So a radio transmitter adjusted to 4.5 KHz with a single tone may exceed the maximum 5 KHz channel. As the two tones combine and mix, they will exceed the output level of any individual tone. When making the adjustments for DTMF, the technician must remember that DTMF signals are comprised of not just a single tone, but of two tones begin generated at the same time. When set at this level, the audio is transmitted at just below the maximum set by the FCC. The calibrated tone is normally 1000 Hz, and most service technicians set the level for two-way radios to 4.5 KHz on a wideband channel which has a maximum allowable setting of 5 KHz deviation. You then adjust the radios “deviation” (loudness control) while observing the test equipment until the adjustment meets the desired setting. To make audio adjustments to a transmitter, an expensive meter called a “Communications Signal Monitor” is used to send a calibrated tone. That is less than 5% of maximum levels and the decoders operated just fine. While not recommended, more than once we’ve seen DTMF decoders responding to levels less than 200 Hz deviation on a channel with a 5 KHz maximum. In fact most decoders function much better with a lower audio levels. This means you don’t need to hit them over the head with excessively loud audio. DTMF is very forgiving and adaptable, but there are a few guidelines which should be observed when sending the signals.ĭTMF decoders use active technology to seek out the DTMF tone pairs.
DTMF signals can be sent over copper wire, FM radio, microwave systems and any other medium which can handle normal audio.
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