Falsing
Encyclopedia
In telecommunications, falsing describes a decoder
assuming that it is detecting a valid input when one is not present. This is also known as a false decode. To make the concepts simpler, this article will discuss analog circuits used before digital signal processing.
Examples of decoder falsing include:
Analog tone decoders used in telephone
and two-way radio
systems are designed to work in a balance between expensive, complicated filtering and low cost simplicity. The engineering problem is to make the simplest circuit that will work reliably. A decoder generally tries to filter audio input to strip off every audio component except a sought-after, specific tone. In part, a decoder is a narrow bandpass filter. A signal that gets through the narrow filter is rectified into a DC
voltage which is used to switch something on or off.
Falsing sometimes occurs on a voice circuit when a human voice hits the exact pitch to which the tone decoder is tuned, a condition called talk-off.
For the tone decoder to work reliably, the audio input level must be in the linear range of audio stages, (undistorted). A 1,500 Hz tone fed into an amplifier that distorts the tone could produce a harmonic
at 3,000 Hz, falsely triggering a decoder that is tuned to 3,000 Hz.
, Quik Call I, MF
, or DTMF, pairs of decoders are used and their outputs are connected to logical and
cicuitry. When tones are decoded, they are submitted to the and logic. If both are decoded at the same time, the and logic output shows a decoded pair of tones is present. To false in a dual-tone system, both decoders would have to false at the same moment. Two-out-of-five code
and similar methods provide an additional check in some applications.
Another method is to subject tone decoding to a time constraint. In the case of Quik Call I or a string of DTMF digits, the falsing would have to occur in the exact order required to actuate the decoder. This is much more improbable than falsing in a single tone decoder. In two-tone sequential, tone and voice radio paging, decoders are actuated by decoding one tone and then a second in the proper sequence. Falsing could only occur if the two decoders falsed in a valid sequence within the decoder's time constraints. Examples of systems of two-tone sequential paging are Motorola
Quik Call II, General Electric
Mobile Radio Type 99, and later-model Plectron receivers.
Decoder
A decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode...
assuming that it is detecting a valid input when one is not present. This is also known as a false decode. To make the concepts simpler, this article will discuss analog circuits used before digital signal processing.
Examples of decoder falsing include:
- a telephoneTelephoneThe telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
answering machineAnswering machineThe answering machine or message machine, also known as the telephone answering machine in the UK and some Commonwealth countries) and previously known as an ansaphone, ansafone, or telephone answering device is a device for answering telephones and recording callers' messages.Unlike voicemail,...
detecting dial pulses from a rotary dialRotary dialThe rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...
as ringing voltage. The result being that the answering machine answers in response to dialing. - a two-way radio with an enabled CTCSSCTCSSIn telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes called tone squelch...
decoder turns on the receive audio for one or two syllables of a signal with a close-in-tone-frequency, (but wrong), CTCSS tone. The person listening to the radio occasionally hears nonsense partial words from the receiver's speaker: "et"... "up"... - a ringyRinging (signal)In electronics, signal processing, and video, ringing is unwanted oscillation of a signal, particularly in the step response...
telephone circuit with SF single-frequency signalingSingle-frequency signalingSingle-frequency signaling is line signaling in which dial pulses or supervisory signals are conveyed by a single voice-frequency tone in each direction...
and poor level discipline drops calls because it sees harmonic frequencies or the distorted waveform as a valid "circuit idle" or "on-hook" SF signal. - power line transients cause a telemetry decoder to momentarily decode the power line noise as a false "turn-on" command, causing a remote-controlled water well pump to cycle on and off needlessly.
Analog tone decoders used in telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
and two-way radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
systems are designed to work in a balance between expensive, complicated filtering and low cost simplicity. The engineering problem is to make the simplest circuit that will work reliably. A decoder generally tries to filter audio input to strip off every audio component except a sought-after, specific tone. In part, a decoder is a narrow bandpass filter. A signal that gets through the narrow filter is rectified into a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltage which is used to switch something on or off.
Falsing sometimes occurs on a voice circuit when a human voice hits the exact pitch to which the tone decoder is tuned, a condition called talk-off.
For the tone decoder to work reliably, the audio input level must be in the linear range of audio stages, (undistorted). A 1,500 Hz tone fed into an amplifier that distorts the tone could produce a harmonic
Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
at 3,000 Hz, falsely triggering a decoder that is tuned to 3,000 Hz.
Reducing the possibility of falsing
Some systems that use tone signaling require higher reliability and less probability of falsing. One method of reducing falsing uses formats with simultaneous, paired tones. In decoding dual tones such as ICAO's SelCallSelcall
Selcall is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones...
, Quik Call I, MF
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
, or DTMF, pairs of decoders are used and their outputs are connected to logical and
Logical conjunction
In logic and mathematics, a two-place logical operator and, also known as logical conjunction, results in true if both of its operands are true, otherwise the value of false....
cicuitry. When tones are decoded, they are submitted to the and logic. If both are decoded at the same time, the and logic output shows a decoded pair of tones is present. To false in a dual-tone system, both decoders would have to false at the same moment. Two-out-of-five code
Two-out-of-five code
In telecommunication, a two-out-of-five code is an m of n code that provides exactly ten possible combinations, and thus is popular for representing decimal digits using five bits...
and similar methods provide an additional check in some applications.
Another method is to subject tone decoding to a time constraint. In the case of Quik Call I or a string of DTMF digits, the falsing would have to occur in the exact order required to actuate the decoder. This is much more improbable than falsing in a single tone decoder. In two-tone sequential, tone and voice radio paging, decoders are actuated by decoding one tone and then a second in the proper sequence. Falsing could only occur if the two decoders falsed in a valid sequence within the decoder's time constraints. Examples of systems of two-tone sequential paging are Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
Quik Call II, General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
Mobile Radio Type 99, and later-model Plectron receivers.