USB DONE RIGHT: Two magic boxes that let computer audio ...

AAC This is the music compression
format used by the world’s largest music
store, iTunes, and many assume that one
of the A’s stands for “Apple.” In fact it
stands for Advanced Audio Coding, and
it is an accepted standard. Like MP3,
which AAC was intended to replace, it
is a lossy system. See codec.
Absolute phase A music
system can be connected in either absolute
phase or reverse phase. With absolute
phase, the wave from a drumbeat,
for instance, will begin with motion
toward the listeners rather than away.
Though this can be audible, it is difficult
to ensure that all components of the
chain are in phase. Some amplifiers and
preamplifiers reverse phase (though of
course if the signal gets phase-reversed
twice, it is then back in phase. Some
recordings are phase-reversed too.
AB tests This method of evaluating
audio systems involve quick comparisons,
on the theory that auditory
memory is short. The ABX test, favored
today, includes two systems, A and B,
but also X, which is identical to A and B,
though the subject is not told which. It is
common to set up such tests with levels
closely matched, and with the person
conducting the test not knowing which
product is which (the double blind test).
Readers of UHF will know that AB and
ABX tests are controversial.
Acoustic suspension
A loudspeaker configuration that placed
a highly compliant (soft) woofer in a
small sealed enclosure. The stiffness
of the air in the enclosure would compensate
for the floppiness of the woofer
cone, and the combination would allow
reproduction of deeper bass than had
previously seemed possible. The famous
AR2 speaker used the acoustic suspension
principle, and was much copied.
32 ULTRA HIGH FIDELITY Magazine
Nuts&Bolts
High Definition
by Paul Bergman
Acoustic suspension was not without
problems. These speakers were inefficient,
requiring huge amplifier power. If
their power handling ability was limited,
they had poor dynamic range. A more
esoteric problem was imbalance between
the stiffness of the air at the rear of the
cone and the normal atmospheric pressure
at the front. The result was a sound
wave that was not quite symmetrical.
Aliasing Best known in digital
recording, aliasing is an undesirable
phenomenon that occurs when there are
signal frequencies that are more than
half the digital sampling frequency (44.1
kHz in the case of the Compact Disc). A
sharp filter is used to take out any signal
frequencies beyond 20 kHz. However
aliasing also occurs in film, whose “sampling
rate” is a very low 24 Hz (24 images
per second). Aliasing is the reason that
you see cars whose wheels seem to be
Paul Bergman brings
back his lexicon
feature from UHF’s
early issues…only
much expanded!
turning backward.
Analog This word became
common some decades ago in opposition
to “digital.” In audio, a sound wave
is recorded in a format analogous to the
sound wave itself: a modulated record
groove, a magnetic pattern on tape, etc.
Note that the electrical signal itself is
an analog of the acoustic wave. Digital
systems use a numerical description of
the signal, by periodic sampling of the
analog wave, using binary digits (1’s
and O’s).
Antiskating A lateral force
applied to a turntable’s tone arm, with
a spring, a weight or a magnet, to compensate
for the natural skating force on
the arm. If a tone arm were straight and
the cartridge mounted parallel to the
arm, there would be no skating force.
However such an arm would undergo
major tracking errors as it arced across
the record. Most arms therefore are
curved or bent, or the cartridge is
mounted at an angle. The geometry
results in a force pushing the arm toward
the record centre. Because actual skating
force depends on several factors, including
the composition of the record itself,
antiskating can only be approximate.
Balanced line This is a
wiring scheme almost universally used
in studios, and not uncommon in high
end home audio. A balanced line includes
three wires: a ground, a “hot” signal
wire, and a third wire that carries the
same signal but in opposite phase. An
advantage is that any induced noise can
be expected to influence both signal
wires, but noise will be cancelled out
at the receiving end. A balanced circuit
should therefore have high common mode
rejection, of any noise that is common
to both.
Bass reflex A loudspeaker
enclosure that uses resonance (a Helmholtz
resonator) to reverse the phase of
the speaker’s backwave so that it can
emerge in phase with the front wave.