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
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
that it is theoretically possible to isolate and jam one of
instead of on a discrete frequency.
the small groups of an FDM system.
narrowband, continuous-wave interferer