Introduction to Acoustics

nificant design considerations. At the other end of the
chain, power amplifiers usually have a fairly easy time
with dynamic range, but high required power-handling
capability and the need to drive sometimes ill-behaved
loudspeaker loads make power amplifier design an art
unto itself, with the 1948 Williamson amplifier being
just one early example of a notable design [18.48].
The advent of the transistor, by Brattain, Bardeen,
and Shockley of Bell Laboratories in 1947, sparked
a revolution in active devices, and stands as one of the
foremost inventions of the 20th century [18.49,50]. Operational
differences between tubes and transistors were
sufficiently great that, to an extent, the discipline of amplifier
design had to be reinvented from the ground up,
and it was some time before solid-state amplifiers were
accepted in some high-end audio systems.
In time, the development of analog integrated
circuits gave rise to the multi-transistor operational amplifier,
an amazingly versatile device that facilitated
cost-effective analog processors of great sophistication
and complexity. These days, entire analog audio subsystems
can be implemented on monolithic chips with
many transistors and associated components.
Special mention should be made of one particularly
challenging amplifier variant, the voltage-controlled
variable-gain amplifier (VCA). Such a device, basically
an analog multiplier, is essential for exercising automatic
control of signal level, a common building block in
many audio signal processors. Where fixed-gain amplifiers
depend on fixed, highly linear passive components
such as resistors to maintain operating point and linearity,
VCAs require linear active elements, like FETs.
In the late 1960s, Barry Blesser designed a novel variable
pulse width multiplier, and a few years later, David
Blackmer of dbx, Inc. designed a wide-range variabletransconductance
VCA, elements of which are still used
in commercial VCAs [18.51]. The complexity and cost
of VCAs limited their use somewhat, which is perhaps
ironic in these days of digital audio, where DSP chips
typically perform millions of multiplications per second.
18.4.5 Magnetic and Optical Media
Part of the motivation for the use of magnetic and optical
recording on linear media (tape, film) undoubtedly
came from the desire to avoid the wear inherent in the
playback grooved records. Both media involve the use
of narrow apertures to record and play audio striations
perpendicular to the motion of a linear medium.
Magnetic recording got its start in 1898, with Valdemar
Poulsen recording on steel wire, an especially
Audio and Electroacoustics 18.4 Audio Components 767
notable accomplishment considering that the invention
of the triode amplifier tube was still several years away.
The use of magnetic tape originated with O’Neill’s
patent of paper tape coated with iron oxide, in 1926,
followed by the introduction of plastic tape by BASF
in 1935. A serious nonlinearity problem with magnetic
tape was largely resolved in 1939 with the development
of alternating-current (AC) recording bias. Considerable
refinement was achieved during and following World
War II, and consumer reel-to-reel tape recording became
a reality in the 1950s. There followed a series
of formats which expanded the performance envelope
and consumer friendliness, most notably the compact
cassette and the venerable eight-track cartridge.
Optical recording on film was first investigated
around 1901 [18.9], five years before the vacuum tube,
and started to receive significant research and development
attention starting around 1915, with the work
of Arnold and others at Bell Laboratories, followed
by a number of efforts by RCA and other groups. Lee
De Forest may have come up with the first viable optical
sound system, Phonofilm, in 1922 [18.52]. By 1928, synchronized
optical sound on film was used commercially
on Disney’s Steamboat Willie cartoon.
Both of these formats rely on some sort of device
with a narrow aperture to record and playback, and
therefore both face a tradeoff between high-frequency
response, corresponding to the smallest resolvable feature,
and media speed/playback time.
Although there may be no vibrating mechanical elements
in either the magnetic system or optical playback,
both media exhibit difficulty reaching the full audible
bandwidth, or in maintaining ruler-flat response, although,
properly designed and adjusted, they can come
fairly close. Beyond that, both media bear some similarities
to modern record-based reproduction, including
the use of fixed equalization, modest, gentle distortion
at typical operating levels, a typical dynamic range on
the order of 50–70 dB, and both absolute and dynamic
speed error sensitivity.
The viability of these formats was significantly
enhanced by the invention in 1965 of analog noise
reduction by Ray Dolby. Noise-reduction systems
usually consist of a compressor/encoder and an
expander/decoder. The compressor boosts low-level
signals to keep them above the noise floor of the channel,
while the expander restores the signals to their
proper level, reducing the noise in the process. The
Dolby A-type noise-reduction system and follow-on
systems exploited a range of solid-state circuit tools,
including active filters, precision rectifiers, and lin-
Part E 18.4