Transcript EE302: Lesson 2 Gain and decibels

EET260
Introduction 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.
voltage

1
0
1
1
0
1
5V
0V
time
Analog systems
Analog systems use electrical signals that vary
continuously, not having discrete values
 Analog
voltage
signals are electrical representations of
signals from nature (pressure, light, sound, etc.)
 Examples of analog systems: AM/FM radio,
cassettes, telephone, VCR, standard television
Voltage (V)

time
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.
0
1
1
0
Voltage (V)
1
Time (sec)
analog signal with noise
digital signal with noise
1
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.
original analog signal
signal at repeater 1
microwave
relay stations
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.
original analog signal
signal at repeater 1
microwave
relay stations
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.
Parallel outputs
Data input
D0
Q0
C
Clock input
Q0
Q1
D1
Q1
C
Q2
D2
Q2
C
Q3
D4
Q4
C
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
Voltage (V)
A/D conversion these continuously variable
signals are changed into a series of binary numbers.
01101010100111001101010101111
Time (sec)
A/D conversion
The first step in A/D conversion is a process of
sampling the analog signal at regular time
intervals.
sample points
Voltage (V)

sampling frequency
1
f 
T
Time (sec)
sampling period (T)
A/D conversion

How often do we need to sample the signal?
 How
Voltage (V)
Voltage (V)
large does our sampling frequency f need to be
in order to accurately represent the signal?
Time (sec )
Voltage (V)
Time (sec )
Voltage (V)
high sampling rate
Time (sec )
Time (sec )
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
f N  2 fm
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.
Example Problem 1
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?
1
0.25
0.2
Voltage (V)
Voltage (V)
0.5
0
0.15
0.1
-0.5
0.05
-1
0
1
1.0005 1.001 1.0015 1.002 1.0025 1.003 1.0035 1.004 1.0045 1.005
Time (sec)
0
1000
2000
3000
Frequency (Hz)
4000
5000
6000
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.
use 8-bit encoding  28 = 256 levels
 CD audio use 16-bit encoding  216 = 65,536 levels
 Telephones