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Reply-To: julian@bongo.info.com
To: telecom@eecs.nwu.edu
Subject: How Phones Work
Message-Id: <9108050607.AA04605@bongo.info.com>
Date: 5 Aug 91 06:07:47 PDT (Mon)
From: Julian Macassey
Resent-Date: Tue, 6 Aug 91 23:50:03 CDT
Resent-From: telecom@eecs.nwu.edu
Resent-To: ptownson@gaak.LCS.MIT.EDU
Status: RO
Dear Patrick,
as requested, here is my introductory article on phones:
----------cut and slash at will -------------------------------
UNDERSTANDING TELEPHONES
by
Julian Macassey, N6ARE
First Published
in
Ham Radio Magazine
September 1985
Everybody has one, but what makes it work?
Although telephones and telephone company practices may vary
underlying the way they work remain unchanged.
Every telephone consists of three separate subassemblies,
each capable of independent operation. These assemblies are the
Together, these parts - as well as any additional devices such as
modems, dialers, and answering machines - are attached to the
The phone line
A telephone is usually connected to the telephone exchange
by about three miles (4.83 km) of a twisted pair of No.22 (AWG)
or 0.5 mm copper wires, known by your phone company as "the
loop". Although copper is a good conductor, it does have
thousand feet at 77 degrees F (25 degrees C). In the United
States, wire resistance is measured in Ohms per thousand feet;
telephone companies describe loop length in kilofeet (thousands
of feet). In other parts of the world, wire resistance is
usually expressed as Ohms per kilometer.
Because telephone apparatus is generally considered to be
current driven, all phone measurements refer to current
consumption, not voltage. The length of the wire connecting the
current that can be drawn by anything attached at the
In the United States, the voltage applied to the line to
that telephones are peculiar in that the signal line is also the
thus assuring a hum-free supply and complete independence from
the electric company, which may be especially useful during power
outages.
At the telephone exchange the DC voltage and audio signal
are separated by directing the audio signal through 2 uF
capacitors and blocking the audio from the power supply with a 5-
Henry choke in each line. Usually these two chokes are the coil
exchange; in the United States, this relay is known as the "A"
Ohms.
We can find out how well a phone line is operating by using
Ohm's law and an ammeter. The DC resistance of any device
attached to the phone line is often quoted in telephone company
between 150 to 1,000 Ohms. You can measure the DC resistance of
your phone with an Ohmmeter. Note this is DC resistance, not
mpedance.
Using these figures you can estimate the distance between
your telephone and the telephone exchange. In the United States,
the telephone company guarantees you no lower current than 20 mA
- or what is known to your phone company as a "long loop." A
"short loop" will draw 50 to 70 mA, and an average loop, about 35
mA. Some countries will consider their maximum loop as low as 12
mA. In practice, United States telephones are usually capable of
consider your phone in use and feed dial tone down the line with
currents as low as 8 mA, even though the telephone may not be
able to operate.
Although the telephone company has supplied plenty of nice
clean DC direct to your home, don't assume you have a free
battery for your own circuits. The telephone company wants the
DC resistance of your line to be about 10 megOhms when there's no
apparatus in use ("on hook," in telephone company jargon); you
can draw no more than 5 microamperes while the phone is in that
current, but you will need that current to power your phone, any
current you might draw for other purposes would tend to lower the
The phone line has an impedance composed of distributed
according to the length of the loop, the type of insulation of
the wire, and whether the wire is aerial cable, buried cable, or
bare parallel wires strung on telephone poles. For calculation
and specification purposes, the impedance is normally assumed to
be 600 to 900 Ohms. If the instrument attached to the phone line
(Radio Amateurs will recognize return loss as SWR.) A mismatch
on telephone lines results in echo and whistling, which the phone
company calls "singing" and owners of very cheap telephones may
matched to the phone line by placing resistors in parallel or
course, but will make the device usable.
A phone line is balanced feed, with each side equally
balanced to ground. Any imbalance will introduce hum and noise
to the phone line and increase susceptibility to RFI.
The balance of the phone line is known to your telephone
company as "longitudinal balance." If both impedance match and
balance to ground are kept in mind, any device attached to the
transmission lines and devices will ensure good performance in
If you live in the United States, the two phone wires
connected to your telephone should be red and green. (In other
s negative and the green wire is positive. Your telephone
company calls the green wire "Tip" and the red wire "Ring". (In
other parts of the world, these wires may be called "A" and "B".)
