3-6. CHANNEL SEPARATION
When several channels are transmitted over one facility (usually wire, cable, or radio), provisions must be
made to prevent interference from nearby channels. To achieve channel separation, FDM and TDM employ
a. Channel Separation in FDM. The amount of separation required between adjacent FDM carrier channels
depends on the characteristics of the filters used to separate the channels. A filter with a sharp cutoff can be used
with narrow channel separation; a filter with a gradual cutoff requires a wider separation. The separation between
channels is called a guard band. There must always be a guard band to give a margin of safety against mutual
b. Channel Separation in TDM. Each pulse in the rapidly cycling pulse train represents one channel of
communication. The sending and receiving terminals must be completely synchronized to insure the return of
each pulse to the correct channel. Failure to synchronize equipment will cause garble to appear in the receiving
channels. Channel separation depends on the cycling rate of the equipment and the number of channels in
a. Equalization. In a correctly operating carrier system, all channel signals are applied to the line at
approximately equal levels. If the line characteristics are entirely uniform, the received signals will exhibit the
same proportion of signal strengths, although reduced in power. However, the line characteristics will not permit
this desirable condition to exist. The highest frequencies will be attenuated (reduced in power) the most, with the
lowest frequencies being affected the least. The solution to the problem is to insert at the receiving terminal
electrical characteristics that will present to the incoming signals a mirror image of line characteristics. The net
result is a leveling out of all received signal strengths. An amplifier is then needed to raise the received line
signals to the level required for use in the receiving terminal.
b. Amplification. Power loss in the line reduces the useful received power. Amplifiers are used to raise the
power level so as to compensate for line loss. Line amplifiers, called repeaters, are placed at strategic points
along the line or cable to compensate for line loss in each unit length of cable or line. The unit length depends on
the loss characteristics of the cable or line. An amplifier must also follow each equalizer, because an equalizer
always introduces loss.
a. Channel Signal. The bandwidth of a given channel of carrier is related to the type of intelligence being
transferred. For example, a teletypewriter carrier signal is narrowband and therefore requires little band space.
On the other hand, voice and facsimile require a much larger band than teletypewriter to accommodate most of
the complex wave pattern that makes up speech. A still wider band is required for data transmission because of
the very high keying rate of data signals.