Collection of Hack-Phreak Scene Programs/cleanhpvac.zip/cleanhpvac/HOWPHONE.ZIP/HOWPHONE.TXT

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Subject: How Phones Work
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Date: 5 Aug 91 06:07:47 PDT (Mon)
From: Julian Macassey <[e-mail]>
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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
dramatically  from one locality to another, the basic  principles
underlying the way they work remain unchanged.

     Every  telephone consists of three  separate  subassemblies,
each capable of independent operation.  These assemblies are  the
speech  network, the dialing mechanism, and the ringer  or  bell.
Together, these parts - as well as any additional devices such as
modems,  dialers,  and answering machines - are attached  to  the
phone line.


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
resistance.   The resistance of No.22 AWG wire is 16.46 Ohms  per
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
subscriber to the telephone exchange affects the total amount  of
current   that  can  be  drawn  by  anything  attached   at   the
subscriber's end of the line.

     In  the  United States, the voltage applied to the  line  to
drive  the telephone is 48 VDC; some countries use 50 VDC.   Note
that telephones are peculiar in that the signal line is also  the
power  supply line.  The voltage is supplied by lead acid  cells,
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
windings  of  a  relay  that switches  your  phone  line  at  the
exchange;  in the United States, this relay is known as  the  "A"
relay (see fig.1).  The resistance of each of these chokes is 200
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
specifications  as  200  Ohms; this will vary  in  practice  from
between  150 to 1,000 Ohms. You can measure the DC resistance  of
your  phone  with an Ohmmeter. Note this is  DC  resistance,  not
impedance.


     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
working  at  currents  as  low as 14  mA.   Some  exchanges  will
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
state.   When  the phone is in use, or "off hook," you  can  draw
current, but you will need that current to power your phone,  any
current you might draw for other purposes would tend to lower the
signal level.

     The  phone  line has an impedance  composed  of  distributed
resistance, capacitance, and inductance.  The impedance will vary
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
should  be of the wrong impedance, you would get a  mismatch,  or
what  telephone  company  personnel refer to  as  "return  loss."
(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
have  come  to expect.  A mismatched device can, by the  way,  be
matched  to  the phone line by placing resistors in  parallel  or
series  with  the line to bring the impedance of  the  device  to
within the desired limits.  This will cause some signal loss,  of
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
phone  line  will perform well, just as the correct  matching  of
transmission  lines and devices will ensure good  performance  in
radio practice.

     If  you  live  in the United States,  the  two  phone  wires
connected  to your telephone should be red and green.  (In  other
parts  of the world they may be different colors.)  The red  wire
is  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;
sometimes  there's  6.8 VAC on this pair to light  the  dials  of
Princess type phones.  If you have two separate phone lines  (not
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
response  of  the line; the devices used to do  this  are  called
"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
special   telephones   to   allow  the   telephone   company   to
differentiate  between  subscribers.  Very long  lines  may  have
amplifiers,  sometimes  called "loop extenders"  on  them.   Some
telephone  companies  use a system called  "subscriber  carrier,"
which is basically an RF system in which your telephone signal is
heterodyned  up  to around 100 Khz and then  sent  along  another
subscriber's "twisted pair."

     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
wire/four  wire network" - takes the incoming signal and feeds it
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
device  with a carbon microphone; some newer phones use  discrete
transistors or ICs.

     One  of  the advantages of an LC network is that it  has  no
semiconductors,   is  not  voltage  sensitive,  and   will   work
continuously  as  the voltage across the line is  reduced.   Many
transistorized phones stop working as the voltage approaches 3 to
4 Volts.

