Virtual Systems 143938.1 The Design of Sound Systems
Sound systems are made of three primary components:
- Input transducers.
- Signal processing.
- Output transducers
38.1.1 Analog Systems
Transducers are devices that convert energy from one
form into another.
The primary type of input transducer used in sound
systems is the microphone. It converts the form of
acoustic energy we call sound into electrical energy
carrying the same information. Other common audio
input transducers include the magnetic tape head, the
optical sensor, the radio receiver, and the phonograph
pickup cartridge. Tape recorders, floppy and hard drives
use magnetic heads to transform analog or digital
magnetic patterns on the magnetic media into electrical
signals. Optical free-space links, optical fiber receivers,
and CD and DVD players all use optical sensors to turn
optical energy into electrical energy. Radio receivers
turn carefully selected portions of radio frequency
energy into electrical energy. Phonograph cartridges
turn the mechanical motion of the grooves in a record
into electrical energy.
Similarly, the most common type of output trans-
ducer used in sound systems is the loudspeaker. It
converts electrical energy back into the form of acous-
tical energy we call sound. Other common output trans-
ducers include headphones, magnetic tape heads, lasers,
radio transmitters, and record cutting heads. Head-
phones are specialized electrical to acoustic transducers,
which are intended to produce sound for one person
only. Tape recorders, floppy and hard drives use
magnetic heads to transform electrical signals into
magnetic patterns on the magnetic media. Optical
free-space links, optical fiber transmitters, CDR,
CDRW, DVD±RW, and BD recorders all use lasers to
turn electrical energy into optical energy. Radio trans-
mitters turn electrical signals into radio frequency
energy. Phonograph cutting heads turn electrical energy
into the mechanical motion of the grooves in a record.
In general, we can’t just connect a microphone to a
loudspeaker and have a usable sound system. While
there are exceptions such as “sound powered” tele-
phones, in almost all cases there needs to be something
that falls under the general heading of signal processing
to connect the two.
In its most simplified form this processing might
only consist of amplification. In general, microphones
have low electrical power output levels, while loud-
speakers require more electrical input power in order to
produce the desired acoustic output level. Thus the
processing required is amplification.
The next most common form of audio signal
processing is the level control. This is used to adjust the
amount of amplification to match the requirements of
the system at this moment.
Multiple inputs to the signal processing are often
each equipped with their own level control, and the
outputs of the level controls combined. This forms the
most basic audio mixer.
Much of the rest of what is done in signal processing
can be classified as processing that is intended to
compensate for the limitations of the input and output
transducers, the environment of the input and output
transducers, and/or the humans using and experiencing
the sound system. Such processing includes, among
other things, equalization, dynamics processing, and
signal delay.
Equalization includes shelving, parametric, graphic,
and the subcategories of filtering, and crossovers among
others. Common filters include high pass, low pass, and
all pass. Crossovers are made of filters used to separate
the audio into frequency bands.
Dynamics processing is different in that the parame-
ters of the processing vary in ways that are dependent
on the current and past signal. Dynamics processors
include compressors, limiters, gates, expanders, auto-
matic gain controls (AGC), duckers, and ambient level
controlled devices.
Signal delays produce an output some amount of
time (usually fixed) after the signal enters the device.
The biggest early breakthrough in sound systems
was the development of analog electrical signal
processing. No longer was the designer limited to some
sort of mechanical or acoustic system. This was taken a
large step forward with the development of vacuum
tube-based electronic circuitry
Later, transistor circuitry allowed smaller product
sizes and more complex processing to be done. The
development of analog integrated circuits accelerated
this trend.
Analog signal processing had its limitations,
however. Certain types of processing such as signal
delays and reverbs were very difficult to produce. Every
time the signal was recorded or transmitted, quality was
lost. Cascades of circuitry required to meet the ever
more complex requirements of sound systems had