Analog and digital
by Chris Woodford. Last updated: October 5, 2021.
Back in the late 1970s, one of the most exciting things you could own was a
digital watch. Instead of trying to figure out the time from
slowly rotating hands, as you had to do with an old-style analog
watch, you simply read the numbers off a digital display. Since then,
we've got more used to the idea of digital technology. Now pretty
much everything seems to be digital, from television and radio to
music players, cameras, cellphones, and even books. What's the
difference between analog and digital technology? Which is best?
Let's take a closer look!
Photo: Analog or digital? This elegant Swiss watch shows the time with hands moving round a dial, so it's an example of what we call analog technology. The microwave display behind it shows my cooking time
(13 minutes 10 seconds) in digital (numeric) format.
What is analog technology?
People accept digital things easily enough, often by thinking of them
as electronic, computerized, and perhaps not even worth
trying to understand. But the concept of analog technology often seems more
baffling—especially when people try to explain it in pages like
this. So what's it all about?
What does analog actually mean?
If you have an analog watch, it tells the time with hands that sweep around
a dial: the position of the hands is a measurement of the time. How
much the hands move is directly related to what time it is. So if the
hour hand sweeps across two segments of the dial, it's showing that
twice as much time has elapsed compared to if it had moved only one
segment. That sounds incredibly obvious, but it's much more subtle
than it first seems. The point is that the hand's movements over the
dial are a way of representing passing time. It's not the same
thing as time itself: it's a representation or an analogy
of time. The same is true when you measure something with a ruler. If
you measure the length of your finger and mark it on the surface of a
wooden ruler, that little strip of wood or plastic you're looking at
(a small segment of the ruler) is the same length as your finger. It
isn't your finger, of course—it's a representation of your finger:
another analogy. That's really what the term analog means.
Photo: This dial thermometer shows temperature with a pointer and dial. If you prefer a more
subtle definition, it uses its pointer to show a representation (or analogy) of the temperature on
the dial.
Analog measurements
Until computers started to dominate science and technology in the early
decades of the 20th century, virtually every measuring instrument was
analog. If you wanted to measure an electric current, you did it with
a moving-coil meter that had a little pointer moving over a dial. The more the
pointer moved up the dial, the higher the current in your circuit.
The pointer was an analogy of the current. All kinds of other
measuring devices worked in a similar way, from weighing machines and
speedometers to
sound-level meters and seismographs
(earthquake-plotting machines).
Analog information
However, analog technology isn't just about measuring things or using dials
and pointers. When we say something is analog, we often simply mean
that it's not digital: the job it does, or the information it
handles, doesn't involve processing numbers electronically. An
old-style film camera is sometimes referred to as example of analog
technology. You capture an image on a piece of transparent plastic
"film" coated with silver-based chemicals, which react to light. When
the film is developed (chemically processed in a lab), it's used to
print a representation of the scene you photographed. In other words,
the picture you get is an analogy of the scene you wanted to
record. The same is true of recording sounds with an old-fashioned
cassette recorder. The recording you make is a collection of
magnetized areas on a long reel of plastic tape. Together, they
represent an analogy of the sounds you originally heard.
What is digital technology?
Digital is entirely different. Instead of storing words, pictures, and sounds
as representations on things like plastic film or magnetic tape, we
first convert the information into numbers (digits) and display or
store the numbers instead.
Digital measurements
Photo: A small LCD display on a pocket calculator. Most digital
devices now use LCD displays like this, which are cheap to manufacture and easy to read.
Many scientific instruments now measure things digitally (automatically
showing readings on LCD displays) instead of using analog pointers
and dials. Thermometers,
blood-pressure meters, multimeters (for measuring electric current and voltage), and bathroom scales
are just a few of the common measuring devices
that are now likely to give you an instant digital reading. Digital
displays are generally quicker and easier to read than analog ones;
whether they're more accurate depends on how the measurement is
actually made and displayed.
Digital information
Photo: Ebooks owe their advantages to digital technology: they can store the equivalent of thousands of paper books in a thin electronic device that fits in your book. Not only that, they can download digital books from the Internet, which saves an analog trek to your local bookstore or library!
All kinds of everyday technology also works using digital rather than
analog technology. Cellphones, for example, transmit and receive
calls by converting the sounds of a person's voice into numbers and
then sending the numbers from one place to another in the form of radio waves.
Used this way, digital technology has many advantages. It's easier to
store information in digital form and it generally takes up less
room. You'll need several shelves to store 400 vinyl, analog LP records, but with
an MP3 player you can put the same amount
of music in your pocket! Electronic book (ebook) readers are similar: typically, they can
store a couple of thousand books—around 50 shelves worth—in a space smaller
than a single paperback! Digital information
is generally more secure: cellphone conversations are encrypted before
transmission—something easy to do when information is in numeric
form to begin with. You can also edit and play about with digital
information very easily. Few of us are talented enough to
redraw a picture by Rembrandt or Leonardo in a slightly different
style. But anyone can edit a photo (in digital form) in a
computer graphics
program, which works by manipulating the numbers that
represent the image rather than the image itself.
