| Sales Brochure - DLE510 Introduced in 1960.
Originally made by TMC.
 The Loudspeaking Telephone No. 1
(LST No. 1) is transistor operated. Current for the amplifiers is drawn
from the telephone line and therefore a separate power supply is not required.
It
comprises three items:-
(a) Control-unit No. 8A with Connector...
(b) Telephone No. 708 with Connector...
(c) Amplifier No. 138A or B
The three units are interconnected by means of plug-ended cords.
The control unit contains the microphone, indicator lamp, dial, volume control key and
on/off key. Loudspeaking calls are controlled completely from this unit. To make a call
using the loudspeaker, the on/off key is operated and the required number is dialled. When
the call is answered, the received level of speech can be adjusted by means of the
3-position volume control key. To terminate the call, the on/off key is restored and the
volume control key is moved to the low volume position to prevent howl on re-use.
On early models with Control-unit No. 8A (Mark 1), change-over to the handset
whilst a loudspeaking call is in progress is effected simply by lifting the
handset from the telephone; the call is switched over automatically and the
on/off key is self restored. To revert to the loudspeaking condition it is
necessary to depress the on/off key on the control unit whilst the handset is
returned to the telephone. On current models with Control-unit No. 8A
(Mark 2), the self-restoring feature has been withdrawn because of
functional and mechanical weaknesses of the relay-released key. To change-over to the
handset whilst a loudspeaking call is in progress the handset is lifted and then the
on/off key is restored manually. To revert to the loudspeaking condition the on/off key is
re-operated and then the handset returned to the telephone.
Mark 1 control units had an omni-directional microphone. Mark 2 units have a microphone the
sensitivity of which is directional. Maximum pick-up is from the front and rear of the
unit and minimum pick-up is from the sides. When this type of microphone is used there is
less acoustic feedback from the loudspeaker provided the telephone unit (which contains
the loudspeaker) is correctly positioned to one side of the control unit.
The Mark 2 control unit has slotted openings in an escutcheon plate fitted to the rear of
the unit to preserve the directional properties of the microphone. The lamp, which would
otherwise obstruct the frontal opening and hence interfere with the performance of the
microphone, has been moved to a position below the dial on the front edge of the unit. The
microphone is resiliently mounted to reduce the amount of noise transmitted to line when
the control unit is handled.
Because the directional-type microphone is less sensitive than the one used in
Mark 1 units, a pre-amplifier has been added to the Mark 2 control unit and the
output level adjusted 3 dB higher than that of Mark 1. Under ordinary acoustic conditions about 3 dB
increase in receive gain will also be realised because of the improved acoustic stability.
The volume control key of the Mk. 2 unit has three locking positions instead of two
locking positions and one non-locking position. Mark 1 and Mark 2 units are
interchangeable.
The telephone instrument is a standard 700-type telephone modified by the removal of the
dial in place of which is fitted a loudspeaker and its covering grille. On early models an
extra gravity switch was fitted for the automatic on/off key restoration facility. The
switch is omitted from current models but the appropriate terminals are strapped through
to maintain circuit compatibility.
Amplifier No. 138B supersedes Amplifier No. 138A. The amplifier will usually be mounted in
an inconspicuous position under the desk or on a wall.
Amplifier No. 138B
Loudspeaking Telephone No. 1A is similar to the No. 1 except that Telephone
No. 716 with Connector... is used instead of Telephone No. 708
with Connector... Telephone No. 716 has provision for the optional fitting of four
push-buttons. Amplifier No. 138B is used.
When working to a normal handset at the other end of the line the performance of this type
of loudspeaking telephone is satisfactory except under conditions of high total line
attenuation. In such circumstances the high gain positions of the volume control key
should be tried but, depending upon the local conditions, it may be found that oscillation
occurs. If this should happen then the handset must be used.
Field of use
The equipment is designed for use on direct exchange lines on 50V exchange systems
(including shared service) and should also function satisfactorily on other exchange
systems where the line current through the telephone is not less than 30 ma.
It may be used as an extension instrument on a P.B.X. and also on the following extension
plans:-
Extension Plans 1 and 1A: Main or extension
Extension Plan 9: External and internal extensions on P.M.B.X. only, with or without
recall
House Exchange System No. 3: Non-multiple extensions
TELECOMMUNICATIONS INSTRUCTION
C MARKETING INSTALLATION
3 Internal
B2001
Issue 2, July 1973
LOUDSPEAKING TELEPHONES
Nos. 1 AND 1A
Installation
LST No. 1 with Control-unit No. 8A (Mark 1)
GENERAL
This Instruction gives details of the installation of Loudspeaking Telephones Nos. 1 and
1A. Reference should be made to B2000 for information on the acoustic problems associated
with their use.
