Baseband Transmission
Before it can be transmitted, the information or intelligence must be converted to an
electronic signal compatible with the medium. For example, a microphone changes voice
signals (sound waves) into an analog voltage of varying frequency and amplitude. This
signal is then passed over wires to a speaker or headphones. This is the way the telephone
system works.
A video camera generates an analog signal that represents the light variations along one
scan line of the picture. This analog signal is usually transmitted over a coaxial cable. Binary
data is generated by a keyboard attached to a computer. The computer stores the data and
processes it in some way. The data is then transmitted on cables to peripherals such as a
printer or to other computers over a LAN. Regardless of whether the original information
or intelligence signals are analog or digital, they are all referred to as baseband signals.
In a communication system, baseband information signals can be sent directly and
unmodii ed over the medium or can be used to modulate a carrier for transmission over
the medium. Putting the original voice, video, or digital signals directly into the medium
is referred to as baseband transmission. For example, in many telephone and intercom
systems, it is the voice itself that is placed on the wires and transmitted over some distance to the receiver. In most computer networks, the digital signals are applied directly
to coaxial or twisted-pair cables for transmission to another computer
In many instances, baseband signals are incompatible with the medium. Although it
is theoretically possible to transmit voice signals directly by radio, realistically it is
impractical. As a result, the baseband information signal, be it audio, video, or data, is
normally used to modulate a high-frequency signal called a carrier. The higher- frequency
carriers radiate into space more efi ciently than the baseband signals themselves. Such
wireless signals consist of both electric and magnetic i elds. These electromagnetic signals, which are able to travel through space for long distances, are also referred to as
radio-frequency (RF) waves, or just radio waves.
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Broadband Transmission
Modulation is the process of having a baseband voice, video, or digital signal modify
another, higher-frequency signal, the carrier. The process is illustrated in Fig. 1-7. The
information or intelligence to be sent is said to be impressed upon the carrier. The carrier
is usually a sine wave generated by an oscillator. The carrier is fed to a circuit called a
modulator along with the baseband intelligence signal. The intelligence signal changes
the carrier in a unique way. The modulated carrier is amplii ed and sent to the antenna
for transmission. This process is called broadband transmission.
Consider the common mathematical expression for a sine wave:
Figure 1-7 Modulation at the transmitter
Figure 1-8 Types of modulation. (a) Amplitude modulation. (b) Frequency modulation
The three ways to make the baseband signal change the carrier sine wave are to vary
its amplitude, vary its frequency, or vary its phase angle. The two most common methods of modulation are amplitude modulation (AM) and frequency modulation (FM). In
AM, the baseband information signal called the modulating signal varies the amplitude
of the higher-frequency carrier signal, as shown in Fig. 1-8(a). It changes the Vp
part of
the equation. In FM, the information signal varies the frequency of the carrier, as shown
in Fig. 1-8(b). The carrier amplitude remains constant. FM varies the value of f in the
i rst angle term inside the parentheses. Varying the phase angle produces phase modulation (PM). Here, the second term inside the parentheses (θ) is made to vary by the
intelligence signal. Phase modulation produces frequency modulation; therefore, the PM
signal is similar in appearance to a frequency-modulated carrier. Two common examples
of transmitting digital data by modulation are given in Fig. 1-9. In Fig. 1-9(a), the data
is converted to frequency-varying tones. This is called frequency-shift keying (FSK). In
Fig. 1-9(b), the data introduces a 180º-phase shift. This is called phase-shift keying
(PSK). Devices called modems (modulator-demodulator) translate the data from digital
to analog and back again. Both FM and PM are forms of angle modulation.
At the receiver, the carrier with the intelligence signal is amplii ed and then demodulated to extract the original baseband signal. Another name for the demodulation process
is detection. (See Fig. 1-10.)
Figure 1-9 Transmitting binary data in analog form. (a) FSK. (b) PSK
Figure 1-10 Recovering the intelligence signal at the receiver. 
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