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Unformatted text preview: tion (FM).
Two binary values of digital data are
represented by two differ frequencies while the amplitude and phase are kept
Also known as frequency shift key, (FSK), this method is less
susceptible to error than amplitude modulation. Another advantage oft method is
that it is easier to design devices based on FSK because discrimination between two frequencies is simpler than detecting phase changes. Thus for medium speed
communications (1200 to 2400 bits per second) the FSK scheme is preferred.
3. Phase Modulation (PM). Two binary values or digital data are represented by
the shift in phase of the carrier signal. That is, a sine wave with phase = 0°
represents a digital 1 and a sine wave with phase = 180° represents a digital 0.
This technique is more noise resistant and efficient than both amplitude
modulation and frequency modulation. For higher speed transmission of 9600 bits
per second, phase modulation is more reliable and is preferred to other methods.
The process of modulation and demodulation, that is, the conversion of digital data
to analog form and vice-versa, is carried out by a special device called a modem
(modulator/demodulator). Hence, when an analog facility is used for data
communication between two digital devices (say a terminal and a computer), two
modems are required, one near each digital device. As shown in Figure 17.9, the
digital signal generated at the terminal is converted to analog form by the
modulator of the modem placed near it. The analog signal is transmitted through
the telephone line, which is converted to digital form by the demodulator of the
modem placed near the computer. This digital data is processed by the computer.
The processed digital data is modulated to analog form and returned via the
telephone line to the terminal where the analog signals are demodulated to digital
form for display on the terminal. Thus, the modem is an essential piece of
hardware for any application in which two digital devices (say two computers)
want to communi...
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- Spring '14