Figure 3-5. Examples of channel distribution for FDM and TDM.

TM 11-490-5

Figure 3-5. Examples of channel distribution for FDM and TDM.

would affect every audio channel in the same manner.

(2) Time division multiplex.

A random pulse interferer would also interfere with all

(a ) In a TDM system each audio

channels equally. A pulse interferer operating at the

channel is assigned a discrete time slot. Since TDM

synchronizing frequency of the system could

systems are digital in form, the characteristics and

theoretically interfere with one channel only; however,

sequencing of the time slots are determined by the

this occurrence is improbable.

analog-to-digital conversion system employed and the

(3) Multiple multiplex.

Both types of

number and nature of the audio channels being

multiplexing systems can be double multiplexed. That

multiplexed. In the case of current tactical pulse code

is, a small group of audio channels can be multiplexed

modulation (PCM) systems, the sample rate for the

and then combined or remultiplexed with other small

analog-to-digital conversion is 8,000 samples per

groups of audio channels into a large group of

second. Since 6 bits are allocated to each sample, a

multiplexed channels for transmission over the RF link.

total of 48,000 bits per second (48 kb/s) are allocated for

This approach enables small groups of channels to be

each audio channel. Channel separation is related to

extracted from the larger group at an intermediate

time rather than frequency. Therefore, a method of

communications site in a multiple relay system for local

scynchronizing and maintaining the time relationship of

distribution, and other local channels reinserted in the

the transmitted and received signal is required.

larger group without disturbing or demultiplexing the

(b ) Because the channel separation is a

total number of channels being transferred. This does

function of time, the information on each channel is

not change the interference mechanism itself, except

transmitted on the total RF bandwidth of the transmitter

that it is theoretically possible to isolate and jam one of

instead of on a discrete frequency.

Therefore, a

the small groups of an FDM system.

narrowband, continuous-wave interferer

3-5