Information Technology Reference
In-Depth Information
In 1877, Thomas Edison created a brilliant device known as a phonograph, which is shown
in Figure 1-1.
Sound-Collecting Horn
Cylinder Coated with Tinfoil
Figure 1-1
Replica of Edison's Phonograph
This device was able to record sounds by pressing a needle into a cylinder covered with
tinfoil on a rhythmic basis as a person spoke into a sound-collecting horn. The phono-
graph could then play back this sound by moving the needle at a steady speed back over
the indentions made in the tinfoil. This “archaic” form of recording is one representation
of an analog signal.
An analog signal uses a property of the device that captures the audio signal to convey
audio information. In the case of Edison's phonograph, the property was the various in-
dentions in tinfoil. In today's world, where everything is connected through some form of
cabling, electric currents are used to send analog signals. When you speak into an analog
phone, the sounds that come out of your mouth are converted into electricity. The volume
and pitch that you use when speaking result in different variations of electrical current.
Electrical voltage, frequency, current, and charge are all used in some combination to con-
vey the properties of your voice. Figure 1-2 illustrates perhaps a more familiar view of us-
ing electrical signals to capture the properties of voice.
