"Testing and Clearing Crosstalk"

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From Illinois Bell manual:

TESTING AND CLEARING CROSSTALK

1. GENERAL

1.01 This section covers methods of
       locating and correcting crosstalk.

1.02 Crosstalk is the existence of con-
      versation, music, or other audible
signals in circuits other than the one over
which those signals are being transmitted.
Crosstalk is usually caused by one of the
following conditions:

(a) Unbalanced cable pairs (1.03).

(b) Unbalanced apparatus (1.04).

(c) Common impedance in two or more
    circuits (1.05).

(d) Extension of one wire  of a pair (1.06).

(e) Inductive or capacitive coupling
    between apparatus  (1.07).

(f)  Excessive volume level (1.08).

1.03  A cable pair is unbalanced when the
       two wires are not alike as to their
resistances or capacitance with respect to
other circuits.

1.04  Apparatus is unbalanced when it con-
       tains unequal inductances, resistances
or capacitances in the battery and ground
sides of the wiring. Supervisory relays
connected only with the ring side of talking
circuits, being of low resistance, usually
do not cause serious unbalance. Station
line circuits are not designed to be balanced
in the signaling condition. This occasion-
ally causes reports of "Crosstalk before
operator answers.

1.05 When separate circuits have an apprec-
       ciable impedance in common with each
other, crosstalk often results. This
occasionally occurs when the battery supply
pairs to a P. B. X. from the central office
battery or a centralized battery and the
fuses form a common resistance to all
talking circuits using the battery supply.
Common impedance of coupling may also
be found in P. B. X.'s having local batteries.

1.06  Circuits which have but one wire of a
       pair extended between keys or other
apparatus will cause crosstalk; the amount
depending upon the length of the single wire
extension. This type of crosstalk is
sometimes found in key equipments and key
telephone systems.

1.07 Iron core apparatus, such as repeating
      coils or retard coils, produce magnetic
fields; and if the magnetic fields of two
different circuits and overlap due to close
physical proximity, there will be coupling
and possible crosstalk. Capacitive coupling
may occur when two capacitors housed within
a single can are used in separate circuits.

1.08 Excessive volume level in a circuit
      carrying conversation, music, or other
signals may cause crosstalk in other circuits
in the same cable.

Section 500-110-904IB, Issue A, April, 1958, Illinois Bell Telephone Co., Chicago Area--Plant Dept.

Found in Key Telephone Systems, Illinois Bell Telephone Co., Volume 1, October 1969

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Continued/ Suite

2. TESTING FOR CROSSTALK -- REMEDIAL MEASURES


2.01  Consider crosstalk as serious when four
        consecutive words, or the subject of
the message is understood. At times it may
be necessary in order to satisfy some cus-
tomers, to reduce crosstalk when the inter-
ference is of lower volume than as described
above.

2.02 Crosstalk, even though classed as
       intermittent trouble because the
disturbing line is not always in use, usually
will require correction.

2.03 Determine from the test center and the
       usual test procedures that the com-
plaining customer's line is free from grounds
or crosses, and from unbalance due to unequal
conductor resistance or capacitance. When
none of the above trouble conditions are
found to be the cause of the complaint,
interview the customer to learn as much as
possible about the conditions existing at
the time the trouble occurred. The following
information will aid in isolating the source
of the crosstalk.

(a)  Frequency of crosstalk. (On every
      call, time of day, daily, weekly.)

(b)  Type of call. (Incoming, outgoing,
      long distance, tie trunk, etc.)

(c)  Amount of crosstalk. (Faint, medium,
      loud. Intelligible or not.)

(d)  Identification. (Telephone number if
      overheard, type of message, voice,
music, etc.)

2.04 Because the customer hears crosstalk
       only occasionally, his observations may
be misleading, and it is necessary to verify
the details of the complaint to the extent
possible. For example, a customer may hear
other conversation picked up by the far end
transmitter from elsewhere in the room and
assume that it is crosstalk.

