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| © Eugene Brennan |
Analogue vs. Digital
We
often hear about analogue and digital in the context of communications,
sound recording, cameras, TV, radio, and electronic devices. But what
exactly is the difference, and is digital better than analogue? Why has
digital replaced analogue in audio, digital imaging, and electronic
communication?
In this article, I hope to shed light on the mystery.
How Are Analogue and Digital Displays Used?
Although
it has nothing to do with the real distinction between analogue and
digital, these terms are often used to refer to the type of displays on
clocks, measuring devices, and electronic instruments.
An analogue
display usually involves some form of pointer which indicates a value
depending on its position or angle on a scale. Examples of analogue
displays are traditional watches and clocks, weighing scales,
speedometers, and old-style moving coil or moving iron voltmeters and
ammeters. See the photos below for examples.
A digital display
indicates the value of a parameter directly by actually showing a
number. This number can be produced on an LED, LCD, fluorescent, or
Nixie tube (cold cathode) display. Even the displays on old-fashioned
electromechanical cash registers could be thought of as digital, but the
term is usually reserved for electronic devices.
Note: the US English spelling of analogue is "analog".
Examples of Analogue Displays
On an analogue display, the value is indicated by a line or pointer. See the examples of analogue displays in the photos below.
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| Analogue voltmeter. © Eugene Brennan |
Example of a Digital Display
On
a digital display, a value is displayed as a series of 1 or more
numerical or alphanumerical digits. See the examples of digital displays
in the photos below.
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| My vintage 1977 calculator. This has a miniature vacuum fluorescent display. © Eugene Brennan |
Analogue and Digital Quantities
Another
distinction between analogue and digital is in the field of electronics
and signals. In the real world, many varying parameters can be
considered analogue. So, for instance, the variation of temperature in a
room over time is an analogue quantity, as is the voltage of a battery as
it discharges.
The characteristic of an analogue parameter or
quantity is that its value varies continuously within a range. So the
temperature in a room could vary anywhere between 10 and 20 degrees C.
The
state of a light switch is either on or off. This is an example of a
digital quantity. The switch doesn't exist in an in-between state; it is
either on or off. Digital technology is fundamentally based on this
idea of on and off states of "switches".
What Is a Signal?
The
function of a signal is to convey information about the behavior of
some phenomenon. An example is the output of a temperature sensor which
provides information about the temperature in a room.
The
fluctuations in light level which travel down fiber optic cables are
signals (signals don't have to be electrical). The information could be a
telephone conversation, internet data, or a TV program. The state of
the switch in the example above, measured over time, can be thought of
as a digital signal.
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| Analogue and digital signals. The analogue signal varies continuously over time, the digital single is either "high" or "low". © Eugene Brennan |
What Is a Sensor?
A sensor converts a real-world parameter such as temperature, pressure, or light level into a signal. This signal may then get amplified and/or processed before being used for some other purpose. Some examples:
- In
instrumentation, the signal from a sensor could drive a display
indicating the value being measured, e.g., speedometer in a vehicle,
temperature gauge, or voltmeter
- In communications, sound or pictures from microphones or cameras are
converted to an electrical signal which eventually gets transmitted as
radio waves.
How Do Digital Signals Work?
A
digital signal has two states or levels, high and low. The signals in a
digital computer or instrumentation such as a digital voltmeter are
two-state. A computer doesn't understand analogue signal levels and just
works on high and low states, effectively ones and zeros.
For a more thorough explanation of this and an explanation of the binary number system, see my article "Why is Binary Used in Computers?"
What Does Sampling Mean?
Sampling
is a process that converts a real-world signal to a digital format
which can then be processed by a computer, instrumentation, or other
digital electronics technology.
Some examples:
- A digital multimeter (DMM) samples a voltage, converts it to digital, and the digital signals are used to drive a display.
- An image projected by the lens of a digital camera falls on the CCD
(Charged Coupled Device) at the back of the camera. The image is
converted to numbers and stored in the camera's flash memory.
- A scanner scans a document. The resultant image is stored in memory.
- A sound recording is made. The audio signal is converted to digital and saved on a hard disk or printed onto a CD.
Analogue to Digital Conversion
As
digital electronics and computers were developed, applications arose
for reading signals from the real world. The process to do this is
called sampling, whereby an analogue signal is converted to a binary number understandable by a computer.
A device that does this is called an analogue-to-digital converter (ADC). An
ADC measures the level of the analogue signal at regular intervals, known
as the sampling frequency. The voltage range over which the ADC works
(e.g., 0 to 5V or 0 to 10V) is broken up into equally spaced ranges and a
binary number is assigned to each of these ranges.
