Analog
or Digital Transmission?
Analog
signals are continuously variable signals where
the
information in the signal is contained in the
amplitude
of the signal over time. Digital signals are
sampled at
regular time intervals and the amplitude
converted to
a number - digital bytes - so the information is
transmitted as a digital number in binary - "1s" and "0s".
Analog signals are the natural form of most data, e.g. your voice, the sound of a musical instrument, etc,,but
are subject to degradation by noise in the transmission
system. As an analog signal is attenuated in a
cable, the signal to noise ratio becomes worse so the
quality of the signal degrades. Digital signals can be
transmitted long distances without degradation as
the signal is less sensitive to noise.
Here are some examples:
For a century, the
phone system used a simple analog current loop over copper wires, where
a microphone varied the current in the loop and the variation in
current was converted to a sound by a speaker at the other end. You may
hear this called POTS for plain old telephone service. As the
distance between microphone and speaker gets longer, the signal is
attenuated and the wires pick up electrical noise, so the signal at the
end becomes noisy.
The speaker reproduces the attenuated signal by amplifying it and
amplifies the noise also. The noise is at least annoying and at worst
makes the signal unintelligible.
If the signal is digitized by an analog-to-digital converter (A-D)
and transmitted as binary bits - 1s and 0s - the signal will still be
attenuated and can still pick up noise, but the noise will not affect
the ability of the digital-to-analog converter (D-A) at the receiving
end from converting the signal back to its original quality. So digital
signals have the advantage of being able to be sent over larger
distances and in noisy environments without signal degradation.
Binary bits have another advantage. The bits can be multiplexed, mixing
bits from two signal streams that can be separated out at the receiver
to transmit two separate signals on one transmission path. The red bits
above are from a different signal than the blue bits but the bits can be
mixed on the transmission path.
In 1948 Bell Labs mathematician Claude Shannon published a paper called
“A Mathematical Theory of Communications.” Shannon’s paper said that the
solution to transmitting information farther and faster was to digitize
the information; convert the analog electrical signal to digital, a
series of “1s” and “0s,” binary data like used in digital computers.
Converting analog signals to digital requires special electronics to
sample the analog signal at sequential times and convert the signal to
binary digits. The sampling must be done at exact periods and at
relatively high speeds. To digitize the analog voice signal on the phone
which is in the frequency range of 0 – 4,000 hertz (cycles per second)
requires sampling the signal 8,000 times per second.
Implementing Shannon’s principles in the phone system could not be done
overnight. There were many technologies that needed developing before it
could become practical. Digitization of the phone system had to wait
until the development of semiconductors and integrated circuits in the
1960s and 1970s.
The conversion of the phone networks to digital was occurring at the
same time as the conversion from copper wires, radio waves and
satellites to fiber optics.
Table
of Contents: The FOA Reference Guide To Fiber Optics
