Introduction to Digital Signal Processing - Digital Signal Processing: Fundamentals and Applications - page 2

Band-limited
signal

Digital
signal

Processed
digital signal

Output
signal

Analog
input

Analog
output

Analog
filter

Reconstruction
filter

ADC DSP DAC

FIGURE 1.1

A digital signal processing scheme.

The basic concept of DSP is illustrated by the simplified block diagram in Figure 1.1 , which
consists of an analog filter, an analog-to-digital conversion (ADC) unit, a digital signal (DS) processor,
a digital-to-analog conversion (DAC) unit, and a reconstruction (anti-image) filter.
As shown in the diagram, the analog input signal, which is continuous in time and amplitude, is
generally encountered in the world around us. Examples of such analog signals include current,
voltage, temperature, pressure, and light intensity. Usually a transducer (sensor) is used to convert the
nonelectrical signal to the analog electrical signal (voltage). This analog signal is fed to an analog
filter, which is applied to limit the frequency range of analog signals prior to the sampling process. The
purpose of filtering is to significantly attenuate aliasing distortion , which will be explained in the next
chapter. The band-limited signal at the output of the analog filter is then sampled and converted via the
ADC unit into the digital signal, which is discrete both in time and in amplitude. The DS processor
then accepts the digital signal and processes the digital data according to DSP rules such as lowpass,
highpass, and bandpass digital filtering, or other algorithms for different applications. Notice that the
DS processor unit is a special type of digital computer and can be a general-purpose digital computer,
a microprocessor, or an advanced microcontroller; furthermore, DSP rules can be implemented using
software in general.
With the DS processor and corresponding software, a processed digital output signal is gener-
ated. This signal behaves in a manner according to the specific algorithm used. The next block in
Figure 1.1 , the DAC unit, converts the processed digital signal to an analog output signal. As shown,
the signal is continuous in time and discrete in amplitude (usually a sample-and-hold signal, to be
discussed in Chapter 2). The final block in Figure 1.1 is designated as a function to smooth the DAC
output voltage levels back to the analog signal via a reconstruction (anti-image) filter for real-world
applications.
In general, the analog signal process does not require software, an algorithm, ADC, and DAC. The
processing relies wholly on the electrical and electronic devices such as resistors, capacitors, tran-
sistors, operational amplifiers, and integrated circuits (ICs).
DSP systems, on the other hand, use software, digital processing, and algorithms; thus they have
a great deal of flexibility, less noise interference, and no signal distortion in various applications.
However, as shown in Figure 1.1 , DSP systems still require minimum analog processing such as the
anti-aliasing and reconstruction filters, which are musts for converting real-world information into
digital form and digital signals back into real-world information.
Note that there are many real-world DSP applications that do not require DAC, such as data
acquisition and digital information display, speech recognition, data encoding, and so on. Similarly,
DSP applications that need no ADC include CD players, text-to-speech synthesis, and digital tone
generators, among others. We will review some of them in the following sections.