Loudspeaking Telephone No. 1 incorporates a telephone instrument capable of being fitted
with a single push button, whilst the instrument of Loudspeaking Telephone No. 1A may have
up to four push buttons. The type of loudspeaking telephone to be fitted in individual
applications is indicated in the relevant N diagram.
(a) Loudspeaking Telephone No. 1 (Mk II) comprises the following items:-
Telephone No. 708, Grey, with Connector 1018A
Control-unit No. 8A, Grey, with Connector 1014B
Amplifier No. 138B
(b) Loudspeaking Telephone No. 1A (Mk I) comprises the following items:-
Telephone No. 716, Grey, with Connector 1018A
Control-unit No. 8A, Grey, with Connector 1014B
Amplifier No. 138B
The separate items are supplied complete with cords and plugs for inter-connection and
additional items are not normally required. If connectors longer than 72in are required
for control-units or telephones, the following are available:
Connector 1014B, 144 in
Connector 1014B, 216 in
Connector 1018A, 144 in
Connector 1018A, 216 in
LAYOUT
Arrange with the subscriber the most suitable layout on his desk for the control unit and
the telephone, bearing in mind the following considerations:
(a) To ensure the best possible quality and level of transmission, the control-unit should
be placed at the front of the desk as near to the subscriber as possible so that the
speaking distance from the microphone is a minimum. This also places the controls and dial
in the most convenient position for making a call.
(b) The subscriber should be instructed always to speak closely to the microphone, a
distance of 15in - 18in (380mm - 460mm) being reasonable.
(c) The loudspeaker should be separated from the microphone by at least 2ft (600mm) to
prevent "howling". The Telephone No. 708 or 716 containing the loudspeaker
should therefore be positioned at the required distance from the control-unit but within
reasonable reach of the subscriber for the occasions when use of the handset is required. The control-unit must be positioned to one side of the telephone instrument so that
advantage is taken of the directional properties of the microphone.
(d) Choose a suitable position for mounting the Amplifier No. 138B where it is unlikely to
be knocked and damaged. An inconspicuous position on a wall or under the desk will be
suitable but it must be mounted within reach of the connecting cords from the control-unit
and the telephone. The amplifier should be kept away from heating appliances and direct
sunlight to avoid overheating.
INSTALLATION
Diagram N4472 details the connections required for direct exchange lines, shared service and
switchboard extension lines. For other applications, diagrams in the series N4451-4499
refer. Procedure as follows:
(a) Remove the cover of the Amplifier No. 138B. Three terminal blocks, one 4-way, one
5-way and one 7-way, numbered 1-16, are provided in the amplifier. The straps on these
blocks are set initially for a DEL installation as shown in Diagram
N4472.
(b) Connect the cords from the control-unit and telephone to the appropriate plug and
socket in the amplifier and anchor them by their grommets in the two outer slots
provided. The separate grommet packed with the amplifier is for use with the line cord and
should be located in the centre slot.
(c) Before connecting the exchange line to the line terminals on the terminal block,
proceed as follows:-
(i) Measure the line current with a Resistor Coil No. 35A - 300 ohm in place of the
loudspeaking telephone. If the equipment is installed on a PBX extension line with through
clearing facilities at the PBX, two measurements of line current should be made, firstly
with the extension line connected through the PBX switchboard to another extension, and
then the extension connected through to the main exchange.
The current measured should not exceed 90 ma or the amplifier may be damaged. If this
value is exceeded (due, for instance, to the use of special speaking bridge arrangements)
then two equal Resistors, Coil, No. 35A should be inserted in the A and B lines to limit
the current to this value. At line currents lower than 30 ma there will be progressive
deterioration in the loudspeaking performance.
(ii) On shared service, measure the earth current under calling conditions using the 300
ohm resistor in place of the instrument. If the current exceeds 90 ma a Resistor, Coil,
No. 35A, 200 ohms, should be included in the earth circuit in accordance with Diagram N 4472.
(iii) Measure the line loop resistance. The gain of the 'send' amplifier should be
adjusted to suit the line loop conditions, by means of a strap provided on terminals 12,
13 or 14 in the amplifier. The strap is initially set between terminals 12 and 13 to give
the correct output for lines having a loop resistance less than 500 ohms. For lines
exceeding 500ohms loop resistance, change the strap from terminals 12 and 13 to terminals
13 and 14.
(d) Connect up the installation in accordance with the relevant N diagram.
TESTS
(a) Place the control-unit and telephone in their correct positions on the desk and
operate the ON key. The supervisory lamp should glow, and in automatic areas dial tone
should be heard from the loudspeaker. Remove the dial tone by dialling a suitable digit. Set the 3-position volume control key to the low volume position, and adjust the
continuously variable volume control in the amplifier (marked REC VOL) to a setting just
below the 'howl' point. In CB areas this adjustment should be made under conditions of
calling the exchange, i.e. the operator withholding the plug from the line.
