telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...

, single-frequency signaling (SF or SF tone) is

line signaling Line signaling is a class of telecommunications signaling protocols. Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called ...

in which dial pulses or supervisory signals are conveyed by a single-frequency tone in each direction in the voice-band. SF and similar systems were used in 20th-century

carrier system A carrier system is a transmission system that transmission (telecommunications), transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the ...

s. An SF signaling unit converts DC signaling (usually, at least in

long-distance Long distance or Long-distance may refer to: *Long-distance calling *Long-distance operator *Long-distance relationship * Long-distance train *Long-distance anchor pylon, see dead-end tower Footpaths *Long-distance trail *European long-dista ...

circuits, E&M signaling) to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for

transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...

over an AC path, ''e.g.'', a carrier system. The SF tone is present in the

on-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing ...

or idle state and absent during the seized state. In the seized state, dial pulses are conveyed by bursts of SF tone, corresponding to the interruptions in dc continuity created by a

rotary dial A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone numb ...

or other DC dialing mechanism. The SF tone may occupy a small portion of the

user Ancient Egyptian roles * User (ancient Egyptian official), an ancient Egyptian nomarch (governor) of the Eighth Dynasty * Useramen, an ancient Egyptian vizier also called "User" Other uses * User (computing), a person (or software) using an ...

data channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for informa ...

spectrum, ''e.g.,'' 1600 Hz or

2600 Hz 2600 hertz (2600 Hz) is a frequency in hertz (cycles per second) that was used in telecommunication signaling in mid-20th century long-distance telephone networks using carrier systems. Tone signaling carrier systems operated in the sta ...

(SF "

in-band signaling In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even o ...

)". There may be a

notch filter In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the inverse of a ''band-pass filter''. A notch filter is ...

at the precise SF

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

, either filtering the circuit at all times or only when the circuit is

off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing ...

, to prevent the user from inadvertently disconnecting a

call Call or Calls may refer to: Arts, entertainment, and media Games * Call (poker), a bet matching an opponent's * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage Music and dance * Call (band), from L ...

if the users voice has a sufficiently strong spectral content at the SF frequency, a

falsing In telecommunications, falsing is a signaling error condition when a signal decoder detects a valid input although the implied protocol function was not intended. This is also known as a false decode. Other forms are referred to as talk-off. Sig ...

condition known as ''talk-off''. Notoriously, this property was exploited by

blue box A blue box is an Electronics, electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voi ...

ers and other toll fraudsters. The SF tone may also be just outside the user voice band, ''e.g.,'' 3600 Hz. The

Defense Data Network The Defense Data Network (DDN) was a computer networking effort of the United States Department of Defense from 1983 through 1995. It was based on ARPANET technology. History As an experiment, from 1971 to 1977, the Worldwide Military Command a ...

(DDN) transmitted DC line signaling pulses or supervisory signals, or both, over carrier channels or

cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...

pairs on a

four-wire circuit In telecommunications, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its ...

basis using a 2600 Hz

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

tone. The conversion into tones, or vice versa, is done by SF signal units. SF was developed in the early 20th century and standardized in middle century. In-Band Single-Frequency Signaling
by A. Weaver and N. A. Newell, Bell System Technical Journal, June 7, 1954 It declined in the 1970s due to the adoption of

T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell Syst ...

, and was largely abandoned late in the century in favor of common-channel signaling. SF tones can transmit data that only consists of everyday

telephone numbers A telephone number is the address of a telecommunication endpoint, such as a telephone, in a telephone network, such as the public switched telephone network (PSTN). A telephone number typically consists of a sequence of digits, but histori ...

, numbers which only have 0-9 in them. How many pulses are transmitted in one time is part of the number that is dialed. One pulse is 1, two pulses are 2, three pulses are 3 and so on, with 10 pulses being 0.

References

* {{telecommunications Telephony signals