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. Signal decoders used in communication systems, such as telephony and two-way radio systems, detect communication protocol states by recognizing a variety of electrical, optical, or acoustic conditions. Misinterpretation of those conditions leads to communication errors. Proper detection of signaling is a compromise between acceptable error rates and cost of implementation. The engineering problem is to produce the simplest circuit that works 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. Examples of decoder falsing include: a telephone answering machine detects dial pulses from a rotary dial as ringing voltage, with the result that the answering machine answers in response to dialing. a two-way radio with an enabled CTCSS 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 ringy telephone circuit with SF single-frequency signaling 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.