To adjust the volume control, first slacken the locking screw visible through the
aperture, rotate the slotted wings of the control and finally retighten the locking screw.
(b) Make several test calls, both local and junction, and check that the transmission and
loudspeaking reception are satisfactory when speaking closely, i.e. approximately 18in
(460mm) from the microphone.
(c) On a loudspeaking call, lift the handset and restore the ON key. Check that:-
(i) the indicator lamp is darkened, and
(ii) the call is switched from the loudspeaking equipment to the handset.
(d) While the call is switched to the handset, test for correct restoration to the
loudspeaking equipment, by depressing the ON key and restoring the handset to the
switch-hook cradle. Check that:-
(i) the call is switched back to the loudspeaking equipment, and
(ii) the indicator lamp glows.
(e) Perform all the usual tests to a standard telephone, such as dial and bell tests.
(f) Anchor the two cords securely inside the amplifier and replace the cover.
NOTE:- The high volume position of the volume control is provided to give extra receive
gain on calls of high line attenuation. On normal calls it is expected that howling will
occur if this position is used.
Formerly EI Telephones, Stations, F3002
Diagram SA4130, SA4177 and SA4174 - LST No 1 & 1A.
Diagram SA4179 - LST 1A Mk 1.
Diagram N4458 and 91565 - Control-unit No. 8A.
Diagram SA4182 - Control-unit No. 8A (Marks 1 and 2)
Installation Instructions (PDF)
An extract from
THE POST OFFICE ELECTRICAL ENGINEERS' JOURNAL
Volume 54, Part 1 - APRIL 1961
A Loudspeaking Telephone without Voice Switching
Loudspeaking Telephone
No. 1
Loudspeaking telephones enable subscribers to converse
during calls with both hands free, and it is evident that the demand for
such an instrument is sufficient to justify development of a Post Office
model. This article describes the first of a range of Post Office
loudspeaking telephones and discusses its performance and limitations.
INTRODUCTION
In 1932, the Post Office developed a loudspeaking-telephone equipment
suitable for use with 2-wire lines, and several were installed on
privately-rented circuits to provide loudspeaking conference facilities.
Others were installed at private branch exchanges for use on
extension-to-extension calls, and were barred from use on calls over the
public network. Each equipment consisted of a microphone and loudspeaker
mounted side by side in a desk cabinet, with their respective sending and
receiving amplifiers and power supply housed in a separate box. To prevent
unwanted oscillation (howling) due to coupling between the microphone and
loudspeaker, either the sending amplifier or receiving amplifier was
automatically suppressed, depending on the direction of transmission at a
given moment. The suppression was achieved by deriving voltages from the
speech signals and using them to control variable attenuation networks; the
system was therefore said to be "voice-switched".
Several modified designs were produced experimentally up to 1939, but during
1939-1945 most of the loudspeaking telephones were recovered. Until 1955 no
loudspeaking telephones were approved for connexion to the public network;
this restriction was necessary because of the inferior performance of the
loudspeaking telephones at that time compared with the standard telephone.
In 1955 it was decided that the Post Office would develop a "non-switched"
loudspeaking telephone (subsequently named Loudspeaking-Telephone No. 1)
suitable for use with 2-wire lines of the public network. A non-switched
system is one in which the sending-amplifier and receiving-amplifier paths
function simultaneously. The advantage of such a system is that, having no
complex switching networks, it is cheaper to produce and easier to maintain
than a switched system. Although under certain conditions its performance is
inferior to that of a voice-switched instrument, it was considered that
there would be sufficient demand to justify development. Ticket agencies,
reception desks and information offices, where it is advantageous to have
both hands free during calls but where expensive apparatus is not justified,
are probably suitable premises for the provision of non-switched
loudspeaking telephones.
In a voice-switched equipment the receiving amplifier is suppressed when the
sending amplifier is in use, and vice versa. The receiving amplifier may be
rendered inoperative, or "suppressed", by using part of the output voltage
of the sending amplifier to operate a relay or to control a
variable-attenuation network.
The use of voice switching introduces several problems:-
-
The voice-switching circuits can considerably
increase the cost of the equipment.
-
It is desirable that a distant subscriber should be able to break into
the conversation by overriding the suppression bias in the near-end
loudspeaking equipment. This "break-in" facility and other design features
increase the complexity of the switching networks and may cause maintenance
difficulties.
-
First syllables in sentences may be clipped due to slight delay in
operation of the switches; under adverse conditions a complete word may be
lost.
-
When a voice-switched loudspeaking telephone is being used in a noisy
locality the noise sent to line is suddenly switched off when the user stops
talking. This creates the disturbing impression at the distant end that the
circuit has been disconnected.