2.05 Following is a simple method of testing
       for crosstalk:  Request the test center
to connect 1000 cycle tone to the line on
which crosstalk has been reported. Then
using a 1011-B test set with the monitor key
operated, test from ground to the individual
conductors for the presence of tone on the
other pairs in the same local or house cable,
and on the aerial or underground cable pairs
appearing in the terminal.    If tone is heard
on a pair on which there is a cross-
connection, isolate the fault by testing with
the cross-connection momentarily opened. It
may be necessary, in order to cover a larger
cable pair count, to test from other termi-
nals in the same cable.

   Note:  The 1000 cycle tone supplied when
           dialing (C.  O.  Name) 9996 is not reliable
           as a tone source for trouble locating
           purposes.  Since the tone is interrupted
           for one second in every ten, the absence
           of tone when testing will not be
           conclusive.

2.06  Care for faults located in house cables
        on which no records are maintained by
the Assignment forces and other local cables,
such as jumper, P. B. X., etc., in accordance
with existing instructions. When crosstalk
is isolated to aerial, underground, or house
cables on which assignment records are main-
tained, call the test deskman for referral to
Cable Test.

P. B. X. Trunk to Trunk Crosstalk

2.07  When crosstalk is found between adja-
        cent trunk circuits of a P. B. X. there
is a possibility of coupling between the
ringing capacitors if they are in the same
case.  It is advisable, in such instances, to
replace the double capacitors with individual
capacitors in accordance with existing
practices.

P. B. X. Station to Station Crosstalk

2.08  Crosstalk coupling between station
         signaling circuits of a P. B. X. general-
ly is due to an impedance common to both cir-
cuits in the battery supply circuit. In
some cases the coupling is sufficient to
permit conversation between two stations
waiting for the operator to answer. This
type of cross-talk can be heard by connecting
a 1011-B test set, with the monitor key
operated, across the battery and ground supply
at the switchboard side of the fuses. Because
this condition usually disappears when the
operator answers, very few P. B. X. customers
object to it. However, when the seriousness
of the condition or a customer's complaint
warrants, arrange for the installation of
electrolytic capacitors across the battery
and ground supply in accordance with existing
practices.

P. B. X. Cord to Operator's Circuit Crosstalk

2.09  Crosstalk between an operator's tele-
         phone circuit and the cord circuits is
generally due to capacitances between the
springs of the listening keys. Inasmuch as
this crosstalk is usually very faint and
heard only by the operator when all keys are
normal, it generally will not require cor-
rection. This crosstalk may be balanced
by connecting small capacitors between the
tip of the operator's telephone circuit and
the ring of each cord circuit. Drawing
C-2444 covers such an installation. Another
method is shown on Drawing C-2912 which con-
sists of a relay arranged to short circuit
the operator's receiver when all listening
keys are normal. The C-2912 circuit cannot
be used on P. B. X.'s on which the operator's
telephone and dial circuits have been modi-
fied for long range use. Crosstalk traced to
this source which requires remedial action
shall be referred to the supervisor who will
arrange to have the necessary Plant Repair
Order for the installation of circuit C-2444
or C-2912, as appropriate, prepared and for-
warded to the Transmission Engineer for ap-
proval.

Key Equipment

2.10  If crosstalk is reported or suspected
        on a key equipment or key telephone
system it is important to see that station
equipment is connected in accordance with
practices covering this equipment.

2.11  In arranging hold key circuits, both
        wires of the pair must be extended
to the termination point to avoid unbalances.

2.12  Early 100-type key equipment were
        furnished without capacitors across the
windings of the "Hold" relays. Where cross-
talk is traced to this cause, it will be
necessary to arrange for the installation of
the necessary capacitors.

2.13  In HCK sets the induction coil carries
        a magnetic shield to avoid a coupling
between the coil and the ringer. This pre-
vents a customer being held or waiting to be
answered, from hearing crosstalk from another
line in use.

Customer Services

2.14  Customer owned equipment, such as re-
         corders and amplifiers, may cause
crosstalk if unbalanced at the point of con-
nection to the telephone circuit. A customer
owned amplifier may cause crosstalk if it
places too high a volume  level on the
telephone circuit.

2.15 Troubles traced to customer owned
        equipment, such as unbalances and ex-
cessive volume, are to be reported to the
test center for referral to the Commercial
Department.

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