SUBSCRIBER LOOP DESIGN
current flows; when open, current stops flowing. If, for instance, a 5 is dialed, there will
be five break-and-make operations which represent the digit 5.
The Subset Mouthpiece or Transmitter
The mouthpiece converts acoustic energy (i.e., sound) into equivalent electric energy
by means of a carbon granule transmitter. The transmitter requires a direct-current (dc)
voltage, a minimum of 3 to 5 volts, across its electrodes. We call this talk battery. In
modern systems it is supplied over the subscriber loop and derives from a battery source
at the local serving switch as illustrated in Figure 5.3.
Current deriving from the local switch supply flows through the carbon grains or
granules that are contained just below a diaphragm that holds the carbon granules in place.
This current flow occurs when the telephone is taken off-hook (i.e., out of its cradle). When
sound impinges on the diaphragm of the transmitter, resulting variations of air pressure are
transferred from the diaphragm to the carbon granules, and the resistance of the electrical
path through the carbon changes in proportion to the pressure. A pulsating direct current
in the subscriber loop results. The frequency response of the carbon transmitter peaks
between 800 and 1000 Hz. This is illustrated in Figure 5.2.
The Subset Earpiece or Receiver
A typical receiver consists of a diaphragm of magnetic material, often soft iron alloy,
placed in a steady magnetic field supplied by a permanent magnet, and a varying magnetic
field, caused by the voice currents flowing through the voice coils. Such voice currents are
alternating (ac) in nature and originate at the far-end telephone transmitter. These currents
cause the magnetic field of the receiver to alternately increase and decrease, making the
diaphragm move and respond to the variations. As a result, an acoustic pressure wave is set
up, reproducing, more or less exactly, the original sound wave from the distant telephone
transmitter. The telephone receiver, as a converter of electrical energy to acoustic energy,
has a comparatively low efficiency, on the order of 2% to 3%.
Sidetone is the sound of the talker's voice heard in his own receiver. The sidetone level
must be controlled. When the level is high, the natural human reaction is for the talker
to lower his voice. Thus by regulating the sidetone, talker levels can be regulated. If too
much sidetone is fed back to the receiver, the output level of the transmitter is reduced,
owing to the talker lowering his/her voice, thereby reducing the level (voice volume) at
the distant receiver, deteriorating performance.
SUBSCRIBER LOOP DESIGN
Basic Design Considerations
We speak of the telephone subscriber as the user of the subset. As we mention in
Section 1.3, telephone subscribers are connected via a subscriber loop to a local serv-
ing switch that can connect a call to another subscriber served by that same switch or
via other switches through the PSTN to a distant called subscriber. The conventional
subscriber loop is a wire pair. Present commercial telephone service provides for both
transmission and reception on the same pair of wires that connect the subscriber to her/his
local serving switch. In other words, it is two-wire operation.
The subscriber loop is a dc loop in that it is a wire pair supplying a metallic path
Metallic path is a path that is "metal," usually copper or aluminum. It may be composed of a wire pair or
coaxial cable. We could have a radio path or a fiber-optic path.