-
Excessive room noise can hold the voice switch operated and thus
prevent break-in by the distant subscriber, or, alternatively, it can cause
false break-in.
-
It is difficult to maintain the correct relationship between the
suppression and break-in biases for varying line conditions.
-
It is difficult to ensure satisfactory operation of the voice-switched
networks on circuits with high attenuation where the received speech signals
are at a low level.
These disadvantages can be reduced to a minimum by
careful design, and many commercial equipments now available give good
results under average line and room conditions. They are not, however,
suitable for use on the public-exchange network where a variety of line
conditions are encountered.
The outstanding feature of the voice-switched type of
loudspeaking telephone is that it can be used to give loudspeaking reception
at both ends of a line. In contrast, the non-voice-switched loudspeaking
telephone cannot be used in this way except on very short lines, and will
normally be used as a loudspeaking equipment only when a handset is used at
the distant end.
At the present time there is no loudspeaking telephone of either type which
will work satisfactorily on all calls in the British Post Office network,
but with further development this objective may eventually be attained.
Present voice-switched instruments are usually capable of working over
higher-attenuation connexions than the non-switched type. Thus, the choice
of instrument for any particular subscriber depends upon a number of
factors. If more than one loudspeaking telephone is required on a single
installation, or if calls are to be made over high-attenuation connexions,
then a voice-switched loudspeaking telephone must be used. If, however,
simple hands-free operation at an economic rental is required then a single
non-voice-switched type will be satisfactory.
The Post Office is introducing models of both the non-switched and
voice-switched types and these will be known as Loudspeaking-Telephone No. 1
and Loudspeaking-Telephone No. 2, respectively. The remainder of this
article is concerned with a description of the Loudspeaking-Telephone No. 1
(Fig. 1).
LOUDSPEAKING-TELEPHONE No. 1
The schematic diagram of a loudspeaking-telephone equipment is shown in Fig.
2. A conventional telephone handset is provided for privacy or for use in
the event of failure of the loudspeaking equipment.
There are several ways in which the items comprising a loudspeaking
telephone can be grouped without violating the important requirement that
the microphone and loudspeaker should be about 2ft to 3ft apart to reduce
sufficiently the acoustic feedback.
After careful consideration it was decided that the best arrangement was to
combine in a single control unit all the controls necessary for setting up a
call (the on/off key, dial, volume-control key, and indicator lamp),
together with the microphone. This unit would then be drawn forward by the
user whenever a call was made, and this would ensure that the user would
speak close to the microphone (which is desirable in order to reduce to a
minimum the effects of room reverberations).
FIG. 1 - THE NEW LOUDSPEAKING TELEPHONE
The Loudspeaking-Telephone No. 1 consists of a Telephone No. 708, a Control
Unit No. 8A, and an Amplifier Unit No. 138A. The Amplifier Unit No. 138A,
which is mounted separately, is not shown in this photograph.
The Control-unit No. 8A as shown in this picture is probably a
pre-production model.
FIG. 2 - SCHEMATIC DIAGRAM OF A LOUDSPEAKING TELEPHONE
The loudspeaker would be fitted in the telephone instrument in place of the
dial, and this unit would normally be placed at the back of the desk since
on normal calls the handset would not be used. The two amplifiers and hybrid
transformer would be fitted in a case, which could be placed in a position
remote from the subscriber's desk or table. Thus, in the arrangement chosen
for the Loudspeaking-Telephone No. 1, three separate units are provided;
these are the control unit, the telephone and the amplifier, the first two
of which are shown in Fig. 1.
The loudspeaking telephone is designed to work from 50-volt exchanges, and
the transistor amplifier draws its power from the subscriber's line so that
it does not need separate power supplies.
Control Unit
The microphone lies behind the grille at the top of the control unit
(Control Unit No. 8A), together with the indicator lamp, which is also
mounted in this aperture. The on/off key is on the right-hand side. The
left-hand key is a 3-position volume control. The bottom position of this
key (position 3) gives maximum receiving-amplifier gain, which can be used
advantageously under certain conditions. There is, however, a possibility of
howling occurring if the key is left in this position during the setting up
of a call. For this reason the bottom position of the key has been made
non-locking. All the components, except the microphone and indicator lamp,
are mounted on a framework screwed to the base plate, as shown in Fig. 3.
The outer case of the unit can be readily removed from the base, and this
permits easy access for maintenance.
FIG. 3 - CONTROL UNIT
A special feature of the on/off key is that, although it can be moved
manually to the on or off positions as for a normal 2-position locking key,
when in the on position it can be restored electrically to the off position.
By this means a call can be switched automatically by a relay from the
loudspeaking equipment to the conventional telephone by lifting the handset.
