Signals and Systems - Electrical Engineering

6 C H A P T E R 0: From the Ground Up!

DAC

Laser

Sensor

Audio

amplifier

Speaker

FIGURE 0.2

When playing a CD, the CD player follows the tracks in the disc, focusing a laser on them, as the CD is spun.

The laser shines a light that is reflected by the pits and bumps put on the surface of the disc and corresponding

to the coded digital signal from an acoustic signal. A sensor detects the reflected light and converts it into a

digital signal, which is then converted into an analog signal by the DAC. When amplified and fed to the speakers

such a signal sounds like the originally recorded acoustic signal.

pits and bumps corresponding to the ones and zeros from the quantization and encoding processes

are impressed on the surface of the disc. Such pits and bumps will be detected by the CD player and

converted back into an analog signal that approximates the original signal when the CD is played.

The transformation into an analog signal uses a DAC.

As we will see in Chapter 7, an audio signal is sampled at a rate of about 44,000 samples/second

(sec) (corresponding to a maximum frequency around 22 KHz for a typical audio signal) and each of

these samples is represented by a certain number of bits (typically 8 bits/sample). The need for stereo

sound requires that two channels be recorded. Overall, the number of bits representing the signal is

very large and needs to be compressed and especially encoded. The resulting data, in the form of pits

and bumps impressed on the CD surface, are put into a spiral track that goes from the inside to the

outside of the disc.

Besides the binary-to-analog conversion, the CD player exemplifies a very interesting control system

(see Figure 0.2). Indeed, the player must: (1) rotate the disc at different speeds depending on the

location of the track within the CD being read, (2) focus a laser and a lens system to read the pits

and bumps on the disc, and (3) move the laser to follow the track being read. To understand the

exactness required, consider that the width of the track and the high of the bumps is typically less

than a micrometer( 10 −^6 meters or 3.937× 10 −^5 inches)and a nanometer( 10 −^9 meters or 3.937×

10 −^8 inches), respectively.

0.2.2 Software-Defined Radio and Cognitive Radio...............................

Software-defined radio and cognitive radio are important emerging technologies in wireless commu-

nications [43]. In software-defined radio (SDR), some of the radio functions typically implemented

in hardware are converted into software [64]. By providing smart processing to SDRs, cognitive radio

(CR) will provide the flexibility needed to more efficiently use the radio frequency spectrum and to

make available new services to users. In the United States the Federal Communication Commission

(FCC), and likewise in other parts of the world the corresponding agencies, allocates the bands for