the number of bits used to
digitization Any conversion of analog information into a digital form.
DSP (digital signal processing) A technology for signal
processing that combines algorithms and
fast number-crunching digital hardware, and is capable of high-performance and flexibility.
dynamic microphone A microphone design where a wire
coil (the voice coil) is attached to a small
diaphragm such that sound pressure causes the coil to move in a magnetic field, thus creating an
electrical voltage proportional to the sound pressure. Works in almost the exact opposite of a
dynamic loudspeaker where an electrical voltage is applied to the voice coil attached to a large cone
(diaphragm) causing it to move in a magnetic field, thus creating a change in the immediate sound
pressure. In fact, under the right circumstances, both elements will operate as the other, i.e., a
dynamic loudspeaker will act as a microphone and a dynamic microphone will act as a loudspeaker
-- although not too loud.
dynamic range The ratio of the loudest (undistorted)
signal to that of the quietest (discernible) signal
in a unit or system as expressed in decibels (dB). Dynamic range is another way of stating the
maximum S/N ratio.With reference to signal processing equipment, the maximum output signal is
restricted by the size of the power supplies, i.e., it cannot swing more voltage than is available. While
the minimum output signal is determined by the noise floor of the unit, i.e., it cannot put out a
discernible signal smaller than the noise. Professional-grade analog signal processing equipment can
output maximum levels of +26 dBu, with the best noise floors being down around -94 dBu. This
gives a maximum dynamic range of 120 dB - pretty impressive numbers, which coincide nicely with
the 120 dB dynamic range of normal human hearing (from just audible to uncomfortably loud).
dyne A unit of force, equal to the force required to
impart an acceleration of one centimeter per
second per second to a mass of one gram. Old usage for sound pressure.
electret microphone A microphone
design similar to that of condenser mics except ultiziing a
permanent electrical charge, thus eliminating the need for an external polarizing voltage. This is done
by using a material call an electret [acronym for electricity + magnet] that holds a permanent charge
(similar to a permanent magnet, i.e., a solid dielectric that exhibits persistent dielectric polarization).
Because electret elements exhibit extremely high output impedance, they often employ an integral
built-in impedance converter (usually a single JFET) that requires external power to operate. This
low voltage power is often supplied single-ended over an unbalanced connection, or it may operate
from standard phantom power.
filter Any of various electric,
electronic, acoustic, or optical devices used to reject signals,
vibrations, or radiations of certain frequencies while passing others. Think sieve: pass what you want,
reject all else. For audio use the most common electronic filter is a bandpass filter, characterized by
three parameters: center frequency, amplitude (or magnitude), and bandwidth. Bandpass filters
form the heart of audio graphic equalizers and parametric equalizers.
FIR (finite impulse-response) filter A commonly used
type of digital filter. Digitized samples of
the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite
number of previous inputs. An FIR filter can be designed to have completely linear phase (i.e.,
constant time delay, regardless of frequency). FIR filters designed for frequencies much lower that
the sample rate and/or with sharp transitions are computationally intensive, with large time delays.
Popularly used for adaptive filters.
frequency 1. The property or condition of occurring
at frequent intervals. 2. Mathematics.
Physics. The number of times a specified phenomenon occurs within a specified interval, as: a. The
number of repetitions of a complete sequence of values of a periodic function per unit variation of an
independent variable. b. The number of complete cycles of a periodic process occurring per unit
time. c. The number of repetitions per unit time of a complete waveform, as of an electric current.
hertz Abbr. Hz. A unit of frequency
equal to one cycle per second. [After Heinrich Rudolf
high-pass filter A filter having a passband extending
from some finite cutoff frequency (not zero) up
to infinite frequency.
IIR (infinite impulse-response) filter
A commonly used type of digital filter. This recursive
structure accepts as inputs digitized samples of the audio signal, and then each output point is
computed on the basis of a weighted sum of past output (feedback) terms, as well as past input
values. An IIR filter is more efficient than its FIR counterpart, but poses more challenging design
issues. Its strength is in not requiring as much DSP power as FIR, while its weakness is not having
linear group delay and possible instabilities.
impedance The numerical measure of the complex reaction
to current flow that a general electrical
network has when excited with an ac voltage.
inverse square law Sound Pressure Level. Sound propagates
in all directions to form a spherical
field, thus sound energy is inversely proportional to the square of the distance, i.e., doubling the
distance quarters the sound energy (the inverse square law), so SPL is attenuated 6dB for each
kHz (kilohertz) One thousand (1,000) cycles per second.