This is achieved by arranging that when the key is moved to on, it remains
locked by a latch attached to a relay armature. The latch can be disengaged
by either electrically operating the relay or manually restoring the key to
off. Electrical operation of the relay causes the armature to lift the latch
out of the locking niche, and a restoring spring then moves the key handle
to the off position. The relay functions correctly on all subscribers' lines
up to the maximum line resistance of 1,000ohms.
The electrical circuit of the relay is shown in Fig. 4.
FIG. 4 - CIRCUIT OF CONTROL-UNIT CHANGE-OVER RELAY
While the loudspeaking telephone is in use, i.e. the
control key is in the on position, the K relay armature is released and the
1,500uF capacitor is
fully charged by the line current. When the handset is lifted the capacitor
discharges into the relay coil and relay K operates, releasing the latch,
which moves the key to off while contacts K1 and K2 switch the line over to
the telephone. Relay K remains mechanically locked in its operated position.
It is desirable on such equipment to have an indicator lamp showing when the
equipment is switched on, but it is difficult to derive the necessary power
entirely from the line. Nevertheless, a 12volt 20mA lamp has been
specially designed which gives a satisfactory glow down to about 14mA and,
although its light output is only just sufficient on 1,000ohm subscribers'
lines, it is ample on most lines.
Telephone
The telephone (Telephone No. 708) is a Telephone No. 706 modified by
mounting a 3in. loudspeaker in place of the dial and providing an
additional switch-hook spring-set. The loudspeaker is of modern design,
especially in that the permanent magnet is housed within the cone, resulting
in a 3 in. loudspeaker only 0.75in. deep. This technique has been made
possible by the advent of modern magnetic materials from which can be
produced small permanent magnets with very high flux-densities. The magnetic
path is completed through the frame of the loudspeaker, which is of
comparatively heavy gauge material (0-08 in.) to provide a low reluctance
path. The voice coil is multi-layer, and is wound so that it can work
between the inner pole-piece of the magnet and the outer pole-piece, which
is formed by the 0.08in. thick housing. The telephone is shown with the
case removed in Fig. 5.
Amplifier
For convenience of production and maintenance, the amplifier (Amplifier No.
138A) is manufactured as three separate units: sending amplifier, receiving
amplifier and automatic balance-control (ABC) unit. These are wired to
appropriate sockets and line terminals
for interconnexion with the desk telephone and control unit. The complete
amplifier and the three separated units are shown in Fig. 6.
FIG. 5 - TELEPHONE
OPERATION
The equipment is simple to operate. Due to the use of transistors it is
unnecessary for the loudspeaking telephone to be switched on some time
before it is to be used. To make a call the subscriber operates the on key
and commences dialling on hearing dialling tone from the loudspeaker. The
indicator lamp glows when the on/off key is operated. The call is terminated
by restoration of the on/off key. During the call the volume-control key is
normally left in position 1 (low) or position 2 (medium). If, during
conversation, the handset is required, it is lifted from its cradle and the
line is automatically switched to the telephone, and the indicator lamp is
extinguished. The call is cleared by replacement of the handset.
Occasionally, it may be required to change from the handset to the
loudspeaking telephone; to do this the subscriber must operate the key
before replacing the handset.
When the loudness of the received speech is inadequate on
position 2 of the volume control the subscriber can move the volume control
to position 3 for maximum volume, but if the hybrid balance is insufficient
the loudspeaking telephone may howl. The subscriber should then revert to
the handset.
TRANSMISSION FEATURES
There is little difference between the fundamental circuits of the
non-voice-switched loudspeaking telephone and a conventional handset
telephone. Each consists of a pair of transducers for the transmission and
reception of speech, coupled through a hybrid transformer to a 2-wire line.
Where the two instruments differ greatly is in their sending and receiving
channel sensitivities.
For the loudspeaking telephone, the distance from the mouth to the
microphone is at least 18in., and the reproducer is not directly against the
ear as with the telephone receiver but may be 3ft or more away. Electrical
amplification is therefore required in both the sending and receiving paths
of the loudspeaking telephone in order to give results comparable with the
standard handset.
FIG. 6 - AMPLIFIER
An electromagnetic microphone is used because of its superior frequency
response compared with the carbon-granule transmitter, and also because of
its smaller size. Its low sensitivity is no handicap as the
sending-amplifier gain can compensate for this.
With these considerations in mind, it is obvious that the design should be
such that Nh is as high as possible over the frequency range in use and on
all connexions, and that the sending-frequency and receiving-frequency
characteristics produce maximum articulation efficiency with maximum
stability. Na should also be as high as possible but, as its value is
governed to a large extent by the disposition of the items on the
subscriber's desk, little control can be exercised by the designer.
In addition, the frequency characteristics must be so arranged that when
instability does occur the frequency of the resulting �howl� does not
interfere with voice-frequency signalling systems, which may be associated
with the particular telephone connexion in use. At present it appears that
howling frequencies between approximately 1,000c/s and 2,000c/s meet this
requirement.