Most installations have another pair of wires, yellow and black.
These wires can be used for many different purposes, if they are
used at all. Some party lines use the yellow wire as a ground;
extensions) in your home, you will find the yellow and black pair
carrying a second telephone line. In this case, black is "Tip"
and yellow is "Ring."
The above description applies to a standard line with a DC
connection between your end of the line and the telephone
exchange. Most phone lines in the world are of this type, known
as a "metallic line." In a metallic line, there may or may not
be inductance devices placed in the line to alter the frequency
"loading coils." (Note: if they impair the operation of your
modem, your telephone company can remove them.) Other types of
lines are party lines, which may be metallic lines but require
amplifiers, sometimes called "loop extenders" on them. Some
telephone companies use a system called "subscriber carrier,"
If you have questions about your telephone line, you can
call your telephone company; depending on the company and who you
can reach, you may be able to obtain a wealth of information.
The Speech Network
The speech network - also known as the "hybrid" or the "two
to the earpiece and takes the microphone output and feeds it down
the line. The standard network used all over the world is an LC
transistors or ICs.
One of the advantages of an LC network is that it has no
continuously as the voltage across the line is reduced. Many
transistorized phones stop working as the voltage approaches 3 to
When a telephone is taken off the hook, the line voltage
length of the loop. If another telephone in parallel is taken
off the hook, the current consumption of the line will remain the
tend not to pass this test, although some manufacturers use ICs
that will pass. Although some European telephone companies claim
that phones working in parallel is "technically impossible," and
telephones will work in parallel.
While low levels of audio may be difficult to hear, overly
loud audio can be painful. Consequently, a well designed
telephone will automatically adjust its transmit and receive
levels to allow for the attenuation - or lack of it - caused by
the length of the loop. This adjustment is called "loop
compensation." In the United States, telephone manufacturers
achieve this compensation with silicon carbide varistors that
consume any excess current from a short loop (see fig. 2).
Although some telephones using ICs have built-in loop
compensation, many do not; the latter have been designed to
they provide low volume on long loops, and are too loud on short
loops. Various countries have different specifications for
transmit and receive levels; some European countries require a
level if used on European lines without modification.
Because a telephone is a duplex device, both transmitting
and receiving on the same pair of wires, the speech network must
ensure that not too much of the caller's voice is fed back into
achieved by phasing the signal so that some cancellation occurs
n the speech network before the signal is fed to the receiver.
Callers faced with no sidetone at all will consider the phone
"dead." Too little sidetone will convince callers that they're
not being heard and cause them to shout, "I can hear you. Can
you hear ME?" Too much sidetone causes callers to lower their
voices and not be heard well at the other end of the line.
A telephone on a short loop with no loop compensation will
appear to have too much sidetone, and callers will lower their
voices. In this case, the percentage of sidetone is the same,
but as the overall level is higher the sidetone level will also
be higher.
The Dial
There are two types of dials in use around the world. The
most common one is called pulse, loop disconnect, or rotary; the
oldest form of dialing, it's been with us since the 1920's. The
other dialing method, more modern and much loved by Radio
Amateurs is called Touch-tone, Dual Tone Multi-Frequency (DTMF)
or Multi-Frequency (MF) in Europe. In the U.S. MF means single
tones used for system control.
Pulse dialing is traditionally accomplished with a rotary
closes a switch in series with your phone and the line. It works
by actually disconnecting or "hanging up" the telephone at
"disconnected" once. Dial a seven and you'll be "disconnected"
countries invert the system so "1" causes ten "disconnects" and
cause six disconnects and 0, eleven disconnects. There are even
all other digits are plus one, making a 5 cause six disconnects
and 9, ten disconnects.