loudness The SPL of a standard
sound which appears to be as loud as the unknown. Loudness
level is measured in phons and equals the equivalent SPL in dB of the standard. [For example, a
sound judged as loud as a 40 dB-SPL 1 kHz tone has a loudness level of 40 phons. Also, it takes
10 phons (an increase of 10 dB-SPL) to be judged twice as loud.]
low-pass filter A filter having a passband extending
from DC (zero Hz) to some finite cutoff
frequency (not infinite). A filter with a characteristic that allows all frequencies below a specified
rolloff frequency to pass and attenuate all frequencies above.
mixer At its simplest level, an
audio device used to add (combine or sum) multiple inputs into
two outputs, complete with level controls on all inputs. From here signal processing is added to each
of the inputs and outputs until behemoth monsters with as many as 64 inputs are created -- at a cost
of around 10-20 kilobucks per input for fully digitized and automated boards. At these price points a
mixer becomes a recording console.
notch filter A special type of
cut-only equalizer used to attenuate (only, no boosting provisions
exist) a narrow band of frequencies. Three controls: frequency, bandwidth and depth, determine
the notch. Simplified units provide only a frequency control, with bandwidth and depth fixed
internally. Used most often in acoustic feedback control to eliminate a small band of frequencies
where the system wants to howl (feedback).
octave 1. Audio. The interval between any two frequencies having a ratio of 2 to 1. 2. Music a. The interval of eight diatonic degrees between two tones, one of which has twice as many vibrations per second as the other. b. A tone that is eight full tones above or below another given tone. c. An organ stop that produces tones an octave above those usually produced by the keys played.
ohm Abbr. R, (Greek upper-case omega). A unit of electrical
resistance equal to that of a
conductor in which a current of one ampere is produced by a potential of one volt across its
terminals. [After Georg Simon Ohm.]
one-third octave 1. Term referring to frequencies spaced
every one-third of an octave apart.
One-third of an octave represents a frequency 1.26-times above a reference, or 0.794-times below
the same reference. The math goes like this: 1/3-octave = 2E1/3 = 1.260; and the reciprocal,
1/1.260 = 0.794. Therefore, for example, a frequency 1/3-octave above a 1 kHz reference equals
1.26 kHz (which is rounded-off to the ANSI-ISO preferred frequency of "1.25 kHz" for equalizers
and analyzers), while a frequency 1/3-octave below 1 kHz equals 794 Hz (labeled "800 Hz").
Mathematically it is significant to note that, to a very close degree, 2E1/3 equals 10E1/10 (1.2599
vs. 1.2589). This bit of natural niceness allows the same frequency divisions to be used to divide
and mark an octave into one-thirds and a decade into one-tenths. 2. Term used to express the
bandwidth of equalizers and other filters that are 1/3-octave wide at their -3 dB (half-power) points.
3. Approximates the smallest region (bandwidth) humans reliably detect change. See: critical bands.
Compare with: third-octave
op amp (operational amplifier) An analog integrated
circuit device characterized as having two
opposite polarity inputs and one output, used as the basic building block in analog signal processing.
pascal Abbr. Pa A unit of pressure equal to one newton per square meter.
passband The range of frequencies passed by an audio
low-pass, high-pass or bandpass filter.
Normally measured at the -3 dB point: the frequency point where the amplitude response is
attenuated 3 dB (decibels) relative to the level of the main passband. For a bandpass filter two
points are referenced: the upper and lower -3dB points. The -3dB point represents the frequency
where the output power has been reduced by one-half. [Technical details: -3dB represents a
multiplier of 0.707. If the voltage is reduced by 0.707, the current is also reduced by 0.707 (ohms
law), and since power equals voltage-times-current, 0.707 times 0.707 equals 0.5, or half-power.]
period Abbr. T, t 1. The period of a periodic function
is the smallest time interval over which the
function repeats itself. [For example, the period of a sine wave is the amount of time, T, it takes for
the waveform to pass through 360 degrees. Also, it is the reciprocal of the frequency itself: i.e., T =
phantom power The term given to the standardized scheme
of providing power supply voltage to
certain microphones using the same two lines as the balanced audio path. The internationl standard is
IEC 268-15, derived from the original German standard DIN 45 596. It specifies three DC voltage
levels of 48 volts, 24 volts and 12 volts, delivered through 6.8 k ohms, 1.2 k ohms, and 680 ohms
matched resistors respectively, capable of delivering 10-15 ma. The design calls for both signal
conductors to have the same DC potential. This allows the use of microphone connections either for
microphones without built-in preamps, such as dynamic types, or for microphones with built-in
preamps such as condenser and electret types.