D.C. SUPPLY FOR AMPLIFIERS AND INDICATOR LAMP
The resistance of subscribers' lines connected to a 50volt exchange can lie
between zero and 1,000ohms, and the voltage at the telephone terminals can
vary, correspondingly, from 21volts to 8volts.
To limit variations in the d.c. supply to the amplifiers and indicator lamp,
regulation is necessary. The circuit arrangement is shown in Fig. 7. On
1,000ohm lines the p.d. across rectifier MR 1-5 is 8volts, and at this
voltage the rectifier is designed to draw a negligible shunting current of
under 5mA. On zero-resistance lines the voltage across rectifiers MR 1-5
rises, and the rectifier then draws a shunting current of about 30mA,
causing an additional voltage drop in the exchange transmission bridge so
that the d.c. supply-line voltage does not exceed 14volts. The d.c. supply
line is decoupled from the subscriber's line by inductor L1 and capacitor
C6. The supply is connected in parallel with
the line as this simplifies the difficulty of meeting the signalling
requirement that the maximum telephone resistance on 1,000-ohm lines shall
not exceed 330 ohms. Rectifier MR6 is necessary to maintain correct polarity
for the amplifiers, irrespective of the line polarity.
FIG. 7 - D.C. SUPPLY FOR AMPLIFIERS AND INDICATOR LAMP
AMPLIFIER
The amplifier is subdivided into three units: the sending amplifier, the
receiving amplifier, and the automatic balance control unit, as shown in
Fig. 6.
Sending Amplifier
The sending amplifier is a conventional 3-stage transistor amplifier. When
the talker is 2ft from the microphone the amplifier is capable of
transmitting a speech signal to line at a slightly lower level than that
which would be obtained from a handset. It has a nominal maximum gain of
76db between 300c/s and 3,500c/s, and can be used with ambient temperatures
not exceeding 45�C.
When the amplifier is received from the manufacturer the gain is set for use
with lines of less than 500ohms resistance. An L-type attenuator in the
output circuit can be switched by a link to increase by 4 db the level
transmitted to line for use with lines of greater than 500ohms resistance.
Receiving Amplifier
This amplifier, using four transistors, provides a maximum output of 200mW,
with less than 5 per cent total distortion, to the 3ohm loudspeaker when the
d.c. supply is 9volts. The output stage operates in Class B push-pull and
requires a low-impedance d.c. supply, which is provided by capacitor C6
(1,500uF). During peak signals, the amplifier power is drawn from this
capacitor rather than from the higher-resistance supply of the line,
capacitor C6 being recharged when the signal voltage falls.
It is probable that the output transformer used will be dispensed with in
future designs of the receiving amplifier because miniature loudspeakers are
becoming available with voice-coils wound to the higher impedances (e.g.
40-70ohms) required by transformer-less output stages.
Automatic Balance-Control Unit
The principal purpose of this unit, apart from linking the sending and
receiving channels with the line, is to keep the hybrid attenuation Nh (Fig.
2) as high as possible.
A basic hybrid circuit is shown in Fig. 8 (a). The transformer consists of
three windings; windings 2 and 3 have equal turns and are connected in
series-aiding. When a generator is applied to winding 1, currents i1 and i2
flow in the networks and, clearly, when ZL = Zb , i1 = i2 and, because the
two currents in Zr are equal and opposite, the net current in Zr is zero.
This is the balanced condition, i.e. the generator has delivered power into
both ZL and Zb , but none into Zr.
FIG. 8 - HYBRID CIRCUIT
The practical arrangement in a loudspeaking telephone is
shown in Fig. 8 (b). The attenuation Nh (the attenuation between P1 and P2)
is very high when ZL = Zb The amplifier gains can, therefore, be made high
without instability occurring, and the telephone can provide
adequately-high-level signals in both directions.
Unfortunately, the impedance ZL presented by the subscriber's line varies
over a very wide range in both modulus and phase, and it changes
considerably with frequency. Furthermore, on any single call it changes at
least once because the impedance of the circuit when the connexion is
established is different from that while the call is being set up. This
condition can be illustrated by considering a call being set up for a
subscriber with a very short fine to a manual exchange or private branch
exchange. While waiting for the operator to answer, and also under certain
other conditions, the value of ZL is extremely high, amounting, in fact,
almost to a disconnexion; Nh is then very low.
Thus, the subscriber must reduce the receiving-amplifier
gain to avoid instability while the call is being set up but will have to
readjust it to provide sufficient loudspeaker output once the call is
established. This detracts from the "hands-free" facility which the
telephone should provide. However, such violent impedance variations do not
occur for the subscriber on a long line because the impedance of the local
line is always present between his telephone and the exchange.
