THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
■ BY LOUIS E. FRENZEL W5LEF
Q & A ABOUT DIGITAL RADIO
I was talking to a friend of mine recently and he asked if I had ever built
a crystal radio. I said yes, and asked why he wanted to know.
He answered that he had never built one and just wanted
to see if he could. I told him not to delay because pretty soon
there would be no radio stations you could receive on a crystal radio.
Crystal radios are just a simple diode demodulator for amplitude modulation (AM) signals. Typically, crystal
radios are built to receive local AM broadcast stations.
And while those stations still exist, that might not be the
case in the future. Why? Because almost all radios today
have been converted to digital. AM broadcast stations are
one of the few remaining radio services that still use
analog AM. What ensued with my friend was a Q & A
discussion of what digital radio is and how broad it is.
Here is a summary of that discussion.
Q: What do you mean by the term radio?
A: A radio can be a receiver, a transmitter, or (more
commonly today) a transceiver. Radio includes AM and FM
broadcast, TV, any two-way radios, satellites, cell phones, hobby
radios, marine and aircraft radios, military radios, radars, GPS,
and so on. Anything we generally refer to as wireless means
Q: How long has this digitization of radio been going on?
A: For decades actually. It probably started with satellites in
the 1970’s, then moved on to other services over the years.
Things really picked up in the 1980’s and I would guess that
90% of all radio is digital today.
Q: Just why did this move to digital occur?
A: Great question. And there is no one single reason.
Probably the main reason is that digital signals are very robust
and less affected by noise because of their binary nature. Second,
we now have small, cheap, and fast integrated circuits and
microcomputers that can process these signals. Designers found
they could squeeze more signals into a given spectrum band
with digital techniques thereby greatly improving spectral
efficiency in a limited spectrum. Finally, some really great digital
compression techniques were developed that further improve
the ability to cram more signals at higher speeds into a limited
Q: Give me a good example of how some of those benefits
A: The best example is cellular radio. Cell phones started
out in the 1980’s as analog FM signals. Even with a relatively
narrow band ( 30 kHz) modulation, the cell phone carriers found
that their limited spectrum was quickly used up meaning they
could not take on any more new customers. With profit limited,
they because extremely aggressive in creating new digital
technologies to fix that problem. Digital radios solved the problem
initially by using Time Division Multiplexing (TDM) to put three
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