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TM 11-5895-1012-10
these is often called "passive" equalization and the
other "active" equalization. Passive equalization is gen-
erally used when referring to circuits where fixed
equalizers are inserted after the required value is com-
puted. Active equalization is commonly used through-
out the DCS since such equalizers are continuously
variable over wide ranges and may be simply adjusted
by  Technical Control Facility or maintenance
(7) Either method of equalization ignores the
absolute delay and only equalizes the relative delay.
switched 2-wire PBX
Delay equalizers consist of a series of networks with
tion lines will not be
since they do not have
delay characteristics inverse to those of the circuit
king equalized. Each adjustable section provides suf-
generate the echo.
ficient time delay in its frequency band to reduce the
g. Telegraph
relative delay distortion. One unit in common use in
tronic devices
the DCS reduces the relative delay distortion from 3
synchronous o
milliseconds to less than 80 microseconds from 1000
them prior to retransmission. They are usually con-
to 2000 Hz, to less than 260 microseconds from 200 to
structed to accept various modulation rates
1000 Hz, and to less than 500 microseconds from 500
cept signals with up
to 600 Hz, and from 2600 to 2800 Hz. It is important
erative repeaters are
for telegraph circuits,
that delay equalizers, particularly passive equalizers,
be placed adjacent to a constant impedance attenuator
included as part of original circuit design and a con-
or a 600 ohm pad to ensure proper impedance termina-
tinuing requirement for their use generally indicates
circuit or equipment malfunctions which should be lo-
cated and corrected and the regenerator removed from
e. Repeat Coils. A repeat coil is essentially a trans-
the circuit.
former which will transfer electrical energy from one
source to another without metallic connection between
h. Data Modems. This term is a contraction from
the circuits. For direct current purposes, it divides the
the terms modulatordemodulator and all of these
units, which are in use in the DCS in many forms and
circuit into two portions. It was from this transfer
configurations, perform the same purpose. They con-
function that the term "repeating coil" was originally
vert digital signals to an analog form in the modulator
derived, since it was used primarily to repeat
portion and convert analog information into digital
information, rather than change voltage or current
form in the demodulator portion. In certain special
values as in power systems. Repeat coils are usually
cases two modems are used on a back-to-back arrange-
thought of as impedance matching devices, and the use
ment to provide regeneration for data circuits. In this
of the proper ratio coil is a basic consideration in cir-
arrangement the output of one modem will be put into
cuit design
f. Echo Suppression
another modem of the same type which will in turn
(1) Echo. A problem, peculiar to long circuits that
provide a reshaped and retimed output.
terminate in 2-wire instruments, arises from the
2-32. Wideband Con
velocity of propagation of the various facilities used
for telephone communications. The time required for
a. Wideband condi
egories, the first covering that condition required for
the transmission Of voice signals from one point to a
distant termination will vary according to the type of
the interconnection of voice frequency multiplex
equipment with different characteristics; the second
facility and the overall length of the circuit. If, when a
conversation is being carried on, some part of the
for the special conditioning required for 48 kHz chan-
nels utilized for wideband data or secure voice trans-
speaker's voice is returned to him from the distant ter-
missions. The standard parameters for voice frequency
mination, and the time involved is of sufficient dura-
multiplex, frequency division (FDM) which have been
tion, the returned voice signal will have the effect of
established for equipment used in the DCS are:
an annoying echo.
(1) -34.5 dBr send level input.
(2) Echo Tolerance. On circuits that are physically
(2) -12 dBr receive level output.
very short, currents return in such a short period of
time that they take the form of sidetone, which is not
(3) 135 ohm impedance (balanced) for both input
objectionable to the talker. As the time interval in-
and output.
creases, currents returned at the same level assume
(4) 104.08 kHz pilot.
b. Any new vf FDM must operate at the
the form of an echo and become objectionable. Since

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