Photo: Large digital clocks are quick and easy for runners to read. Photo by Jhi L. Scott courtesy of US Navy and Wikimedia Commons.
Which is better, analog or digital?
Just because digital technology has advantages, that doesn't mean it's
always better than analog. An analog watch might be far more accurate
than a digital one if it uses a high-precision movement (gears
and springs) to measure time
passing, and if it has a sweeping second hand it will represent the
time more precisely than a digital watch whose display shows only
hours and minutes. Surprisingly, analog watches can also keep time
better than quartz ones: the day-to-day variations in a mechanical, analog watch tend to
cancel one another out, while those in an electronic quartz watch tend to compound one
another (here's why).
Generally, the most expensive watches in the world are analog ones (of course, that's partly because people prefer the
way they look), though the world's most accurate atomic clocks show
time with digital displays.
Photo: An early analog computer from 1949: machines like this
represented numbers with analog dials, levers, belts, and gears rather than (digital) numbers stored in
electronic memories. Picture courtesy of NASA on the Commons.
One interesting question is whether information stored in digital form
will last as long as analog information. Museums still have paper
documents (and ones written on clay or stone) that are thousands of
years old, but no-one has the first email or cellphone
conversation. Open any book on the history of photography and you'll
see reproductions of early photos taken by Niepce, Daguerre, and
Fox-Talbot. But you won't see any pictures of the first digital
photo: even though it was much more recent, probably no-one knows
what it was or who took it! Lots of people own and cherish plastic
LP records that are decades old, but no-one attaches the same importance
to disposable MP3 music files. A lot of information recorded on early
computer memory devices is completely impossible to read with newer
computers; even floppy disks, commonplace as recently as
the mid-1990s, are impossible to read on modern computers that no longer
have built-in floppy drives.
That's why, though the future may be digital, analog technology will always have its
place!
What is sampling?
It's easy to convert analog information into digital: you do it every time you make a
digital photo, record sound on your computer, or speak over a cellphone.
The process is called analog-to-digital conversion (ADC) or, more informally, sampling.
Sampling simply means "measuring at regular intervals"—and it's easiest to understand with an example.
Let's suppose I'm talking to you on my cellphone. The sound of my voice is really waves of energy that travel through the air to the phone's microphone, which converts them into
electrical signals. The sound waves and the signals are both continuously varying waveforms—they're analog information—
and they look like the upper graph in the diagram.
Artwork: Top: A crude analog sound wave. Middle: A low sampling rate produces a crude digital approximation to the original wave. Bottom: Doubling the sampling rate produces a more accurate digital version of the wave, but generates twice as much digital information (data) that we need to store and transmit.
A cellphone transmits sound in digital form, so those analog waves need to be converted into numbers. How does
that happen? A circuit inside the phone called an analog to digital converter measures the size of the waves many times each second
and stores each measurement as a number. You can see in the middle figure that I've turned the first graph into
a very approximate bar chart. If each bar represents one second of time, we can represent this chart by nine
numbers (one number for the height of each bar): 5-7-7-5-1-1-3-3-5. So by sampling (measuring) the sound wave once per second, we've successfully turned our analog sound wave into digital information. We could send those numbers through the air as radio waves to another phone, which would run the process in reverse and turn the numbers back into sound we could hear.
But do you see the problem? Some information is going to get lost in the process of converting the sound to digital and back
again, because the measurement I've made doesn't precisely capture the shape of the original wave: it's only a crude
approximation. What can I do about this? I could make more measurements, by measuring the sound wave twice as often.
That means doubling what's called the sampling rate. Now, as you can see in the bottom chart, I get twice as many measurements and my sound wave is represented by eighteen numbers: 6-7-7-8-8-7-7-5-2-1-1-2-3-3-4-4-4-4. The more I increase the sampling rate, the more accurate my digital representation of the sound becomes—but the more digital information I create and
the more space I need to store it.
Sampling rate and bit rate
When you download digital music, you might be given the option of downloading the same track at
what are called different bit rates. Broadly speaking, the bit rate is the amount of
information captured each time the music is sampled. So a higher bit rate means more information is captured and
the analog information is turned into digital information more accurately. Higher-quality music tracks may have a higher
bit rate, but the tracks will take up far more space on your computer and take longer to download.
Typically, music is digitally converted for CDs and MP3 tracks with a sampling rate of 44.1kHz (about 44,000 times per second). Why such a high rate? For technical reasons that I won't go into here, the sampling rate needs to be about twice the highest frequency of sound in your wave, and since human hearing is limited to about 20kHz, that suggests we need a sampling rate of at least 40kHz.
The typical bit rate for MP3 tracks is around 128kbps (128,000 binary digits or bits per second), though higher quality tracks
have a bit rate between 128kbps and 256kbps (up to 256,000 bits per second).