Each time the ADC samples the signal, it outputs a binary number,
representative of the level at the instant of sampling. These numbers
can then be stored, used to drive a display, transmitted along a
communication line, etc.
For a 3 bit converter (see diagram below), there are 23
= 8 possible binary numbers. In reality, ADC converters have 6, 12, 16,
or even 24-bit resolution (16 bit at 44kHz sampling rate is used for CD
recordings). Obviously, this means that there are lots more levels, and
the converter can resolve more detail in the signal.
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| Sampling an analogue signal. There are 8 possible levels; the converter outputs a 3 bit number depending on the level of the analogue signal. © Eugene Brennan |
A to D Converter Resolution
As I mentioned before, A to D converters typically have resolutions of 6 to 16 bits, although 24-bit converters are available.
The number of levels which a converter can resolve is equal to 2n
, where n is the number of bits of the converter. So for a 6-bit
converter, there are 64 levels and for a 16-bit converter, there are 216 = 65,536 levels.
A
converter has an input range over which it works, e.g., 0 to 5 volts,
and it is this range that is split up into equal ranges. So to get the
most amount of "detail" out of a signal, the signal should span as much
of this range as possible.
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| Block diagram of a typical ADC. A start conversion signal initiates a conversion. The ADC generates an end of conversion signal on completion. An 8 bit converter can resolve 2 to the power of 8 = 256 different levels. © Eugene Brennan |
The
maths behind this theorem is a bit complicated, but basically, it says
that the sampling rate of a signal must be at least twice the highest
frequency content of the signal.
This is intuitively correct, so,
for instance, the slowly changing signal from a temperature sensor in a
room would need to be sampled much less frequently than an audio signal
from a microphone or video signal from a camera in order to preserve the
rapid changes in the signal.
Reproducing a Signal: Digital to Analogue Converter (DAC)
Once
a signal has been sampled, several things can be done with it. In the
case of instrumentation, e.g., a digital multimeter, data may not need
to be stored. Instead, the digital output signal of the ADC drives the
display on the instrument (through intermediate driver electronics).
Alternatively,
data can be stored in a computer as a set of numbers: in random access
memory (RAM ), on a hard drive, or on a CD or DVD. Sometimes a signal
which has been sampled and stored must be reproduced. An example of this
is an audio recording on a CD. A device called a digital-to-analogue
converter (DAC) is used to reproduce the original signal.
The
DAC does the same job as the ADC in reverse. Each binary number that
was stored during the original recording is input to the DAC at a
playback rate equal to the original sampling frequency. The output of
the DAC is an analogue voltage level. The signal is then filtered to
remove the "staircase" effect due to sampling.
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| Sampled signal is stored as a set of numbers. When this signal is reconverted back to analogue, filtering must be used to remove the "staircase" effect. © Eugene Brennan |
Digital Electronics
Some
electronic devices are almost purely digital and don't need to sample
analogue signals. An example is a digital watch or clock which makes use
of a quartz oscillator. An oscillator is a system that does something at regular intervals, e.g., a swinging pendulum or a flashing light.
The
frequency of a quartz oscillator is very stable and has a relatively
low sensitivity to temperature, unlike the components of a mechanical
clock. The output of the oscillator is a square wave voltage, usually at
32,768 Hz, i.e., a digital signal.
This frequency is a power of 2
and is chosen so that it can be successively divided by 2 by a digital
counter to produce a 1-second pulse. Other counters then generate minute
and hour outputs every 60 and 3600 seconds for driving the digital
display. (Digital quartz watches can also have analogue displays)
Increasing the Signal to Noise Ratio
The
process of storing and transmitting data in digital format is, in
effect, increasing the signal-to-noise ratio (SNR or S/N). There are no
low-level signals, just highs and lows. Digital electronics "decide"
whether a bit is high or low depending on whether it is within a high or
low band of voltages.
What Are the Advantages of Digital Data?
Once a signal is sampled and the data is converted to a series of numbers in memory, lots of things can be done with it.
- Data can be stored and copied indefinitely without degradation.
This is because numbers in memory or on digital media are stored in
binary format as ones and zeros. In memory, electronic switches are
either on or off, representing the storage of 1 or 0. This single piece
of information is known as a bit.
On a CD, either a 1 or 0
can be represented by a pit on the surface of the disk. Contrast this
with an analogue recording on tape (which could be audio or video).
Low-level signals get swamped by noise over time, and noise also gets
introduced during the copying process. This also applies to image
recording on photographic media, such as a traditional negative.
- Clearer audio communication. Because digital
increases the SNR, radio and telephone communication and radio
broadcasts are clearer without the introduction of noise. Increasing the
SNR is somewhat similar to the idea of how the phonetic alphabet can be
used during radio communication or a telephone call to improve
intelligibility. Each letter of a message is represented by a word -
(e.g., "Car" as "Charlie - Alpha - Romeo") so that a transmission can be
understood.
