A DTMF telephone keypad
Dual-tone multi-frequency (
DTMF)
signaling is used for
telecommunication signaling
over analog telephone lines in the voice-frequency band between
telephone handsets and other
communications devices and the
switching center. The version
of DTMF used for telephone
tone dialing is known
by the trademarked term
Touch-Tone (canceled March
13, 1984), and is standardized by
ITU-T
Recommendation
Q.23. It is also known in the UK
as
MF4.Synonyms include
multifrequency
pulsing and
multifrequency signaling.
Other
multi-frequency systems are
used for internal signaling internal within the telephone
network.
As a method of
in-band signaling,
DTMF tones were also used by
cable
television broadcasters to indicate
the start and stop times of local commercial insertion points
during station breaks for the benefit of cable companies. Until
better
out-of-band signaling
equipment was developed in the 1990s, fast, unacknowledged, and
loud DTMF tone sequences could be heard during the commercial
breaks of cable channels in the United States and elsewhere.
History
In the
time preceding the development of DTMF, telephone systems employed
a system commonly referred to as pulse
(Dial Pulse or DP in the U.S.
) or loop
disconnect (LD) signaling to dial numbers, which functions by
rapidly disconnecting and connecting the calling party's telephone line, similar to
flicking a light switch on and off. The repeated connection
and disconnection, as the
dial spins, sounds
like a series of clicks. The exchange equipment counts those clicks
or dial pulses to determine the dialed number. Loop disconnect
range was restricted by telegraphic distortion and other technical
problems, and placing calls over longer distances required either
operator assistance (operators used an earlier kind of
multi-frequency dial) or the provision of
subscriber trunk dialing
equipment.
DTMF, is a method for instructing a telephone switching system of
the telephone number to be dialed, or to issue commands to
switching systems or related telephony equipment.
The DTMF
dialing system traces its roots to a technique developed by
Bell
Labs
in the 1940s called MF (Multi-Frequency) which was
deployed within the AT&T telephone network to direct calls
between switching facilities using in-band signaling. In the
early 1960s, a derivative technique was offered by AT&T through
its
Bell System telephone companies as a
"modern" way for network customers to place calls. In AT&Ts
Compatibility Bulletin No. 105, AT&T described the product as
"a method for pushbutton signaling from customer stations using the
voice transmission path."
The consumer product was marketed by AT&T under the registered
trade name Touch-Tone. Other vendors of compatible telephone
equipment called this same system "Tone" dialing or "DTMF," or used
their own registered trade names such as the "Digitone" of Northern
Electric (now known as
Nortel
Networks).
The DTMF system uses eight different frequency signals transmitted
in pairs to represent sixteen different numbers, symbols and
letters - as detailed below.
#, *, A, B, C, and D
The engineers had envisioned phones being used to access computers,
and surveyed a number of companies to see what they would need for
this role. This led to the addition of the
number sign (#, sometimes called 'octothorpe' or
'pound' in this context) and
asterisk or
"star" (*) keys as well as a group of keys for menu selection: A,
B, C and D. In the end, the lettered keys were dropped from most
phones, and it was many years before these keys became widely used
for
vertical service codes
such as *67 in the United States and Canada to suppress
caller ID.
Public
payphones that accept credit cards
use these additional codes to send the information from the
magnetic strip.
The
U.S. military also used the
letters, relabeled, in their now defunct
Autovon phone system. Here they were used before
dialing the phone in order to give some calls priority, cutting in
over existing calls if need be. The idea was to allow important
traffic to get through every time. The levels of priority available
were Flash Override (A), Flash (B), Immediate (C), and Priority
(D), with Flash Override being the highest priority. Pressing one
of these keys gave your call priority, overriding other
conversations on the network. Pressing C, Immediate, before dialing
would make the switch first look for any free lines, and if all
lines were in use, it would disconnect any non-priority calls, and
then any priority calls. Flash Override will kick every other call
off the trunks between the origin and destination. Consequently, it
was limited to the
White House Communications
Agency.
Precedence dialing is still done on the military phone networks,
but using number combinations (Example: Entering 93 before a number
is a priority call) rather than the separate tones and the
Government
Emergency Telecommunications Service has superseded
Autovon for any civilian priority telco
access.
Present-day uses of the A, B, C and D keys on telephone networks
are few, and exclusive to network control. For example, the A key
is used on some networks to cycle through different carriers at
will (thereby listening in on calls). Their use is probably
prohibited by most carriers. The A, B, C and D tones are used in
amateur radio phone patch and repeater
operations to allow, among other uses, control of the repeater
while connected to an active phone line.
DTMF tones are also used by some cable television networks and
radio networks to signal the local cable company/network station to
insert a local advertisement or station identification. These tones
were often heard during a station ID preceding a local ad insert.
Previously, terrestrial television stations also used DTMF tones to
shut off and turn on remote transmitters.
DTMF signalling tones can also be heard at the start or end of some
VHS (Video Home System) cassette tapes.
Information on the master version of the video tape is encoded in
the DTMF tone. The encoded tone provides information to automatic
duplication machines, such as format, duration and volume levels,
in order to replicate the original video as closely as
possible.
DTMF tones are sometimes used in
caller ID
systems to transfer the caller ID information, however in the USA
only
Bell 202 modulated
FSK signaling is used to transfer the
data.
A DTMF can be heard on most
Whelen Outdoor
Warning systems.
Keypad
1209 Hz on 697 Hz to make the 1
tone
The DTMF keypad is laid out in a 4×4 matrix, with each row
representing a
low frequency, and each column representing
a
high frequency. Pressing a single key (such as '1' )
will send a
sinusoidal tone for each of
the two frequencies (697 and 1209
hertz (Hz)).
The original keypads had levers inside, so each button activated
two contacts. The multiple tones are the reason for calling the
system multifrequency. These tones are then decoded by the
switching center to determine which key was pressed.
Special tone frequencies
National telephone systems define additional tones to indicate the
status of lines, equipment, or the result of calls with special
tones. Such tones are standardized in each country and may consist
of single or multiple frequencies. Most European countries use a
single frequency, where the United States uses a dual frequency
system, presented in the following table.
The tone frequencies, as defined by the
Precise Tone Plan, are selected such that
harmonics and
intermodulation products will not cause an
unreliable signal. No frequency is a multiple of another, the
difference between any two frequencies does not equal any of the
frequencies, and the sum of any two frequencies does not equal any
of the frequencies. The frequencies were initially designed with a
ratio of 21/19, which is slightly less than a
whole tone. The frequencies may not vary
more than ±1.8% from their nominal frequency, or the switching
center will ignore the signal. The high frequencies may be the same
volume or louder as the low frequencies when sent across the line.
The loudness difference between the high and low frequencies can be
as large as 3
decibels (dB) and is referred
to as "twist." The minimum duration of the tone should be at least
70 ms, although in some countries and applications DTMF receivers
must be able to reliably detect DTMF tones as short as 45ms.
As with other
multi-frequency
receivers, DTMF was originally decoded by tuned filter banks.
Late in the 20th century most were replaced with
digital signal processors. DTMF can
be decoded using the
Goertzel
algorithm.
See also
References
- Notes
- "What are the ABCD tones?" - Tech FAQ
- Bibliography
Further reading
External links
Wikis
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