How Analog and Digital Recording
Works by Marshall
Brain
When CDs
were first introduced in the early 1980s, their single purpose
in life was to hold music in a digital format. In order to
understand how a CD works, you need to first understand how
digital recording and playback works and the difference
between analog and digital technologies.
In this edition of HowStuffWorks,
we will examine analog and digital recording so that you have
a complete understanding of the difference between the two
techniques.
In the Beginning: Etching Tin Thomas
Edison is credited with creating the first device for
recording and playing back sounds in 1877. His approach used a
very simple mechanism to store an analog wave mechanically. In
Edison's original phonograph, a diaphragm directly
controlled a needle, and the needle scratched an analog signal
onto a tinfoil cylinder:
You spoke into Edison's device while rotating the cylinder,
and the needle "recorded" what you said onto the tin. That is,
as the diaphragm vibrated, so did the needle, and those
vibrations impressed themselves onto the tin. To play the
sound back, the needle moved over the groove scratched during
recording. During playback, the vibrations pressed into the
tin caused the needle to vibrate, causing the diaphragm to
vibrate and play the sound.
This system was improved by Emil Berliner in 1887 to
produce the gramophone, which is also a purely
mechanical device using a needle and diaphragm. The
gramophone's major improvement was the use of flat records
with a spiral groove, making mass production of the records
easy. The modern phonograph works the same way, but the
signals read by the needle are amplified electronically rather
than directly vibrating a mechanical diaphragm.
Analog Wave What is it that the needle in
Edison's phonograph is scratching onto the tin cylinder? It is
an analog wave representing the vibrations created by
your voice. For example, here is a graph showing the analog
wave created by saying the word "hello":
This waveform was recorded electronically rather than on
tinfoil, but the principle is the same. What this graph is
showing is, essentially, the position of the microphone's
diaphragm (Y axis) over time (X axis). The
vibrations are very quick -- the diaphragm is vibrating on the
order of 1,000 oscillations per second. This is the
sort of wave scratched onto the tinfoil in Edison's device.
Notice that the waveform for the word "hello" is fairly
complex. A pure tone is simply a sine wave vibrating at a
certain frequency, like this 500-hertz wave (500 hertz = 500
oscillations per second):
You can see that the storage and playback of an analog wave
can be very simple -- scratching onto tin is certainly a
direct and straightforward approach. The problem with the
simple approach is that the fidelity is not very good.
For example, when you use Edison's phonograph, there is a lot
of scratchy noise stored with the intended signal, and the
signal is distorted in several different ways. Also, if you
play a phonograph repeatedly, eventually it will wear out --
when the needle passes over the groove it changes it slightly
(and eventually erases it).
Digital Data In a CD (and any
other digital recording technology), the goal is to create a
recording with very high fidelity (very high similarity
between the original signal and the reproduced signal) and
perfect reproduction (the recording sounds the same
every single time you play it no matter how many times you
play it).
To accomplish these two goals, digital recording converts
the analog wave into a stream of numbers and records the
numbers instead of the wave. The conversion is done by a
device called an analog-to-digital converter (ADC). To
play back the music, the stream of numbers is converted back
to an analog wave by a digital-to-analog converter
(DAC). The analog wave produced by the DAC is amplified
and fed to the speakers
to produce the sound.
The analog wave produced by the DAC will be the same every
time, as long as the numbers are not corrupted. The analog
wave produced by the DAC will also be very similar to the
original analog wave if the analog-to-digital converter
sampled at a high rate and produced accurate numbers.
You can understand why CDs have such high fidelity if you
understand the analog-to-digital conversion process better.
Let's say you have a sound wave, and you wish to sample it
with an ADC. Here is a typical wave (assume here that each
tick on the horizontal axis represents one-thousandth of a
second):
When you sample the wave with an analog-to-digital
converter, you have control over two variables:
The sampling rate - Controls how many samples are
taken per second
The sampling precision - Controls how many
different gradations (quantization levels) are possible when
taking the sample
In the following figure, let's assume that the sampling
rate is 1,000 per second and the precision is 10:
The green rectangles represent samples. Every
one-thousandth of a second, the ADC looks at the wave and
picks the closest number between 0 and 9. The number chosen is
shown along the bottom of the figure. These numbers are a
digital representation of the original wave. When the DAC
recreates the wave from these numbers, you get the blue line
shown in the following figure:
You can see that the blue line lost quite a bit of the
detail originally found in the red line, and that means the
fidelity of the reproduced wave is not very good. This is the
sampling error. You reduce sampling error by increasing
both the sampling rate and the precision. In the following
figure, both the rate and the precision have been improved by
a factor of 2 (20 gradations at a rate of 2,000 samples per
second):
In the following figure, the rate and the precision have
been doubled again (40 gradations at 4,000 samples per
second):
You can see that as the rate and precision increase, the
fidelity (the similarity between the original wave and the
DAC's output) improves. In the case of CD sound, fidelity is
an important goal, so the sampling rate is 44,100 samples per
second and the number of gradations is 65,536. At this level,
the output of the DAC so closely matches the original waveform
that the sound is essentially "perfect" to most human
ears.
CD Storage Capacity One thing about the CD's
sampling rate and precision is that it produces a lot of data.
On a CD, the digital numbers produced by the ADC are stored as
bytes,
and it takes 2 bytes to represent 65,536 gradations. There are
two sound streams being recorded (one for each of the speakers
on a stereo system). A CD can store up to 74 minutes of music,
so the total amount of digital data that must be stored on a
CD is:
That is a lot of bytes! To store that many bytes on a cheap
piece of plastic that is tough enough to survive the abuse
most people put a CD through is no small task, especially when
you consider that the first CDs came out in 1980. Read How CDs Work
for the complete story!
For lots more information, check out the links on the next
page.
CD manufacturers If you want to create a few CDs,
you use a CD burner. If you want to create a few thousand CDs,
you go with a CD manufacturer. Here are a couple of CD
manufacturers with informative Web sites:
