Motorola Timeport P8190, Service Manual

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© 2000 Motorola, Inc.
Cellular Overview Service Manual
19
voice samples as viewed on oscilloscopes, are
clearly shown to be nothing more than vary-
ing voltages produced by microphones. In-
stantaneous samples are discrete voltages.
It has been shown that if the sampling rate
is fast enough, it is possible to make a faith-
ful representation of each conversation.
If these samples are then compressed, it is
possible for more than a single conversation
to occur on a single medium (such as a radio
frequency) by sharing time slots. Here we
see three conversations being shared on six
time slots. The conversations shown are com-
pressed sampled analog audio, not yet digi-
tized.
Digitization of Voltage
A byte, consisting of 8 bits, holds 256 pos-
sible numbers (0 through 255). If the maxi-
mum voltage we might expect from our voice
samples is 2 volts, we can arbitrarily match
those voltages proportionally to our byte. The
result is a binary string representing volt-
ages. We now have digital voice.
Digital Cellular
A B C A B C
Slot
1
Slot
2
Slot
3
Slot
4
Slot
5
Slot
6
.25 volt
.5 volt
.75 volt
1.25 volts
1.5 volts
1.75 volts
2 volts
Slot
1
Slot
2
Slot
3
Slot
4
Slot
5
Slot
6
A B C A A B C
0 volt
1 volt
The very first instantaneous sample has
an amplitude of .625 volts represented by
01001111 (79 decimal).
This instantaneous sample has an amplitude
of 1.125 volts represented by 10001111
(143 decimal).
Conventional Radio
Radio uses transmitters to convert speech to
radio energy and receivers to convert radio
energy back to speech. In conventional ana-
log radio, speech is converted into varying
voltages called audio by a microphone. The
audio is mixed with a carrier frequency, am-
plified, and propagated through space as ra-
dio energy by an antenna. At the remote re-
ceiver an antenna converts the received ra-
dio energy to varying voltages at radio fre-
quencies. The RF energy is beat against the
output from a local oscillator to produce a
difference frequency called the intermediate
IF
Microphone Speaker
Figure 13. Slot Assignments
Figure 14. Digitization of Voltage
Figure 15. Convetional Radio