- Data can be compressed Since data is
stored as numbers, fancy algorithms can be used to compress the data.
This is advantageous because data files can be made smaller and require
less storage space (think JPEG and MP3 which are formats for storing and
compressing images and sound). Digital TV and digital radio also make
use of these compression techniques (MPEG 4 compression for TV).
The
radio frequency spectrum is limited, and only so many channels can be
squeezed onto the spectrum. Compressing TV signals narrows the required
bandwidth for a channel so more channels can be squeezed into a
frequency band ( which may be a good or bad idea! It reminds me of Bruce
Springsteen's song, "57 Channels and Nothin' On).
What Are the Disadvantages of Digital Data?
- A decoding device is required.
Data that is stored on media is usually compressed, encoded, and
formatted in some way. Equipment to read and decode the archived data
may not be available in the future. For instance, if you have any 5 1/4
inch floppy disks with data on them, do you still have a computer that
can read these disks?
Yet it is even possible to read the information
from books written hundreds of years ago and view the illustrations or
view photographs from the mid-nineteenth century. It would also be
relatively easy to build a machine that could play early wax cylinder
phonographic recordings. This is obviously an issue for archivists of
important information.
- Digital data is not necessarily durable. While
digital data on media is theoretically rugged and less likely to be
degraded than an analogue recording, in practice, if the media is damaged,
it may not be possible to read data (e.g., a seriously scratched CD).
However, an analogue tape recording could probably be pieced together and
played.
Similarly, a photograph (a traditional non-digital type,
which is, in effect, a two-dimensional, analogue, optical recording) can
still be viewed if stained or damaged in some other way. The moral of
the story is to always back up your digital data (and back up the
backup!)
What Are TTL and CMOS?
TTL
(Transistor-Transistor Logic) and CMOS (Complementary Metal Oxide
Silicon) are two technologies used to implement switches in the
integrated circuits of digital electronic devices. CMOS has the
advantage that it is low power which, of course, is important for
battery-powered devices.
Digital ICs have input and output pins,
and the voltages on these pins are within designed tolerance bands. For
instance, if a high output of a TTL chip is connected to the input of
another TTL chip, the output voltage must be between 2.7 and 5 volts.
Inputs between 2 and 5 volts are interpreted as high.
Similarly, a
low output must be between 0 and 0.5 volts, even though anything
between 0 and 0.8 is interpreted as low. This means that up to 0.3 volts
of noise can be added to a signal without it being misinterpreted.
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| TTL(Transistor Transistor Logic) is a type of technology used in digital electronics. Output signal levels of TTL ICs must be within the acceptable range of voltages shown above. © Eugene Brennan |
Disclaimer
This
article is accurate and true to the best of the author’s knowledge.
Content is for informational or entertainment purposes only and does not
substitute for personal counsel or professional advice in business,
financial, legal, or technical matters.
© 2014 Eugene Brennan
Comments From Readers
Tutu on October 03, 2017:
Wow
Beautiful article
It is so helpful
Ronald E Franklin from Mechanicsburg, PA on August 26, 2015:
As a former electrical engineer, the info you cover in this hub was once my everyday bread and butter. Congrats on a good overview.
Eugene Brennan (author) from Ireland on August 27, 2015:
Thanks Ron! Hopefully it should shed some light on the mystery. Often words like "analog", "digital", "RAM", "JPEG" etc are used as buzz words but without any understanding of what they mean.
Writer Fox from the wadi near the little river on February 02, 2014:
Very comprehensive explanation! You are a real expert in this topic and you have explained the differences so well. I think many people will find this article useful, especially students. Enjoyed and voted up!
Eugene Brennan (author) from Ireland on February 02, 2014:
Thanks, glad you liked it and it made sense! As with any of these types of topics, in reality, things can get quite complex because of all the communication protocols, storage formats etc which are involved. The basic concept of analogue and digital however is quite simple.
Tim Anthony on January 30, 2014:
A very nice and informative hub. Consider detailing the digital data transmission over wireless networks like wi-fi and bluetooth. The emerging trends, versions and technologies too.
John MacNab from the banks of the St. Lawrence on January 30, 2014:
Brilliant eugbug. I had just finished asking my electronically knowledgeable wife what the difference was and I didn't understand her answer. Your hub explains it perfectly. Voted up+
Eugene Brennan (author) from Ireland on January 30, 2014:
Thanks John and Tim!
I'll add more info to this hub later, but probably deal with bluetooth, comms and wi-fi in a separate article.
