Transmission Bridges

1
Transmission Bridges

• All calls through an exchange require the use of
  a transmission bridge. The main requirements of a
  transmission bridge are
• 1) The provision of line current to both calling
  and called telephones
• 2) The transmission of speech between the phones.

2
The simplest bridge would be the circuit below
Such a bridge, whilst providing very little
speech attenuation, is not very satisfactory for
the following reasons 1) the current supplied
through the resistance of the relay splits to
feed both phones. The line with the higher
resistance receives the least current and vice
versa. This is exactly what is not required as
the high resistance line would provide the lower
speech level at the phone and provide the highest
line attenuation. 2) The relay can only be used
to determine when both telephones have cleared
down. Generally it is necessary to monitor the
two phones separately.
3
A practical bridge needs two relays, each
monitoring one of the connected phones. The
circuit below shows the arrangement in use in the
majority of transmission bridges. This is known
as a Stone bridge.
In such a bridge, relay A supplies line current
to the caller and generally controls the release
of the call. Relay D supplies current to the
called phone and usually triggers metering when
the called phone answers. The capacitors permit
speech currents to pass between the phones with
very little attenuation. The relays generally
have high resistance split nickel iron sleeves
around the soft iron core to reduce speech eddy
currents and therefore speech losses. When a
strowger exchange was being installed by
contract, one of the acceptance tests involved
the measurement of loss across the transmission
bridges in the exchange. It was normal for the
loss at each bridge to be less than 0.5db at
1000Hz although reference books generally accept
that the loss would be just over 1 db.
4
An even more practical transmission bridge
circuit is shown below.
Two further features are added, particularly when
the transmission bridge is going to be
incorporated into a final selector or outgoing
relay set. 1) A third winding on the A relay
coil will permit the connection of tones to the
line by transformer action in the relay. The
line remains balanced when the tone is applied,
an improvement on older circuitry. 2) It is
possible to compensate for the loss in long local
lines to some extent by raising the current
flowing around the line and through the
microphone. This can increase the speech output.
Reducing the resistance of the D relay to 50 50
ohms helps to achieve this. However a 50 ohm
relay is unsuitable on its own as too much
current will flow under short line conditions
and could "fry" a carbon microphone. The
barretter normally has a very low resistance but
as the current flowing through it rises, its
resistance also rises rapidly to cap the rise to
a reasonable level. The barretter also prevents
the 50 ohm D relay from overheating should a full
earth be found on the called line circuit.
5
You will notice that the barretter is only
provided on one side of the bridge. On an "own
exchange" call, the line transmission would be
good enough without any barretter facility. Only
on calls using junction circuits with their
increased transmission loss is it necessary to
attempt to improve speech levels using the
barretter circuitry. Under these circumstances
the barretter is needed on the caller's side of
the bridge at the originating exchange and the
called side of the bridge at the terminating
exchange. The capacitor bridge is
satisfactory for most occasions but it does allow
conditions on one side of the bridge to affect
pulsing relays on the other side. Surges are
transmitted through the capacitors easily. The
capacitors also become connected in series across
the A relay in certain pulsing relay sets
causing some slugging effects.
6
When such effects cannot be tolerated, usually in
junction relay sets employing long distance
signalling techniques, then the Hayes
transformer bridge is preferred.
                                                  
                                                  
                                                  
This bridge separates the speech
transmission and DC relay requirements into
distinct elements. 1) The transformer transmits
the speech currents across the bridge with a loss
of about 1db. However the transformer has to be
quite large as the core must not be near
saturation from the DC flowing through the
windings. 2) The relays now have no effect on
the speech transmission and can be of any type
suitable for the particular purpose. High speed
relays can be employed for pulsing purposes. A
barretter can be Included if required. The
Hayes bridge was also common on the older manual
board systems as the "relays" were often
replaced by cord circuit indicators which would
not have functioned in a capacitor
bridge. Pulsing through a transmission bridge
will be dealt with in the next article.