Here Comes Digital Cell Phone Frequencies
Digital cell phones are the second
generation (2G) of cellular technology. They
use the same radio technology as analog
phones, but they use it in a different way.
Analog systems do not fully utilize the
signal between the phone and the cellular
network -- analog signals cannot be
compressed and manipulated as easily as a
true digital signal. This is the reason why
many cable
companies are switching to digital -- so
they can fit
more channels within a given bandwidth.
It is amazing how much more efficient
digital systems can be.
Digital phones convert your voice into
binary
information (1s and 0s) and then compress it
(see
How
Analog-Digital Recording Works
for details on the conversion process). This
compression
allows between three and 10 digital
cell-phone calls to occupy the space of a
single
analog call.
Many digital cellular systems rely on
frequency-shift keying
(FSK) to send data back and forth over AMPS.
FSK uses
two frequencies,
one for 1s and the other for 0s,
alternating
rapidly between the two to send digital
information between the cell tower and the
phone. Clever modulation and encoding
schemes are required to convert the analog
information to digital, compress it and
convert it back again while maintaining an
acceptable level of voice quality. All of
this means that digital cell phones have to
contain a lot of processing power.
Let's take a good look inside a digital cell
phone.
Inside a Digital Cell Phone
On a "complexity per cubic inch" scale, cell
phones are some of the most intricate
devices people use on a daily basis. Modern
digital cell phones can process
millions of calculations per second
in order to compress and decompress the
voice stream.
|
The parts of a cell phone |
If you take a basic digital cell phone
apart, you find that it contains just a few
individual parts:
·
An amazing circuit board containing the
brains of the phone
·
An antenna
·
A
liquid crystal
display (LCD)
·
A keyboard (not unlike the one you find in a
TV remote
control)
·
A
microphone
·
A
speaker
·
A
battery
The circuit board is the heart of the
system. Here is one from a typical
Nokia
digital phone:
|
The front of the circuit board |
|
The back of the circuit board |
In the photos above, you see several
computer chips. Let's talk about what some
of the individual chips do. The
analog-to-digital
and
digital-to-analog
conversion chips translate the outgoing
audio signal from analog to digital and the
incoming signal from digital back to analog.
You can learn more about A-to-D and D-to-A
conversion and its importance to digital
audio in
How Compact
Discs Work. The
digital signal processor
(DSP) is a highly customized processor
designed to perform signal-manipulation
calculations at high speed.
The
microprocessor
handles all of the housekeeping chores for
the keyboard and display, deals with command
and control signaling with the base station
and also coordinates the rest of the
functions on the board.
|
The microprocessor |
The
ROM
and
Flash memory
chips provide storage for the phone's
operating
system and customizable features,
such as the phone directory. The
radio frequency
(RF) and power
section handles power management and
recharging, and also deals with the hundreds
of FM channels. Finally, the
RF amplifiers
handle signals traveling to and from the
antenna.
|
The display and keypad contacts |
The
display
has grown considerably in size as the number
of features in cell phones have increased.
Most current phones offer built-in phone
directories, calculators and games. And many
of the phones incorporate some type of
PDA or
Web browser.
|
The Flash memory card on the
circuit board |
|
The Flash memory card removed |
Some phones store certain information, such
as the SID and MIN codes, in internal Flash
memory, while others use external cards that
are similar to
SmartMedia
cards.
|
The cell-phone speaker,
microphone and battery backup |
Cell phones have such tiny speakers and
microphones that it is incredible how well
most of them reproduce sound. As you can see
in the picture above, the speaker is about
the size of a dime and the microphone is no
larger than the watch battery beside it.
Speaking of the watch battery, this is used
by the cell phone's
internal clock chip.
What is amazing is that all of that
functionality -- which only 30 years ago
would have filled an entire floor of an
office building -- now fits into a package
that sits comfortably in the palm of your
hand!
In the next section, we'll get into the
cell-phone networking methods.
