Falsing

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.

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 SelCall, Quik Call I, MF, or DTMF, pairs of decoders are used and their outputs are connected to logical and circuitry. 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.

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