Applied physics III
Basic Concept: digital and analogue
by Benson Mbuya
Digital Systems vs Analogue Systems
• A signal is any kind of physical quantity that
—Audible speech is certainly a kind of signal, as it
conveys the thoughts (information) of one person to
another through the physical medium of sound.
• signal will be used primarily in reference to an
electrical quantity of voltage or current that is
used to represent or signify some other physical
• An analogue signal is a continuous signal that contains
time-varying quantities. Unlike a digital signal, which has
a discrete value at each sampling point, an analogue
signal has constant fluctuations.
• Any information may be conveyed by an analog
signal; often such a signal is a measured
response to changes in physical phenomena,
such as sound, light, temperature, position,
or pressure. The physical variable is converted
to an analogue signal by a transducer.
• A digital signal is a physical signal that is a
representation of a sequence of discrete values.
• Digital signals are signals that are represented
by binary numbers, "1" or "0". The 1 and 0
values can correspond to different discrete
Analog vs digital
• Analogy systems are less tolerant to noise,
make good use of bandwidth, and are easy to
• However, analogy signals require hardware
receivers and transmitters that are designed to
perfectly fit the particular transmission.
• If you are working on a new system, and you
decide to change your analogue signal, you
need to completely change your transmitters
Analog vs digital
• Digital signals are more tolerant to noise, but
digital signals can be completely corrupted in
the presence of excess noise.
• In digital signals, noise could cause a 1 to be
interpreted as a 0 and vice versa, which makes
the received data different than the original
• there are systems in place to prevent this sort of
scenario, such as checksums and CRCs, which
tell the receiver when a bit has been corrupted
and ask the transmitter to resend the data.
Analog vs digital
• The primary benefit of digital signals is that they
can be handled by simple, standardized
receivers and transmitters, and the signal can
be then dealt with in software (which is
comparatively cheap to change).
• The difference between Digital and discrete
—Digital quantity may be either 0 or 1, but discrete
may be any numerical value i.e. 0,1....9.
• The analog clock has no physical limit to how
finely it can display the time, as its "hands"
move in a smooth, pauseless fashion. The
digital clock, on the other hand, cannot convey
any unit of time smaller than what its display
will allow for. The type of clock with a "secondhand" that jerks in 1-second intervals is a digital
device with a minimum resolution of one
• The first telephone was very analog, and in fact
the wired phone in your house still works this
Analog and Noise
All electronic circuits
suffer from 'noise'
which is unwanted
signal mixed in with the
Analogue and digital systems
• Analogue systems process analogue
signals which can take any value within a
range, for example the output from
an LDR (light sensor) or a microphone.
• An audio amplifier is an example of an
analogue system. The amplifier produces
an output voltage which can be any
value within the range of its power
• Digital systems contain devices such
as logic gates, flip-flops, shift registers
and counters. A computer is an example
of a digital system.
Sampling and Reconstruction
• The process of converting from analog data to
digital data is called "sampling".
• The process of recreating an analogue signal
from a digital one is called "reconstruction“
• Digitisation of a signal is the process by which
an analogue signal is converted to a digital
• The sampling rate when digitising an analogue
signal is defined as the number of samples per.
second, and is measured in Hertz (Hz), as it is a
frequency. You can calculate the sampling rate
using the formula:
• The higher the sampling rate, the closer the
reconstructed signal is to the original signal, but,
unfortunately, we are limited by the bandwidth
• Theoretically, a sampling rate of twice the highest
frequency of the original signal will result in a perfect
3. Most sounds created by human speech except for 'ss' and 'ff' have a maximum
frequency of 4 kHz. What is a suitable sampling rate for a low-quality telephone?
4. Using a sampling rate of 20 kHz and 3 bits, sample the following signal, and then
produce a reconstructed signal. What is the maximum frequency that can be
perfectly reconstructed using this sampling rate?