Conversion
Digital To Analog Conversion
- Process of changing one of the characteristics of an analog signal based on the info in a digital signal
- When you Tx data from one computer to the other using a public telephone line
- Original data is digital but because telephone wires carry analog signal, original data must be converted
- Digital data must be modulated on an analog signal that has been manipulated to look like two distinct values corresponding to binary 1 to binary 0
- Figure shows the relationship b/w digital info the digital to analog conversion hardware & resultant analog signal
Variation in Characteristics of Sine Wave
- A sine wave is defined by 3 characteristics:
- By changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data
- When we vary any one of these characteristics ,we create a second version of that wave
- If we than say that the original wave represents binary 1,the variation can represent binary 0 or vice versa
- So by changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data
Mechanisms for Modulating Digital Data to Analog Signals
- Any of the three characteristics listed above can be altered in this way, giving us at least 3 mechanisms for modulating digital data into analog signals
- Amplitude shift keying(ASK)
- Frequency shift keying(FSK)
- Phase shift keying (PSK)
Fourth Mechanism
- In addition, there is a fourth and better mechanism that combines changes in both amplitude and phase called Quadratue Amplitude Modulation(QAM)
- QAM is the most efficient of these options and is the mechanism used in all modern modems
Types of digital to analog modulation
Aspects of Digital to Analog Conversion
- Before we discuss specific methods of digital to analog modulation,two basic issues must be defined:
- Bit/Baud rate
- Carrier signal
- Bit Rate & Baud Rate
- Two terms used frequently in data communication
- Bit rate
- Baud rate
- Bit rate: no of bits transmitted during one second
- Baud rate: no of signal units per second that are required to represent that bit
Bit Rate & Baud Rate
- In discussion of computer efficiency, bit rate is more important –we want to know how long it takes to process each piece of info
- In data transmission, however ,we are more concerned with how efficiently we can more that data from place to place, whether in pieces or blocks
- The fewer signal units required, the most efficient the system and less bandwidth required to transmit more bits ,so we are more concerned with baud rate
- The baud rate determines the B.W required to send the signal
Relationship b/w bit rate &band rate
- Bit rate equals the baud rate times the no. of bits represented by each signal units
- The baud rate equals the bit rate divided by the no. of bits represented by each signal shift
- Bit rate is always greater than or equal to Baud rate
Analogy for Bit rate &Baud rate
- In transportation a band is analogous to a car,a bit is analogous to a passenger
- A car can carry one or more passengers
- If1000 cars can go from one point to another carrying only one passenger(only driver),than 1000 passengers are transported
- However, if each car carries four passengers, then 4000 passengers are transported
- Note that the number of cars, not the numbers of passengers determines the traffic and therefore the need for wider highway
- Similarly, the baud determines the required bandwidth, not the bit rate
Example 5.6
An analog signal carries 4 bits in each signal element.If 1000 signal elements are sent per second, find the Baud Rate and Bit Rate?
Solution:
- Baud Rate= Number of Signal Elements
- Baud Rate =1000 bauds/second
- Bit Rate=Baud Rate * Number of bits per signal element
- Bit Rate= 1000 * 4 = 4000 bps
Carrier Signals
- In analog TX. The sending device produces a high frequency signal, that acts as a basis for the information signal
- This base signal is called the Carrier Signal or Carrier Frequency
- The receiving device is tuned to the frequency of the carrier signal that it expects from the sender
- I=Digital info is then modulated on the carrier signal by modifying one or more of its characteristics (Amplitude, Frequency, Phase)
- This kind of modification is called Modulation and info signal is called a Modulating Signal
Amplitude Shift Keying (ASK)
- In ASK, the strength of carrier signal is varied to represent binary 1 or 0
- Both frequency and phase remain constant, while the amplitude changes
- Which voltage represents 1 and which represents 0 can be chosen by System Designer
- A bit duration is the period of time that defines one bit
- The peak amplitude of the signal during each bit duration is constant and its value depends on the bit (1 or 0)
- Speed of transmission during ASK is limited by the physical characteristics of Tx. Medium
Effect Of Noise on ASK
- ASK is highly susceptible to noise interference
- NOISE: Unintentional voltages introduced onto a line by various sources such as Heat or Electromagnetic Radiation from other sources
- These unintentional voltages combine with signal to change the amplitude
- A 1 can be changed to 0 and a 0 to a 1
- ASK relies solely on Amplitude for recognition
- Noise usually affects the amplitude, therefore ASK is the modulating method, that is most affected by Noise
- On-Off Keying (OOK)
- A popular ASK Technique
- In OOK, one of the bit values is represented by no voltage
- The advantage is the reduction in the amount of energy required to transmit Information
- Bandwidth for ASK
- Bandwidth of a signal is total range of frequencies occupied by that signal
- When we decompose an ASK modulated signal, we get a spectrum of many simple frequencies
- The most significant ones are those b/w, fc-Nbaud/2 and fc+Nbaud /2 with carrier frequency fc at the middle
- Bandwidth requirements for ASK are calculated using the formula:
- BW=(1+d) * Nbaud
- BW = Bandwidth
- Nbaud= Baud Rate
- d= factor related to condition of line (min.value = 0)
Example 5.8
Find minimum bandwidth required for an ASK signal TX at 2000 bps. TX. Mode is half duplex
Solution
- In ASK, Baud Rate= Bit Rate
- Therefore, Baud Rate = 2000
- Also ASK requires a minimum bandwidth equal to its Baud Rate
- Therefore Minimum BW = 2000 Hz
Summary
- Digital to Analog Conversion
- Bit Rate and Baud Rate
- Carrier Signals
- Amplitude Shift Keying (ASK)
Reading Sections
- Section 5.3, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
