Analog Transmission Chapter 5 1 Digital To Analog

Analog Transmission Chapter 5 1

Digital To Analog Transmission 2

Reason of Conversion � Digital data Analog Signal � Allows us of the public telephone system � Allows use of optical fiber � Analog Data Analog Signal � Easy � Telephone system was primarily analog 3

Digital-to-Analog Conversion �Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. 4

Digital-to-analog conversion 5

Aspects of D/A Conversion � Data Element: � Smallest piece of information � Signal Element: � Smallest piece of signal � Signal Rate (Baud Rate): It is supposed to be vehicle. � Number of signals elements per second. � Baud rate determines the bandwidth required to send the signal. � Data Rate (Bit Rate): It is supposed to be passenger. � Number of bits per second. (in bps) � Describes a medium capacity. 6

Carrier Signal Vs. Modulator Signals �Carrier signal (carrier frequency) �In analog signal, the sending device produces a high- frequency signal that acts as a basis for the information signal. �Receiving device is tuned to the frequency of the carrier signal that it expects from the sender. �Digital information then modulates the carrier signal by modifying one or more of its characteristics (amplitude, frequency, or phase). This kind of modification is called modulation (or shift keying, and the information signal is called the modulating signal). 7

Modulation & Demodulation �The technique of superimposing the message signal on the carrier is known as modulation. �The process of conveying a message signal, (for example a digital bit stream or an analog audio signal), inside another signal that can be physically transmitted. �The reverse operation (demodulation/detection) is performed at the receiving end. 8

Overview of Modulation Phone Line Serial link Modem Computer Digital Analog

Baseband Vs. Broadband � In Baseband, data is sent as digital signals through the media as a single channel that uses the entire bandwidth of the media. � Baseband communication is bi-directional, which means that the same channel can be used to send and receive signals. � In Baseband frequency-division multiplexing is not possible. � Multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. � Broadband sends information in the form of an analog signal. Each transmission is assigned to a portion of the bandwidth, hence multiple transmissions are possible at the same time. � Broadband communication is unidirectional, so in order to send and receive, two pathways are needed. � This can be accomplished either by assigning a frequency for sending and assigning a frequency for receiving along the same cable or by using two cables, one for sending and one for receiving. In broadband frequency-division multiplexing is possible 10

1. Amplitude Shift Keying �Amplitude Shift Keying: Amplitude of carrier signal is varied to create signal elements. �Both Frequency and Phase remain constant. �The strength of the carrier signal is varied to represent binary 1 or 0. �Types � BASK (Binary Amplitude Shift Keying) or OOK (on-off keying) � MASK (Multiple Amplitude Shift Keying) 11

1. Amplitude Shift Keying n Peak amplitude of the signal during each bit duration is constant, and its value depends on the bit (0 or 1). n ASK is susceptible to noise interference. n Noise: Unintentional voltages introduced by various sources i-e heat or electromagnetic signals, etc 12

2. Frequency Shift Keying �Frequency Shift Keying (FSK) �The frequency of the carrier signal is varied to represent data. �Frequency of the modulated signal is constant for the duration of one signal element, but could change for the next signal element. �Peak Amplitude and Phase remain constant for all signal elements. �Frequency of the carrier signal is varied to represent binary 1 or 0. 13

2. Frequency Shift Keying n FSK avoids most of the problems from noise. 14

3. Phase Shift Keying n The phase of the carrier is varied to represent binary 1 or 0. n Both peak amplitude and frequency remain constant. n If we start with a phase of 00 to represent binary 0, then we can change the phase to 180 to send binary 1. n The phase of the signal during each bit duration is constant, and its value depends on the bit (0 or 1). 15

Phase Shift Keying n 2 -PSK or binary PSK: Two different phases (00 and 1800) are used. Constellation or phase-state diagram shows the relationship by illustrating only the phases. 16

4. Quadrature Amplitude Modulation n Combination of ASK and PSK We could have x variations in phase and y variations in amplitude, giving us x times y possible variations and the corresponding number of bits per variation. Because amplitude changes are susceptible to noise and require greater shift differences than do phase changes. 17

4. Quadrature Amplitude Modulation 18

Analog To Analog Transmission Or Analog Modulation 19

Analog-to-Analog Transmission n Modulation of an analog signal or analog-to-analog conversion is the representation of analog information by an analog signal. 20

Analog-to-Analog Transmission 21

1. Amplitude Modulation n Carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. n n Frequency and phase of the carrier remains the same. Bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal. n Bandwidth of audio signal (speech and music) is usually 5 KHz. Therefore, an AM radio station needs a minimum bandwidth of 10 KHz. Federal Communications Commission (FCC) allows 10 KHz for each AM station. 22

1. Amplitude Modulation 23

2. Frequency Modulation n Frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. n Peak amplitude and phase of the carrier signal remains constant. n Bandwidth of an FM signal is equal to 10 times the bandwidth of the modulating signal and, like AM bandwidths, covers a range centred on the carrier frequency. BWt = 10 x BWm. n Bandwidth of an analog signal (speech and music) broadcast in stereo is almost 15 KHz. Each FM radio station, needs a minimum bandwidth of 150 KHz. FCC allows 200 KHz for each station to provide some room for guard bands. 24

2. Frequency Modulation • FM stations are allowed carrier frequencies anywhere between 88 and 108 MHz. • Stations must be separated by at least 200 KHz to keep their bandwidths from overlapping. • To create even more privacy, FCC requires that in a given area, only alternate bandwidth allocations may be used. Others remain unused to prevent any possibility of two stations interfering with each other. 25

2. Frequency Modulation 26

3. Phase Modulation • Phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. • Due to simpler hardware requirements, PM is used in some systems as an alternative to frequency modulation. • Peak amplitude and frequency of the carrier signal remain constant. 27