Considering Fig. 8 (a) again, it may at first be thought advantageous to add
a shunt or series network to ZL to reduce the effect of its variations.
Balancing is then improved, but the gain due to the improved balance is
usually offset by the power loss in the added network and necessitates an
increased gain in both the sending and receiving amplifiers. In general,
therefore, increased stability is not achieved. One exception is the example
mentioned, where a subscriber on a very short line is setting up a call and
the impedance across the telephone terminals approaches an open circuit. On
such a call a fairly-high-impedance shunt (2,200ohms in series with 0.1uF)
across the line terminals improves stability by some 5db on a disconnexion,
yet results in a total sending loss plus receiving loss of less than 2 db.
The increased stability of some 3db may seem small, but it is of great value
in an instrument of this type because it avoids the inconvenience to the
subscriber of repeated volume-control changes. The shunt network is
designated R1, C1 in Fig. 9, which shows the complete circuit of the ABC
unit. This has a basic hybrid network similar to that shown in Fig. 8 except
that the line winding is split into two equal parts for cancellation of hum,
which may be induced from external sources. On long fines the shunt loss of
R1, C1 is not required and is automatically removed. This is achieved by the
network MR4, MR5, R2, C2 in a manner similar to that used in the automatic
regulator of the Telephone No. 706, as described elsewhere. When connected
via a zero-resistance fine to a 50volt exchange, the network impedance
between points P3 and P4 is about 12ohms, while on 1,000ohm fines it rises
to over 5,000ohms. Thus, the network R1, C1 is effectively in shunt with the
fine when the latter is short, disconnected when the fine is long, and pro
rata on intermediate lines. Rectifiers MR4 and MR5 also form part of the
d.c. voltage regulator.
FIG. 9 - AUTOMATIC BALANCE-CONTROL UNIT
A similar automatic network is used to control the impedance of the balance.
The network MR1, MR2, R3, C3 exercises this control and effectively shunts
part of the balance network R4, R5, C4 via the resistor R6 (in series with
the capacitors C5, C6 and C7) when on short lines. The balancing arrangement
is designed for the range of impedances found in the Post Office network and
takes into account the network R1, Cl. The change of balance impedance with
subscriber's line length for a 50volt exchange is shown in Fig. 10. Even
this arrangement leaves much to be desired in the aim to achieve perfect
balancing, but the improvement over a fixed 4-element balance is fully
justified in view of the low cost.
Fig. 10 - Change of balance impedance with length of
Subscribers line and with frequency
Circuit details of an automatic balancing unit, which also includes
transmission equalization, are given in the Appendix. This type of unit is
still in the experimental stage but obviously has many advantages for future
designs of non-switched or partially-switched instruments.
PERFORMANCE AND LIMITATIONS IN USE
Little can be said at this stage about the operation of loudspeaking
telephones in the Post Office network because the number tested has been
small. Similar equipments have been used in other countries, and, for
instance, in the U.S.A. the Bell Telephone Co. has introduced the
Speakerphone 6 (a non-voice-switched instrument).
There is little doubt that a major field of usefulness is on private branch
exchanges where extension-to- extension calls are most frequently made.
Under these conditions the incoming signal strength is sufficient for ample
loudness from the loudspeaker when the volume control is set to the low or
medium position. However, when two Loudspeaking-Telephones No. 1 work
together, satisfactory loudspeaking operation at both ends may not be
obtained unless the rooms are quiet and a loud signal is not required.
When using the loudspeaking telephone on junction calls, about 10-15db
junction attenuation is probably the highest which most subscribers would
tolerate before preferring to use the handset. This assumes that room noise
is not excessive.
The received speech at the distant end is at a somewhat lower level than
when generated from a handset, but this is partially compensated for by the
improved quality obtained from an electromagnetic microphone compared with
the carbon-granule transmitter. Because the subscriber will be compelled to
use a handset on the higher-attenuation calls, there is little risk of the
distant-end reception being below standard.
INSTALLATION
The loudspeaking telephone can be used on direct exchange lines, on
extensions, on private branch exchanges, and on certain extension plans.
The 2-wire line is connected to the amplifier unit by terminals, and the
control unit and telephone unit are connected to the amplifier unit by plugs
and sockets and then placed in their working positions. If the subscriber's
line has a resistance exceeding 500 ohms the gain of the sending amplifier
is increased by adjustment of the 2-position link in the ABC unit.
Receiving loudness can be adjusted by two controls in series, one available
to the subscriber, i.e. the volume control key on the control unit, and the
other a pre-set gain control within the amplifier unit. With the
volume-control key in the middle position, the on key is operated and, on
automatic systems, dialling tone should be heard. The pre-set gain control
is then carefully adjusted so that the loudspeaking telephone just does not
howl. On manual systems a similar adjustment is made with only an answering
cord connected. This adjusts the loudspeaking telephone so that it will not
howl on any connexion when the volume control is in the low or medium
positions.
