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
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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
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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.