Motorola M3097 Service Manual download pdf (Page 30)

©1999 Motorola, Inc.

Cellular OverviewService 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 AA 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

1 2 ... 25 26 27 28 29 30 31 32 33 34 35 ... 132 133

Comments to this Manuals

No comments