MAINTENANCE
A component which has caused maintenance engineers misgivings is the
electrolytic capacitor. The transistor circuits, because of their low
impedances, need quite a large number of electrolytic capacitors, but
fortunately of low-voltage working; however, to increase reliability,
high-grade electrolytic capacitors have been used.
At the subscriber's installation the maintenance engineer will renew any of
the three units which becomes faulty, the testing and changing being
simplified by the provision of plugs and sockets within the amplifier unit.
It is undesirable that further maintenance should be done at the
subscriber's premises.
CONCLUSIONS
There is a considerable demand by subscribers for loudspeaking telephones,
mainly from those who are conversant with the facilities obtainable and know
that such items are available. It is expected that the provision of the
Loudspeaking-Telephone No. 1 will satisfy much of this demand by providing
an instrument which, although having certain limitations, can be offered at
a reasonable rental, and will provide the valuable "handsfree" facility.
The loudspeaking telephone requires no mains power supply connexion, and,
for a non-voice-switched loud-speaking telephone, has a high level of
transmission performance. Its cost and estimated maintenance requirements
are low.
APPENDIX
An Automatic Balance-Control Unit Incorporating Transmission Equalization
This unit may be considered as a replacement for the unit described in the
above article. Its principle of operation is somewhat different in that it
employs a fixed balance and an automatically controlled network, in the form
of an L-type attenuator, in tandem with the line to reduce the range of line
impedances which the hybrid circuit is required to balance.
If the maximum permissible length of subscriber's line is I miles and in any
particular case the length of line is x miles, then the automatic circuit
inserts, between the hybrid transformer and the line, a network (series
resistance and shunt capacitance) approximately equivalent to a line of
length (I-x) miles. The hybrid transformer circuit is, therefore, connected
to a similar, very much reduced, range of line impedances whatever the
length of the subscriber's line, and receiving levels in both directions are
automatically compensated.
As in the case of the ABC unit already described, the p.d. between points a
and b (Va-o) in Figure 11 varies according to the
resistance of the subscriber's line (e.g. 14 volts on zero-resistance lines
to 8 volts on 1,000ohm lines). Direct current will therefore flow through
the two parallel paths R1, MR3, etc., and R2, R5, and a p.d. will be
developed between points c and d. This potential biases the rectifiers MR1
and MR2, and resistors R1, R2 and R5 are chosen so that on zero-resistance
lines the bias is either reverse or very low in the forward direction.
However, resistor R5 must be of fairly high resistance as it is connected
directly across the line.
To speech signals, points c and d are at the same potential because they are
linked by the low-reactance capacitor C2; thus, the impedance between points
e and d (Ze-a) is that of the two equal combinations MR1, R3 and MR2, R4 in
parallel. On zero-resistance lines, because MR1 and MR2 are biased to very
high impedances, Ze-d , which is, in effect, a series element in the
subscriber�s line, is approximately equal to R3/2 or R4/2. Thus, the maximum
resistance of the series arm of the L-type attenuator can be made any
practical value by the choice of suitable values for the resistors R3 and
R4.
FIG. 11 - AUTOMATIC BALANCE CONTROL UNIT INCORPORATING
TRANSMISSION EQUALIZATION
The shunt-capacitance arm of the attenuator is controlled
by the network MR4, MR5, R6, C4 in a similar manner to that adopted for the
present ABC unit.
Comparing the electrical conditions on 1,000-ohm lines with those given
above for zero-resistance lines:
(a) Va-b falls from 14 volts to 8 volts.
(b) The voltage across R2 decreases in a similar ratio.
(c) The voltage across R1 decreases in a lower ratio owing to the rise in
resistance of the rectifier chain MR3-MR5 at lower currents.
The voltage across points c and d therefore rises, causing rectifiers MR1
and MR2 to conduct; thus, their impedances fall to values very much smaller
than R3 or R4 (e.g. 10-20 ohms is easily obtained using junction diodes for
MR1 and MR2). The series arm of the attenuator is then effectively
short-circuited. The shunt arm is similarly rendered inoperative by the
considerable rise in impedance of MR4 and MR5.
Although only the extremes of line resistance have been considered, it can
also be shown that an automatically controlled network may be obtained which
closely simulates the difference between a line of a given maximum
resistance and the line in use.
Additional Information
The Control-unit No. 8A was available as Mark 1 and Mark
2 variants. The Mark 1 had a lamp at the top of the case in the centre
of the microphone grill. The Mark 2 had the lamp just below the dial
and also had a Pre-amplifier within the control unit.
LST No. 1 with Control-unit No. 8A (Mark 1)
LST No. 1 with Control-unit No. 8A (Mark 2)
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