EET260 Introduction to digital communication. Digital signals Binary digital signals use two...
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Transcript of EET260 Introduction to digital communication. Digital signals Binary digital signals use two...
EET260Introduction to digital communication
Digital signals Binary digital signals use two discrete voltage
levels to represent binary 1 or 0. Combining multiple bits into words permits us to
represent larger values. Digital circuits operate on digital signals
performing logic and arithmetic functions.
time
volt
age
0 V
5 V
1 0 1 01 1
Analog systems Analog systems use electrical signals that
vary continuously, not having discrete values Analog signals are electrical representations of
signals from nature (pressure, light, sound, etc.)
Examples of analog systems: AM/FM radio, cassettes, telephone, VCR, standard television
Time (sec)
Vo
ltag
e (
V)
time
volt
age
Analog examples
Digital systems Digital systems use electrical signals that
represent discrete, binary values. Digital signals are not representative of signals
that occur in nature (pressure, light, sound, etc.).
Natural signals must be converted into digital format.
Historically, signals in communications systems have been analog but a migration to digital systems has been underway for the last 25 years.
Digital examples
Advantages of digital signals The most important advantage of digital
communications is noise immunity. Receiver circuitry can distinguish between a
binary 0 and 1 with a significant amount of noise.
Time (sec)
Volt
age (
V)
1 0 1 01 1
digital signal with noiseanalog signal with noise
Advantages of digital signals Digital signals can be stripped of any noise in a
process called signal regeneration. Consider a network of relay stations.
microwave relay stations
Advantages of digital signals An analog signal is received, amplified and
retransmitted at each station. However, the noise is also amplified each link.
microwave relay stations
original analog signal
signal at repeater 1
signal at repeater 2
signal at repeater 3
Advantages of digital signals An digital signal is received, regenerated, then
retransmitted at each station. The noise can be eliminated at each repeater.
microwave relay stations
original analog signal
signal at repeater 1
signal at repeater 2
signal at repeater 3
Advantages of digital signals Even if a digital signal does contain bit errors,
many of these errors can be fixed at the receiver through the use of error correcting codes. Error correcting codes allows CDs with minor
scratches to be played without errors. We will discuss such codes later.
scratched CD
Advantages of digital signals Digital signals are easier to multiplex. Multiplexing is the process of allowing multiple
signals to share the same transmission channel.
Advantages of digital signals Digital is the native format for computers. Computers permit the digital signal processing
(DSP) and digital storage of communication signals. DSP allows operations such as filtering, equalization
and mixing to be done numerically without the use of analog circuits.
DSP also permits data compression.
Transmission of digital data There are two ways to move bits from one place
to another: Transmit all bits of a word simultaneously (parallel
transfer). Send only 1 bit at a time (serial transfer).
Serial transmission In serial transmission, each bit of a word is
transmitted sequentially, one after another. The least significant bit (LSB) is transmitted first, and
the most significant bit (MSB) last. Each bit is transmitted for a fixed interval of time.
Serial data can be transmitted faster and over longer distances than parallel data.
Serial buses are now replacing parallel buses in equipment where very high speeds are required.
Parallel transmission Parallel data transmission is extremely fast
because all the bits of the data word are transferred simultaneously. There must be one wire for each bit of information to
be transmitted. Multi-wire cable must be used.
Parallel transmission Parallel data transmission is impractical for long-distance
communication because of: Cost of laying multi-wire cables. Signal attenuation over long distances. At high speeds, capacitance and inductance of multiple wires
distorts the pulse signal. To reduce this, line lengths must be severely shortened.
To achieve clock speeds up to 400 MHz, line lengths must limited to a few inches
Parallel data transmission by radio would be complex and expensive.
One transmitter and one receiver for each bit.
Serial-parallel conversion Because both parallel and serial transmission
occur in computers and other equipment, there must be techniques for converting between parallel and serial and vice versa. Such data conversions are usually taken care of by
shift registers, sequential logic circuits made up of a number of flip-flops connected in cascade.
Q0D0
C
Q1D1
C
Q2D2
C
Q4D4
C
Q0 Q1 Q2 Q3
Data input
Clock input
Parallel outputs
Serial-parallel conversion
Conversion from analog to digital Before we can use digital transmission, we must
convert the signal of interest into a digital format. Translating an analog signal into a digital signal
is called analog-to-digital (A/D) conversion, digitizing a signal, or encoding. The device used to perform this translation is known
as an analog-to-digital converter or ADC.
Conversion from analog to digital An analog signal is a smooth or continuous
voltage or current variation. Through A/D conversion these continuously variable
signals are changed into a series of binary numbers.
Time (sec)
Vol
tage
(V
)
01101010100111001101010101111
A/D conversion The first step in A/D conversion is a process of
sampling the analog signal at regular time intervals.
Time (sec)
Vol
tage
(V
)
sample points
sampling period (T)
sampling frequency
1fT
A/D conversion How often do we need to sample the signal?
How large does our sampling frequency f need to be in order to accurately represent the signal?
Time (sec)
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e (V
)
Time (sec)
Vo
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)
Time (sec)
Vo
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e (V
)
Time (sec)
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e (V
)
high sampling rate
low sampling rate
Minimum sampling frequency The minimum sampling rate required in order to
accurately reconstruct the analog input is given by the Nyquist sampling rate fN given
where fm is the highest frequency of the analog input. The Nyquist rate is a theoretical minimum. In practice, sampling rates are typically 2.5 to 3 times
the Nyquist rate fN.
2N mf f
Consider the signal from the oboe depicted below in time and frequency domain representations.
a. What is the maximum frequency present in the oboe signal?
b. Based upon this, what would be the minimum sampling rate according to Nyquist?
c. What would be practical sampling rate?
Example Problem 1
1 1.0005 1.001 1.0015 1.002 1.0025 1.003 1.0035 1.004 1.0045 1.005-1
-0.5
0
0.5
1
Time (sec)
Volt
age (
V)
0 1000 2000 3000 4000 5000 60000
0.05
0.1
0.15
0.2
0.25
Frequency (Hz)
Volt
age (
V)
A/D conversion The actual analog signal is smooth and
continuous and represents an infinite number of actual voltage values. It is not possible to convert all analog samples to a
precise binary number. Therefore, samples are converted to a binary number
whose value is close to the actual sample value.
A/D conversion The A/D converter can represent only a finite
number of voltage values over a specific range. An A/D converter divides a voltage range into
discrete increments, each of which is represented by a binary number.
A/D conversion The process of mapping the sampled
analog voltage levels to these discrete, binary values is called quantization.
Quantizers are characterized by their number of output levels.
An N-bit quantizer has 2N levels and outputs binary numbers of length N. Telephones use 8-bit encoding 28 = 256 levels CD audio use 16-bit encoding 216 = 65,536 levels