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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