1440 Chapter 38
reduced dynamic range when compared to the indi-
vidual building blocks making them up.
38.1.2 The Introduction of Digital Devices in
Analog Systems
These factors combined together spurred the application
of digital signal processing (DSP) to audio.
DSP is the application of numerical processes to
perform signal processing functions on signals that have
been converted into sequences of numbers. The conver-
sion of an analog signal into a sequence of numbers is
done by an analog to digital converter or A/D. Similarly,
the conversion of a sequence of numbers back into an
analog signal is done by a digital to analog converter, or
D/A.
Delays in the analog world almost always involved
acoustic, mechanical, or magnetic systems. In other
words, you had to use transducers to go from the elec-
trical realm to some alternative form of energy and
back, since it was very difficult to delay the signal
enough to matter for audio systems while staying
strictly in electrical form.
Early digital signal delays had very poor perfor-
mance compared to today’s digital products, but they
were popular since the available analog delays were
often of even worse audio quality, had very short
maximum delay times, and often were not adjustable in
delay time.
Digital signal delays offered much longer delay
times, easy adjustability of the delay time, and often
multiple outputs, Fig. 38-1.
Analog reverbs always required some sort of
mechanical, magnetic, or acoustic system.
The first analog reverbs were simply isolated rooms
built with very reflective surfaces, and equipped with a
loudspeaker for inserting the sound, and one or more
microphones for picking up the reverberated sound.
Obviously, there are major drawbacks to this approach.
The size and cost limited the application of this tech-
nique, as did the difficulty in adjusting the characteris-
tics of the reverberation.
Another analog technique involved vibrating a large
thin steel plate with sound, and picking up the vibra-
tions at multiple locations on the surface of the plate.
This had the advantage of smaller size, and adjustable
reverberation by moving acoustic damping materials
closer to or farther away from the plate.
Smaller yet analog reverbs were made using a gold
foil instead of the steel plate, or by using springs that
were driven at one end and had vibration pickups at
their other end. The gold foil technique resulted in quite
acceptable sound, but the spring-based systems were
often of low cost and barely usable sound quality.
The first digital reverbs were very expensive, and of
a size comparable to that of the gold foil or better spring
systems, but had the advantage of much greater control
over the reverberation characteristics than could be
achieved with the analog systems. As the cost of digital
circuitry has come down over the years, so has the price
and size of DSP-based reverbs.
Analog recording and transmission of sound have
always involved significant reduction in the sound
quality as compared to the original sound. Each time the
sound was rerecorded or retransmitted, the quality was
further reduced.
Digital recording and transmission of sound offered
a dramatic difference. While the conversion of analog
signals into digital always involves some loss, as long
as the signal was kept in digital form and not turned
back into an analog signal, making additional copies or
transmitting the signal did not impose any additional
losses. Therefore, the generational losses associated
with the analog systems we had been using were elimi-
nated.38.2 Digitally Controlled Sound Systems38.2.1 Digitally Controlled Analog DevicesThe physical controls of an audio device have always
constituted a significant portion of the cost of the product.
This was fine for devices such as mixing consoles where
the operator needed instant access to all the controls.
There have always been controls, however, that while
necessary to the initial setup of the sound system, were
best hidden from easy adjustment by the user. These con-
trols were often placed behind security covers to reduce
the chance of their adjustment by unauthorized users.Figure 38-1. Introduced in 1974, the Eventide Clockworks
Model 1745M modular digital audio delay was the first to
use random access memory (RAM) instead of shift registers
for storage of sound. Options included pitch changing and
flanging.