Phone Lines Demystified
From the Engineering staff at Telecom Audio
Phone lines come in many configurations, but they can be summed up as
either PBX lines or “outside” lines.
The PBX System
Many offices and stores have installed low cost electronic PBX (Private
Branch Exchange) or Key telephone systems. The terms "PBX" and
"Key" both refer to hardware that enables several telephones
to be connected to a smaller number of telephone lines. The term "Key"
is now used to describe any small system but it was originally used to
describe the manual keys or push-buttons on systems like the 1A series
Key telephone. Today’s "Key" system is more like a small
PBX with programmable features such as distinctive ringing, hunt groups,
and automatic line selection. A PBX consists of a switch box and punch
block located where the telephone lines come into the building. Special
telephones are connected point-to-point back to the PBX punch block.
Electronic PBX wiring is typically 4 to 8 wires using RJ-11 or RJ-45
modular telephone jacks. These are not standard telephone wires. Even
a simple analog PBX line does not look like a standard phone line. On
an electronic PBX telephone, two wires are often used as control lines,
which send keypress data to the PBX, and ringer and LED data back to the
phone. This control information is required to set up or answer a call.
In many cases, the tones you hear when you dial a number are there for
your own feedback. The “real” digits are sent as data bits
to the PBX which completes the call. The voice path on an analog PBX is
typically referred to as a dry pair. Dry refers to the lack of DC current
or ring voltage found on regular phone lines.
On a digital PBX, your voice is converted to data right in the base
of the phone.
In either case, it is not possible to use phone line type recording adapters
on PBX telephone lines. Telecom Audio offers a line of devices that allow
you to record and play audio from your sound card or tape recorder, through
the handset cord of your telephone, directly in and out of your PBX system
Analog Phone Lines
An “outside line” refers to a direct connection to the telephone
line outside of the building, also referred to as an "analog line",
or "POTS line" (Plain Old Telephone Service), in other words
a standard residence type phone line. The POTS line is the line you will
need for your analog telephone, cordless telephone, fax machine, or modem.
The POTS line consists of two wires called tip and ring. These two wires
provide DC current to power the telephone electronics, AC current to ring
the telephone bell or electronic ringer, and a full duplex balanced voice
path. Full duplex what? Full duplex means that both people can talk at
the same time. Half duplex is more like a walkie-talkie push-to-talk button,
or your old technology speakerphone. Balanced voice path refers to the
fact that the two wires (tip and ring) are only referenced to each other.
In a balanced pair of wires, your voice travels on both wires at the same
time, but one wire is electrically 180 degrees out of phase with the other.
Simply put, when the electrical signal goes up 1 volt on one wire, it
goes down exactly one volt on the other wire. At both ends of the connection
(your telephone and the phone company), the signal on one of the wires
is inverted and then added with the other. Now step back and imagine if
electrical noise from an appliance leaked on to the wires somewhere between
your phone and the phone company. The noise would appear equally and in
phase on both wires. When the signal on both wires was inverted and then
added together 180 degrees out of phase, the electrical noise would cancel
itself out while the voice signal gets stronger. This system allows telephone
signals to travel miles on inexpensive twisted pair wires, without significant
noise getting into your call.
OK, now take out your pocket protector and let’s get technical…
The POTS phone line, with all phones on-hook, should measure around 48
volts DC. Taking a phone off-hook creates a DC signal path across the
pair, which is detected as loop current back at the central office. This
drops the voltage measured at the phone down to about 3 to 9 volts. An
off-hook telephone typically draws about 15 to 20 milliamps of DC current
to operate, at a DC resistance around 180 ohms. The remaining voltage
drop occurs over the copper wire path and over the telephone company circuits.
These circuits provide from 200 to 400 ohms of series resistance to protect
from short circuits and decouple the audio signals.
To ring your telephone, the phone company momentarily applies a 90 VRMS,
20 Hz AC signal to the line. Even with a thousand ohms of line resistance,
this can still pack a bit of a shock so be careful when you are probing
around trying to find a POTS line.
POTS Line Characteristics:
Bandwidth: 180 Hz to 3.2 kHz
The low end is rolled off early to stay away from the 60 Hz power line
noise region. The high end cut off is more critical. Voice on the telephone
network is digitized at 8 kHz sampling rate which means that any signal
above 4 kHz will be aliased back as noise in the voice band. Most voice
CODECs roll off at about -25dB at 4 kHz with a -3dB down point around
3.2 kHz. The phone company decided years ago that the 180 Hz to 3.2 kHz
range would be sufficient for speech intelligibility while allowing them
to multiplex many calls over coax and twisted pair.
Signal to Noise: Approximately 45 dB
This is not as easy to quantify because noise comes in many forms, such
as electrical interference from fluorescent fixtures or hiss from the
many amplifier stages in the voice path. Speech correlated noise can be
introduced from non-linear speech coding and compression algorithms. Crosstalk
from other conversations is another form of noise. The phone company uses
8 bit mulaw nonlinear coding which yields about 12 bits of dynamic range.
The bottom line is that you can never count on more than about 45 dB signal
to noise ratio.