Wideband Con

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

personnel.

(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-

tion.

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

2-36