     When  a  telephone is taken off the hook, the  line  voltage
drops  from 48 Volts to between 9 and 3 Volts, depending  on  the
length  of the loop.  If another telephone in parallel  is  taken
off the hook, the current consumption of the line will remain the
same and the voltage across the terminals of both telephones will
drop.  Bell Telephone specifications state that three  telephones
should  work in parallel on a 20 mA loop;  transistorized  phones
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
discourage  attempts  to make them work that way, some  of  their
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
provide  adequate  volume on the average loop, which  means  that
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
higher transmit level than is standard in the United States so  a
domestically-manufactured telephone may suffer from low  transmit
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
his  or  her  receiver.  This  function,  called  "sidetone,"  is
achieved  by phasing the signal so that some cancellation  occurs
in  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
dial,  which is a speed governed wheel with a cam that opens  and
closes a switch in series with your phone and the line.  It works
by  actually  disconnecting  or "hanging  up"  the  telephone  at
specific intervals.  The United States standard is one disconnect
per   digit,   so  if  you   dial  a  "1,"  your   telephone   is
"disconnected" once.   Dial a seven and you'll be  "disconnected"
seven times; dial a zero, and you'll "hang up " ten times.   Some
countries  invert the system so "1" causes ten "disconnects"  and
0,  one disconnect.  Some add a digit so that dialing a  5  would
cause six disconnects and 0, eleven disconnects.  There are  even
some  systems in which dialing 0 results in one  disconnect,  and
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
15  Pulses Per Second (PPS), the phone company accepted  standard
is  8  to  10 PPS.  Some modern digital exchanges,  free  of  the
mechanical  inertia problems of older systems, will accept a  PPS
rate as high as 20.

     Besides  the PPS rate, the dialing pulses have a  make/break
ratio,  usually  described as a percentage, but  sometimes  as  a
straight  ratio.  The North American standard is  60/40  percent;
most of Europe accepts a standard of 63/37 percent.  This is  the
pulse measured at the telephone, not at the exchange, where  it's
somewhat  different, having traveled through the phone line  with
its  distributed  resistance, capacitance,  and  inductance.   In
practice,  the  make/break  ratio does not  seem  to  affect  the
performance of the dial when attached to a normal loop.  Bear  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
dialing.

     Most pulse dialing phones produced today use a CMOS IC and a
keyboard.  Instead of pushing your finger round in circles,  then
removing  your finger and waiting for the dial to  return  before
dialing the next digit, you punch the button as fast as you want.
The  IC stores the number and pulses it out at the  correct  rate
with the correct make/break ratio and the switching is done  with
a high-voltage switching transistor.  Because the IC has  already
stored the dialed number in order to pulse it out at the  correct
rate,  it's a simple matter for telephone designers to  keep  the
memory  "alive"  and allow the telephone to  store,  recall,  and
redial the Last Number Dialed (LND).  This feature enables you to
redial by picking up the handset and pushing just one button.

Because pulse dialing entails rapid connection and  disconnection
of  the phone line, you can "dial" a telephone that has lost  its
dial,  by  hitting  the hook-switch rapidly.   It  requires  some
practice to do this with consistent success, but it can be  done.
A  more sophisticated approach is to place a Morse key in  series
with  the  line, wire it as normally closed and send  strings  of
dots corresponding to the digits you wish to dial.

     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
phone  lines further than pulse, which can travel only as far  as
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,
where mark and space are sent by the presence or absence of DC or
unmodulated  RF carrier.  Most Radio Amateurs are  familiar  with
DTMF for controlling repeaters and for accessing remote and  auto
phone patches.

     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
group and the four horizontal rows as the low group; the digit  8
is  composed  of 1336 Hz and 852 Hz.  The level of each  tone  is
within  3  dB  of the other, (the telephone  company  calls  this
"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"
has *, #, A, B, C, and D.  Although the letters are not  normally
found  on consumer telephones, the IC in the phone is capable  of
generating them.

     The  * sign is usually called "star" or "asterisk."   The  #
sign,  often referred to as the "pound sign." is actually  called
an  octothorpe.  Although many phone users have never used  these
digits  -  they are not, after all, ordinarily  used  in  dialing
phone  numbers  -  they  are used  for  control  purposes,  phone
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
wait and "time out" before switching your call.

     When DTMF dials first came out they had complicated cams and
switches   for  selecting  the  digits  and  used  a   transistor
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
produce QRM.