Although most exchanges are quite happy with rates of 6 to
s 8 to 10 PPS. Some modern digital exchanges, free of the
mechanical inertia problems of older systems, will accept a PPS
Besides the PPS rate, the dialing pulses have a make/break
most of Europe accepts a standard of 63/37 percent. This is the
ts distributed resistance, capacitance, and inductance. In
mind that each pulse is a switch connect and disconnect across a
complex impedance, so the switching transient often reaches 300
Volts. Try not to have your fingers across the line when
Most pulse dialing phones produced today use a CMOS IC and a
keyboard. Instead of pushing your finger round in circles, then
The IC stores the number and pulses it out at the correct rate
a high-voltage switching transistor. Because the IC has already
memory "alive" and allow the telephone to store, recall, and
Because pulse dialing entails rapid connection and disconnection
of the phone line, you can "dial" a telephone that has lost its
A more sophisticated approach is to place a Morse key in series
Touch tone, the most modern form of dialing, is fast and
less prone to error than pulse dialing. Compared to pulse, its
major advantage is that its audio band signals can travel down
your local exchange. Touch-tone can therefore send signals
around the world via the telephone lines, and can be used to
control phone answering machines and computers. Pulse dialing is
to touch-tone as FSK or AFSK RTTY is to Switched Carrier RTTY,
unmodulated RF carrier. Most Radio Amateurs are familiar with
DTMF for controlling repeaters and for accessing remote and auto
Bell Labs developed DTMF in order to have a dialing system
that could travel across microwave links and work rapidly with
computer controlled exchanges. Each transmitted digit consists
of two separate audio tones that are mixed together (see fig.3).
The four vertical columns on the keypad are known as the high
s composed of 1336 Hz and 852 Hz. The level of each tone is
"Twist"). A complete touch-tone pad has 16 digits, as opposed to
ten on a pulse dial. Besides the numerals 0 to 9, a DTMF "dial"
found on consumer telephones, the IC in the phone is capable of
The * sign is usually called "star" or "asterisk." The #
an octothorpe. Although many phone users have never used these
answering machines, bringing up remote bases, electronic banking,
and repeater control. The one use of the octothorpe that may be
familiar occurs in dialing international calls from phones in the
United States. After dialing the complete number, dialing the
octothorpe lets the exchange know you've finished dialing. It
can now begin routing your call; without the octothorpe, it would
When DTMF dials first came out they had complicated cams and
oscillator with an LC tuning network to generate the tones.
Modern dials use a matrix switch and a CMOS IC that synthesizes
the tones from a 3.57MHz (TV color burst) crystal. This
oscillator runs only during dialing, so it doesn't normally
Standard DTMF dials will produce a tone as long as a key is
a digit can be sent and decoded is about 100 milliseconds (ms).
automatic dialers can do it. A twelve-digit long distance number
can be dialed by an automatic dialer in a little more than a
The output level of DTMF tones from your telephone should be
between 0 and -12 dBm. In telephones, 0 dB is 1 miliwatt over
considered a 600 Ohm load, placing a voltmeter across the line
The Ringer
Simply speaking this is a device that alerts you to an
ncoming call. It may be a bell, light, or warbling tone. The
telephone company sends a ringing signal which is an AC waveform.
Although the common frequency used in the United States is 20 HZ,
t can be any frequency between 15 and 68 Hz. Most of the world
uses frequencies between 20 and 40 Hz. The voltage at the
attached to the line; it could be between 40 and 150 Volts. Note
that ringing voltage can be hazardous; when you're working on a
The telephone company may or may not remove the 48 VDC during
Don't take chances.
The ringing cadence - the timing of ringing to pause -
varies from company to company. In the United States the cadence
s normally 2 seconds of ringing to 4 seconds of pause. An
unanswered phone in the United States will keep ringing until the
caller hangs up. But in some countries, the ringing will "time
out" if the call is not answered.
The most common ringing device is the gong ringer, a
s solely phone-line powered.
Modern telephones tend to use warbling ringers, which are
usually ICs powered by the rectified ringing signal. The audio
transducer is either a piezoceramic disk or a small loudspeaker
via a transformer.
Ringers are isolated from the DC of the phone line by a
capacitor. Gong ringers in the United States use a 0.47 uF
capacitor. Warbling ringers in the United States generally use a
capacitors of the past have been replaced almost exclusively with
capacitors made of Mylar film. Their voltage rating is always
The capacitor and ringer coil, or Zeners in a warbling
up ("on hook") the ringer is across the line; if you have turned
off the ringer you have merely silenced the transducer, not
When the telephone company uses the ringer to test the line,
t sends a low-voltage, low frequency signal down the line
(usually 2 Volts at 10 Hz) to test for continuity. The company
keeps records of the expected signals on your line. This is how
t can tell you have added equipment to your line. If your
telephone has had its ringer disconnected, the telephone company
cannot detect its presence on the line.
