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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.
Click on one of the items below to learn
more.
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