A)
Introduction
a.
Possible data-to-signal conversion combinations
i.
Analog data-to-analog signal – amplitude and
frequency modulation techniques
ii.
Digital data-to-square-wave digital signal –
encoding techniques
iii.
Digital data-to-(a discrete) analog signal –
modulation techniques
iv.
Analog data-to-digital signal – digitization techniques
1.
Digitization
– converting analog data to digital signals
B)
Data and Signals
a.
Data –
entities that convey meaning within a computer or system
i.
Example
1.
Computer file of names and addresses
b.
Signal
– electric or electromagnetic impulses used to encode and transmit data
i.
Example
1.
Transmission of a telephone conversation over a
telephone line
c.
Analog vs. Digital
i.
Analog data and analog signals are represented
as continuous waveforms that can be at an infinite number of points between
some given minimum and maximum. Usually presented as voltages
1.
Noise – unwanted electrical or electromagnetic
energy that degrades the quality of signals and data
ii.
Digital data
and digital signals are composed
of a discrete or fixed number of values, rather than a continuous or infinite
number of values
iii.
A look at how noise distorts digital signals
d.
Fundamentals of signals
i.
Amplitude
– the height of the wave above (or below) a given reference point
1.
Denotes the voltage level of the signal
(measured in volts), can also denote the current level of the signal (measured
in amps), or the power level of the signal (measured in watts
ii.
Frequency
– number of times a signal makes a complete cycle within a given time frame
iii.
Period
– length, or time interval, of one cycle for frequency
iv.
Spectrum –
range of frequencies that a signal spans from minimum to maximum
v.
Bandwidth
– the absolute of the difference between the lowest and highest frequencies
vi.
Because extraneous noise degrades original
signals, an electronic device usually has an effective bandwidth less than its bandwidth
vii.
Phase
– the position of the waveform relative to a given moment of time, or relative
to time zero
viii.
Attenuation
– loss of power or loss of signal strength
1.
When traveling through any type of medium, a
signal always experiences some loss of its power due to friction
ix.
Decibel (dB)
– relative measure of signal loss or gain and is used to measure the
logarithmic loss or gain of a signal
1.
Equation to measure signal loss or gain: 
a.
P2 = ending power level
b.
P1 = beginning power level
x.
Amplification
– signal gains in decibel
C)
Converting Data into Signals
a.
Transmitting analog data with analog signals
i.
Modulation
– process of sending data over a signal by varying its amplitude, frequency, or
phase
b.
Transmitting digital data with square-wave
digital signals: digital encoding schemes
i.
Nonreturn to zero digital encoding schemes
1.
Nonreturn
to zero-level (NRZ-L) – transmits 1s as zero voltages and 0s as positive
voltages
2.
Nonreturn
to zero inverted (NRZI) – voltage change at the beginning of a 1 and no
voltage change at the beginning of a zero
ii.
Manchester
Digital Encoding Schemes
1.
Properties of Manchester encoding scheme
a.
To transmit a 1, the signal changes from low to
high in the middle of the interval, and to transmit a 0, the signal changes
from high to low in the middle of the interval
2. Differential Manchester
a.
If there is a transition at the beginning of the
interval, then a 0 is being transmitted, if there is no transition at the beginning
of the interval, then a 1 is being transmitted
3.
Self-clocking
– the occurrence of a regular transition is similar to second ticking on a
clock
4.
Baud rate
(baud) – the number of times a signal changes value per second
5.
Data rate
is measured in bits per second (bps)
iii.
Bipolar-AMI
encoding scheme – uses three voltage levels
1.
When transmitting a 0, a zero voltage is
transmitted
2.
When transmitting a 1, either a negative or
positive voltage is transmitted
iv.
4B/5B
Digital encoding scheme – takes 4 bits of data, coverts the 4 bits into a
unique 5-bit sequences, and encodes the 5 bits using NRZI
D)
Transmitting digital data with discrete analog
signals
a.
Amplitude
shift keying – amplitude is represented by two different amplitudes for a
signal
i.
Can use more amplitudes in order to achieve
two-bit representation
b.
Frequency
shift keying – uses two different frequency ranges to represent data values
of 0 and 1
i.
Intermodulation distortion – a phenomenon that
occurs when the frequencies of two or more signals mix together and create new
frequencies
c.
Phase shift
keying – represents 0s and 1s by different changes in the phase of a
waveform
i.
Quadrature
phase shift keying – incorporates four different phase angles, each of
which represents 2 bits
1.
a 45-degree phase shift represents 11
2.
a 135-degree phase shift represents 10
3.
a 225-degree phase shift represents 01
4.
a 315-degree phase shift represents 00
ii.
quadrature
amplitude modulation – uses each signal to represent 4 bits
E)
Transmitting analog data with digital signals
a.
Pulse
code modulation (PCM) – a codec, coverts the analog data to a digital
signal by tracking the analog waveform and taking “snapshots” of the analog
data at fixed intervals
i.
The binary value is then transmitted by means of
a digital encoding format
ii.
Pulse
amplitude modulation (PAM) – tracking an analog waveform and converting it
to pulses that represent the wave’s height above a threshold
iii.
Sampling
rate – frequency at which snapshots are taken
b.
Delta modulation – a codec tracks the incoming
analog data by assessing up or down “steps”. During each time period, the codec
determines whether the waveform has risen one delta step or dropped one delta
step. If the waveform rises on delta step, a 1 is transmitted, if the waveform
drops, a 0 is transmitted
i.
Slope overload noise – when the analog waveform
rises or drops too quickly, the
F)
Data Codes – set of all textual characters or
symbols and their corresponding binary patterns
a.
EBCDIC – 8 bit code allowed 256 (28 =
256) possible combinations of textual symbols
b.
ASCII (American Standard Code for Information
Interchange) – 7-bit version that allows for 128 (27 = 128) possible
combinations
c.
Unicode – encoding technique that provides a
unique coding value for every character in every language, no matter the
platform