Because there is only a certain amount of current available
to drive ringers, if you keep adding ringers to your phone line
you will reach a point at which either all ringers will cease to
five normal ringers. A normal ringer is defined as a standard
telephone. Value given to this ringer is Ringer Equivalence
Number (REN) 1. If you look at the FCC registration label of
your telephone, modem, or other device to be connected to the
Other countries have various ways of expressing REN, and
the phones stop ringing, or the phone answering machine won't
than gong ringers, so changing from gong ringers to warbling
Frequency response is the second criterion by which a ringer
s described. In the United States most gong ringers are
electromechanically resonant. They are usually resonant at 20
and 30 Hz (+&- 3 Hz). The FCC refers to this as A so a normal
B and some United States gong ringers are type B. Outside the
United States, gong ringers appear to be non-frequency selective,
or type B.
Because a ringer is supposed to respond to AC waveforms, it
extension phone. This is called "bell tap" in the United States;
n other countries, it's often called "bell tinkle." While
European and Asian phones tend to bell tap, or tinkle, United
States ringers that bell tap are considered defective. The bell
tap is designed out of gong ringers and fine tuned with bias
accomplished by rectifying the AC and filtering it before it
voltage lasts long enough to charge a second capacitor.
Conclusion
This brief primer describing the working parts of a
telephone is intended to provide a better understanding of phone
equipment. Note that most telephone regulatory agencies,
ncluding the FCC, forbid modification of anything that has been
End of text. Figures Follow
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fig 1. The Phone Line
A RELAY
200 Ohms Telephone . Subscriber
------- Exchange .
------- . TIP +
------~~~~~~~--o----------------------o
| 5 H | .
| | .
+| | .
--- | . No 22 AWG wire
--- 48V DC | . up to 10 Miles Long
- | .
--- A RELAY | .
-| 200 Ohms | .
| ------- | .
| ------- | . RING -
------~~~~~~~--|---------o------------o
5 H | | .
Audio 2uF | 2uF | .
coupling 250V --- 250V ---
Capacitors --- ---
| |
o----- \-------- |
|
A RELAY Contacts |
|
o----- \------------------
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fig 2. Telephone Speech Network.
Simplified U.S. Standard "425B". Component Values
may vary between manufacturers. Connections for Dials, Ringers
etc. not shown.
|-------------------|
..|...................|
. | .|
Sidetone balancing. | 0.047uF 250V .|
impedance & loop . | | | .|
compensation. >>> . o----| |-------o .|
. | | | | .|
. | | .|
. | |<| VR2 | .|
. o----| |-------o---.|
. | |>| |.|
. | |.|
. | 68 Ohms |.|
. o---\/\/\/-----| |.|
..|..............|..|.|
| | | |
| . | | |
-----)||(------|---------o (GN)
1)||(5 | | | |
Loop )||( | | | |
TIP Compensation 2)||(6 | | | |
o------ \------o---------)||(------o | | RX O
. | (RR) . || | | | |
. | || 1.5uF | | | |
. \ 180 || --- | | |
. / Ohms || --- | |----o (R)
. \ || 250V | | |
. | || | | |
. VR1 --- . || . | | |
. ^ ^ ----)||(------o--- TX O
. --- | 3)||(7 |
. | | )||( |
RING . | (C) | 4)||(8 22 Ohms |
o----- \-------o---------)||(---o----/\/\/---o (B)
| |
^ | |
Hookswitch ------------
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fig. 3. Standard DTMF pad and Frequencies
(Low ____ ____ ____ ____
Group)| | | | | | | |
697Hz >| 1 | | 2 | | 3 | | A |
|____| |____| |____| |____|
____ ____ ____ ____
| | | | | | | |
770Hz >| 4 | | 5 | | 6 | | B |
|____| |____| |____| |____|
____ ____ ____ ____
| | | | | | | |
825Hz >| 7 | | 8 | | 9 | | C |
|____| |____| |____| |____|
____ ____ ____ ____
| | | | | | | |
941Hz >| * | | 0 | | # | | D |
|____| |____| |____| |____|
^ ^ ^ ^
1209Hz 1336Hz 1477Hz 1633Hz
(High Group)
END
--
Julian Macassey, julian@bongo.info.com N6ARE@K6VE.#SOCAL.CA.USA.NA