Transcript Analogue and Digital Signals

Analogue and Digital Signals
SL – Option F.2
HL – Core 14.1
Binary Number Systems
• “There are 10 kinds of people in the world,
those who understand binary and those
who don’t”
• Digital Signals are encoded/stored using
the binary number system
– This is a system of 1’s and 0’s
– Our normal numbering system uses a
base-10 system while the binary system uses
a base-2 system
Binary Number System
Base 10
3
2
1
0
1000
100
10
1
3
2
1
0
8
4
2
1
Base 2
Decimal
Binary
1
00000001
2
00000010
3
00000011
4
00000100
5
00000101
ASCII Code
A
01000001
B
01000010
C
01000011
D
01000101
Binary Number System
• In the binary number system, each 1 or 0
is called a bit
– There are 8 bits in a byte
• An 8 bit system has 8 places 00000000
• A 128-bit encryption has 128 places
• With the 8-bit system all western characters can be
coded
• Each symbol (letter, number, etc.) takes up 1 byte
of memory
LSB and MSB
• Least significant bit – bit that is farthest to
the right
– Determines whether the number will be even
or odd.
– Changes with any small number change
• Most significant bit – bits farthest to the left
– Don’t change unless there is a large number
change
– Often 000
• 0000101 LSB is 1 and MSB is 0
Converting to Binary
• Find the largest power of 2 that is less that
the decimal
• Subtract that number from the initial
• Repeat until you reach 20
• Convert 67 to binary
0 x 27  1x 26  0 x 25  0 x 24  0 x 23  0 x 22  1x 21  1x 20
– 01000011
Signals
• When talking about electronics we will talk
about ‘signals’
– This is simply the transfer of information from
one place to another
– An electrical signal is when information is sent
by a changing electric field
Analogue Signals
• An analogue signal is a continuous signal
for which the time varying feature
(variable) of the signal is a representation
of some other time varying quantity
– In a circuit with a variable resistor and a light
bulb, as the resistance is decreased, the light
bulb gets brighter
– Electrically the voltage, current and frequency
are most commonly used
Analogue Signals
Analogue Use
• Sound transmission is an example of an
analogue signal
– When you speak into a microphone the
change in pressure causes the inside of the
microphone to vibrate and change the
potential difference
– This signal can be fed through an amplifier
which will increase the strength of the signal
reproducing the signal but louder
Digital Signals
• A digital signal does not vary over time but
rather changes from one discrete value to
another.
– In a circuit with a switch and a light bulb, the
bulb is on when the switch is closed and off
when the switch is open
Digital Signals
Sending Digital Signals
• When computers send signals, binary
numbers are sent as a series of high and
low electrical potentials
– When receiving these signals it is necessary
to know the number of bits
• Digital devices have been standardized (multiples
of 8)
– Timing
• When the signal is sent the computer needs to
know how fast to read the signal
Timing
• If a signal is sent at 20 kb/s then each
pulse lasts 1/20000 s.
– The computer needs to know to read a signal
every 50 μs.
Synchronous Data Transmission
• The sender and receiver clocks are set at
the same rate so the transmission of the
pulses is regular.
– A clock signal is sent with data to synchronize
the clocks
– No clock is 100% accurate so the data is sent
in blocks
Time Division Multiplexing
• Because the data is sent in blocks, data
can be sent seemingly simultaneously
– Different pieces of info are sent in different
blocks
– A code is placed at the beginning of each
block to identify the data
Analogue to Digital Conversion
• Analogue signals can be converted to
digital by breaking the analogue signal into
discrete values (generally pd)
– The more values that are
used, the more accurate
the digital signal will be
but the more storage
space will be required
Analogue to Digital Conversion
• The quantized values (pd) can then be
converted into binary
– If the quantized values range from -6V to +6V
this can be represented most easily by the
numbers 0 – 12
– A 4-bit system can be used to represent these
numbers (0000 – 1100)
– As long as the bit rate is known, the signal
can then be converted back to a pd.
Sampling Rate
• The sampling rate is how often the pd is
measured and recorded
– The higher the sampling rate, the more
accurate the digital conversion will be
– Nyquist theory states that to record a certain
frequency the sample rate needs to be at
least 2x the frequency
• This is so at least 1 peak value and 1 trough value
can be recorded
Sample Rate
• Here there are 22 samples taken for every
cycle. What if there were only 2? 4? Etc.?
Analogue vs. Digital Signals
• Radio waves are analog while signals sent
by computers through wires are digital
• Benefits of digital signals
– Noise
• When a signal is transmitted from point A to point B
changes in the electrical field can affect the signal.
Digital signals can
be unaltered while
analogue signals
are affected much
more.
Analogue vs. Digital Signals
– Compression
• If a musical signal were to be compressed the pitch
of the music would change. Compressing a digital
signal doesn’t affect the data
– Multiplexing is much easier with digital
– Source Independence
• A dig sig is always a series of pulses, regardless of
source. All types of info can be sent on same signal
– Coding
• The signal can be easily coded by changing the
sequence of 1’s and 0’s
Full System
– Input signal from a microphone or other input
– System must compare signal to a quantized
set of potentials to determine signal
– Signal is converted to binary and sent down
wire
– A receiver converts the digital back to
analogue and sends to an output device