phon A unit of apparent loudness, equal in number to
the intensity in decibels of a 1,000 Hz tone
judged to be as loud as the sound being measured.
polarity A signal's electromechanical potential with
respect to a reference potential. For example, if
a loudspeaker cone moves forward when a positive voltage is applied between its red and black
terminals, then it is said to have a positive polarity. A microphone has positive polarity if a positive
pressure on its diaphragm results in a positive output voltage. [Usage Note: polarity vs. phase shift:
polarity refers to a signal's reference NOT to its phase shift. Being 180° out-of-phase and having
inverse polarity are DIFFERENT things. We wrongly say something is out-of-phase when we
mean it is inverted. One takes time; the other does not.]
psychoacoustics The scientific study of the perception of sound.psychoacoustics The scientific study of the perception of sound.
Q (upper-case) Quality factor.
Filters. The selectivity factor, defined to be the ratio of the
frequency f divided by the bandwidth BW.
rarefaction 1. A decrease in density
and pressure in a medium, such as air, caused by the passage
of a sound wave. 2. The region in which this occurs.
root mean square Abbr. rms, RMS Mathematics. The square
root of the average of the squares
of a group of numbers. A useful and more meaningful way of averaging a group of numbers.
slew rate 1. The term used to
define the maximum rate of change of an amplifier's output voltage
with respect to its input voltage. In essence, slew rate is a measure of an amplifier's ability to follow
its input signal. It is measured by applying a large amplitude step function (a signal starting at 0 volts
and "instantaneously" jumping to some large level [without overshoot or ringing], creating a step-like
look on an oscilloscope) to the amplifier under test and measuring the slope of the output waveform.
For a "perfect" step input (i.e., one with a rise time at least 100 times faster than the amplifier under
test), the output will not be vertical; it will exhibit a pronounced slope. The slope is caused by the
amplifier having a finite amount of current available to charge and discharge its internal compensation
S/N or SNR (signal-to-noise ratio) An audio measurement
of the residual noise of a unit, stated
as the ratio of signal level (or power) to noise level (or power), normally expressed in decibels. The
"signal" reference level must be stated. Typically this is either the expected nominal operating level,
say, +4 dBu for professional audio, or the maximum output level, usually around +20 dBu. The noise
is measured using a true RMS type voltmeter over a specified bandwidth, and sometimes using
weighting filters. All these thing must be stated for a S/N spec to have meaning. Simply saying a unit
has a SNR of 90 dB means nothing, without giving the reference level, measurement bandwidth, and
any weighting filers. A system's maximum S/N is called the dynamic range.
sone A subjective unit of loudness, as perceived by
a person with normal hearing, equal to the
loudness of a pure tone having a frequency of 1,000 hertz at 40 decibels sound pressure level.
sound pressure The value of the rapid variation in air
pressure due to a sound wave, measured in
pascals, microbars, or dynes - all used interchangeable, but pascals is now the preferred term.
Instantaneous sound pressure is the peak value of the air pressure, often used in noise control
measurements. Effective sound pressure is the RMS value of the instantaneous sound pressure
taken at a point over a period of time.
sound pressure level or SPL The RMS sound pressure expressed
in dB re 20 microPa (the
lowest threshold of hearing for 1 kHz. [As points of reference, 0 dB-SPL equals the threshold of
hearing, while 140 dB-SPL equals irreparable hearing damage.] See: inverse square law
third-octave Term referring to
frequencies spaced every three octaves apart. For example, the
third-octave above 1 kHz is 8 kHz. Commonly misused to mean one-third octave. While it can be
argued that "third" can also mean one of three equal parts, and as such might be used to correctly
describe one part of an octave spit into three equal parts, it is potentially too confusing. The
preferred term is one-third octave.
volt Abbr. E, also V. The International
System unit of electric potential and electromotive force,
equal to the difference of electric potential between two points on a conducting wire carrying a
constant current of one ampere when the power dissipated between the points is one watt. [After
Count Alessandro Volta.]
wavelength Symbol (Greek lower-case lambda) The distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelength of any frequency may be found by dividing the speed of sound by the frequency.