Manual-7
MOJO GLOSSARY
balanced line
The recommended method of interconnecting audio
equipment. A balanced line requires three conductors: a twisted-pair
for the signal (positive and negative) and an overall shield.
The
shield must be tied to the chassis at both ends for hum-free intercon-
nect.
bandwidth
Abbr.
BW
The numerical difference between the upper
and lower -3 dB points of an audio band.
clipping
What occurs when a unit tries to produce a signal
larger
than its power supply
. The signal takes on a flat-topped, or
clipped
shape. When an amplifier tries to go above its max power, it
clips
.
compressor
A signal processing device used to
reduce the
dynamic range
of the signal passing through it. For instance, an
input dynamic range of 110 dB might pass through a compressor and
exit with a new dynamic range of 70 dB. The modern usage for
compressors is to turn down (or reduce the dynamic range of) just
the loudest signals. Other applications use compressors to control the
creation
of sound. When used in conjunction with microphones and
musical instrument pick-ups, compressors help determine the final
timbre by selectively compressing specific frequencies and wave-
forms.
connectors
Audio equipment uses different styles:
RCA
An
unbalanced
pin connector commonly used on
consumer and some pro equipment; aka
phono plug
XLR
A 3-pin connector common on pro audio equipment.
Preferred for
balanced line
interconnect; aka
Cannon plug
¼" TRS
1.
Stereo
¼" connector consisting of
tip
(T),
ring
(R),
and
sleeve
(S) sections, with T =
left
, R =
right
, and S =
ground/shield
. 2.
Balanced
interconnect with the pos & neg
signal lines tied to T and R respectively and S acting only as an
overall shield. 3.
Insert loop
interconnect with T =
send
, R =
return
, and S =
ground/shield
. [Think:
ring, right, return
]
¼" TS
Mono
¼" connector consisting of
tip
(T) [signal] and
sleeve
(S) [ground & shield] for
unbalanced
wiring.
constant-Q equalizer
(also
constant-bandwidth
)
The
bandwidth remains constant for all boost/cut levels. Since Q and
bandwidth are interrelated, the terms are fully interchangeable.
decibel
Abbr.
dB
(named after
Alexander Graham Bell
). The
preferred method and term for representing the
ratio
of different
audio levels. Being a ratio,
decibels have no units
. Everything is
relative. So it must be relative to some
0 dB reference point
. A suffix
letter is added to distinguish between reference points:
0 dBu
A reference point equal to 0.775 V
+4 dBu
Standard pro reference level equal to 1.23 V
0 dBV
A reference point equal to 1.0 V
-10 dBV
Standard reference level for consumer and some pro
audio use, equal to 0.316 V.
RCA
(phono) connectors are a
good indicator of units operating at -10 dBV
dynamic range
The ratio of the loudest signal to the quietest
signal in a unit or system as expressed in
decibels
(dB).
expander
A signal processing device used to
increase the dynamic
range
of the signal passing through it. Expanders complement
compressors. For example, a compressed input dynamic range of 70
dB might pass through a expander and exit with a new
expanded
dynamic range of 110 dB. Modern expanders usually operate only
below a set threshold point
, i.e., they operate only on low-level
audio. The term
downward expander
describes this type of applica-
tion.
ground
Any electrical reference point for measuring voltage levels.
Usually a large conducting body, such as the earth or an electric
circuit connected to the earth. Chassis should always be at earth
potential.
WARNING: SHOCK HAZARD
Never use an AC line cord ground-lift adapter or cut
off the 3rd pin. It is illegal and dangerous.
headroom
The level in dB between the typical operating level and
clipping
. For example, a n4 dBu system that clips at +20
dBu has 16 dB of
headroom
.
hum
Unwanted sound contaminating audio paths due to EMI
(electro-magnetic interference) caused by AC power-lines &
transformers getting into unbalanced, poorly shielded, or improperly
grounded connecting cables. Hum has a definite smooth (sine wave)
repetitive sound based on the harmonics of 50/60 Hz such as 100/
120 Hz and 150/180 Hz.
interpolating
Term meaning to insert between two points. If a
graphic equalizer’s adjacent bands, when moved together, produce
a smooth response without a dip in the center, they are
interpolat-
ing
between the fixed center frequencies.
levels
Terms used to describe relative audio signal levels:
mic-level
Nominal signal coming directly from a microphone.
Very low, in the microvolts, and requires a preamp with at least 60
dB gain before using with any
line-level
equipment.
line-level
Standard
+4 dBu
or
-10 dBV
audio levels.
instrument-level
Nominal signal from musical instruments
using electrical pick-ups. Varies widely, from very low
mic-levels
to quite large
line-levels
.
limiter
A compressor with a fixed
ratio
of 10:1 or greater. The
dynamic action prevents the audio signal from becoming larger than
the
threshold
setting.
Linkwitz-Riley crossover
The most preferred active crossover
design. It features steep 24 dB/octave slopes, in-phase outputs, and
flat amplitude response. Due to the in-phase outputs the acoustic
lobe resulting when both loudspeakers reproduce the crossover
frequency is always on-axis (not tilted up or down) and has no
peaking.
noise
1.
Interconnect
. Unwanted sounds contaminating audio paths.
RFI (radio frequency interference) caused by broadcast signals
leaking into unbalanced, poorly shielded, or improperly grounded
connecting cables. Also by light dimmers, motor controls and
computers. 2.
Music.
A random mix of audio frequencies not
harmonically related, sounding like radio static.
polarity
A signals electromechanical potential with respect to a
reference. For example, a microphone has
positive polarity
if a
positive pressure on its diaphragm results in a positive output
voltage.
polarity vs. phase shift:
polarity
refers to a signals
reference
NOT to its
phase shift
. Being 180 degrees
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 occurs
over a period of
time
; the other occurs instantaneously.
Q
(upper-case) Quality factor. Defined to be the ratio of the center
frequency
f
divided by the bandwidth
BW
for a bandpass filter.
signal-to-noise ratio
The ratio in dB between a reference level
and the noise floor. For example, a signal-to-noise ratio of 90 dB re
+4 dBu, means the noise floor is 90 dB below a +4 dBu ref.
unbalanced line
An audio interconnect scheme using one wire
with an overall shield. The shield must perform two functions: act as
the return signal path (
ground
) and
to protect the conductor from
noise (
shield
). Consequently this method is vulnerable to hum &
noise problems.
unity gain
A gain setting of one. The level out equals the level in.