     Standard DTMF dials will produce a tone as long as a key  is
depressed.   No  matter  how long you press,  the  tone  will  be
decoded as the appropriate digit.  The shortest duration in which
a  digit can be sent and decoded is about 100 milliseconds  (ms).
It's  pretty  difficult  to dial by hand at  such  a  speed,  but
automatic dialers can do it.  A twelve-digit long distance number
can  be  dialed by an automatic dialer in a little  more  than  a
second - about as long as it takes a pulse dial to send a  single
0 digit.

     The output level of DTMF tones from your telephone should be
between  0 and -12 dBm.  In telephones, 0 dB is 1  miliwatt  over
600  Ohms.   So 0 dB is 0.775 Volts.  Because your  telephone  is
considered  a 600 Ohm load, placing a voltmeter across  the  line
will enable you to measure the level of your tones.


The Ringer

     Simply  speaking  this  is a device that alerts  you  to  an
incoming  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,
it can be any frequency between 15 and 68 Hz.  Most of the  world
uses  frequencies  between  20 and 40 Hz.   The  voltage  at  the
subscribers  end depends upon loop length and number  of  ringers
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
phone line, be sure at least one telephone on the line is off the
hook  (in  use); if any are not, take high  voltage  precautions.
The  telephone  company may or may not remove the 48  VDC  during
ringing;  as  far  as you're concerned, this  is  not  important.
Don't take chances.

     The  ringing  cadence  - the timing of ringing  to  pause  -
varies from company to company.  In the United States the cadence
is  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
solenoid  coil  with a clapper that strikes either  a  single  or
double bell.  A gong ringer is the loudest signaling device  that
is 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
1.0  uF  capacitor.  Telephone companies in other  parts  of  the
world  use  capacitors  between  0.2  and  2.0  uF.   The   paper
capacitors of the past have been replaced almost exclusively with
capacitors  made of Mylar film.  Their voltage rating  is  always
250 Volts.

     The  capacitor  and  ringer coil, or Zeners  in  a  warbling
ringer,  constitute a resonant circuit.  When your phone is  hung
up ("on hook") the ringer is across the line; if you have  turned
off  the  ringer  you have merely silenced  the  transducer,  not
removed the circuit from the line.

     When the telephone company uses the ringer to test the line,
it  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
it  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
ring, some will cease to ring, or some ringers will ring  weakly.
In  the  United States the phone company will guarantee  to  ring
five  normal ringers.  A normal ringer is defined as  a  standard
gong  ringer  as  supplied  in  a  phone  company  standard  desk
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
phone line, you'll see the REN number.  It can be as high as 3.2,
which  means  that device consumes the equivalent  power  of  3.2
standard ringers, or 0.0, which means it consumes no current when
subjected  to  a  ringing  signal.  If  you  have  problems  with
ringing,  total  up your RENs; if the total is  greater  than  5,
disconnect ringers until your REN is at 5 or below.

     Other  countries  have various ways of expressing  REN,  and
some  systems  will handle no more than three of  their  standard
ringers.  But whatever the system, if you add extra equipment and
the  phones  stop ringing, or the phone answering  machine  won't
pick  up  calls,  the solution is disconnect  ringers  until  the
problem  is resolved. Warbling ringers tend to draw less  current
than  gong  ringers, so changing from gong  ringers  to  warbling
ringers may help you spread the sound better.

     Frequency response is the second criterion by which a ringer
is  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
gong ringer is described as REN 1.0A.  The other common frequency
response  is  known as type B.  Type B ringers  will  respond  to
signals between 15.3 and 68.0 Hz.  Warbling ringers are all  type
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
will tend to respond to transients (such as switching transients)
when the phone is hung up, or when the rotary dial is used on  an
extension phone.  This is called "bell tap" in the United States;
in  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
springs.   Warbling  ringers  for use in the  United  States  are
designed  not  to respond to short transients;  this  is  usually
accomplished  by  rectifying the AC and filtering  it  before  it
powers the IC,  then not switching on the output stage unless the
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,
including the FCC, forbid modification of anything that has  been
previously approved or attached to phone lines.

                   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, [e-mail]  N6ARE@K6VE.#SOCAL.CA.USA.NA
742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice [phone]