Chapter 2. BASIC PULSE-CODE MODULATION PRINCIPLE

TM 11-5895-45-14

CHAPTER 2

BASIC PULSE-CODE MODULATION PRINCIPLE

Section I. INTRODUCTION

channel is assigned a time interval in sequence with all

2-1. General.

other channels being multiplexed. These intervals are

a. The TD-204/U or TD-754/G, TD-206/G, and TD-

short and repeated at a high frequency. The samples

660/G or TD-660A/G are pulse-code modulation

taken from each channel are then converted to a form

components used as part of multichannel communication

suitable for transmission in the selected medium. At the

systems.

These systems use radio or cable, or

receiving terminal, the samples are demodulated and

combinations of both as a transmission medium. The

separated into their proper channels by a timing signal

pcm components provide 6 or 12 audio channels in a

from the transmitting terminal.

single transmission channel.

(2) The simplified telephone circuit in figure 2-1

b. In a 12-channel system, separate telephone

illustrates the time division principle. Switches S1 and

signals are converted to pulse-code-modulation (tdm-

S2 are synchronized such that both are in position A at

pcm) pulse trains, for radio transmission. The two pulse

the same time, and in position B at the same time. A

trains are interleaved and transmitted over a single radio

telephone call made on line A is completed only when

channel, and reconverted to telephone signals at a

the switches are in position A. The telephone calls made

distant terminal.

on line B are completed only when the switches are in

c. The 12-channel system also provides for the

position B. When both lines are in use, the switches

capability for pulse trains to be reshaped and retimed at

alternate between position A and position B. If the

a repeater point in the system, and reconverted to

switching rate is low, both conversations will be garbled

telephone signals at another terminal. The repeater can

and unintelligible. If the rate is increased, the signals will

therefore communicate in both directions, and with both

be more intelligible. When the switching rate is higher

terminals, and can be either a radio or cable repeater, or

than voice frequencies, the switching is not detectable.

a combination for radio-to-cable conversion.

(3) The circuit shown in figure 2-2 is a simplified 12-

2-2. Principles of Multiplexing

channel tdm system. The two switches are rotated in

a. General. Multiplexing is a technique used to

synchronism and each channel is sampled once during

transmit simultaneously several channels of voice or

each revolution. Very little distortion occurs and the 12

data over a radio or cable link. Frequency division-

conversations are intelligible when the rotation speed is

multiplexer (fdm) equipment utilizes a subcarrier

rapid enough. Electronic switching is used in the pcm

frequency for each voice or data channel. In time-

components described in this manual and 12 samples

division-multiplexer equipment, each voice or data

are

taken

each

time

frame.

channel shares the transmission time and is

intermittently transmitted.

b. Time Division Multiplexing.

(1) In time division multiplexing, each voice

Figure 2-1. Simplified telephone system showing simple tam.

2-1