Recording Consoles 79527.8 Digital Consoles ....................................................................................................
27.8.1 Introduction to Digital Signal Processing (DSP)
DSP involves the manipulation of real-world signals (for instance, audio signals, video
signals, medical or geophysical data signals) within a digital computer. Why might we
want to do this? Because these signals, once converted into digital form (by means of
an analogue to digital converter), may be manipulated using mathematical techniques to
enhance, change, or display data in a particular way. For instance, the computer might
use height or depth data from a geophysical survey to produce a colored contour map
or the computer might use a series of two-dimensional medical images to build up a
three-dimensional virtual visualization of diseased tissue or bone. Another application,
this time an audio one, might be used to remove noise from a music signal by carefully
measuring the spectrum of the interfering noise signal during a moment of silence (for
instance, during the run-in groove of a record) and then subtracting this spectrum from
the entire signal, thereby removing only the noise—and not the music—from a noisy
record.
DSP systems have been in existence for many years, but, in these older systems, the
computer might take many times longer than the duration of the signal acquisition time to
process the information. For instance, in the case of the noise reduction example, it might
take many hours to process a short musical track. This leads to an important distinction
that must be made in the design, specifi cation, and understanding of DSP systems; that of
nonreal time (where the processing time exceeds the acquisition or presentation time) and
real-time systems, which complete all the required mathematical operations so fast that
the observer is unaware of any delay in the process. When we talk about DSP in digital
audio it is always important to distinguish between real-time and nonreal-time DSP.
Audio outboard equipment that utilizes DSP techniques is, invariably, real time and has
dedicated DSP chips designed to complete data manipulation fast. Nonreal-time DSP is
found in audio processing on a PC or Apple Mac where some complex audio tasks may
take many times the length of the music sample to complete.
27.8.2 Digital Manipulation
So, what kind of digital manipulations might we expect? Let’s think of the functions
that we might expect to perform within a digital sound mixer. First, there is addition.
Clearly, at a fundamental level, that is what a mixer is—an “ adder ” of signals. Second,