DTMF signals can be sent over copper wire, FM radio, microwave systems and any other medium which can handle normal audio. DTMF is very forgiving and adaptable, but there are a few guidelines which should be observed when sending the signals.
DTMF has great hearing.
DTMF decoders use active technology to seek out the DTMF tone pairs. This means you don’t need to hit them over the head with excessively loud audio. In fact most decoders function much better with a lower audio levels. 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. That is less than 5% of maximum levels and the decoders operated just fine.
Adjust Transmit Levels Correctly.
To make audio adjustments to a transmitter, an expensive meter called a “Communications Signal Monitor” is used to send a calibrated tone. You then adjust the radios “deviation” (loudness control) while observing the test equipment until the adjustment meets the desired setting.
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. When set at this level, the audio is transmitted at just below the maximum set by the FCC.
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. As the two tones combine and mix, they will exceed the output level of any individual tone. So a radio transmitter adjusted to 4.5 KHz with a single tone may exceed the maximum 5 KHz channel.
This issue is easy to avoid by either two methods. First, you can simply adjust the deviation of the radio to a lower level. Secondly, you can lower the DTMF audio input to the radio. Lower audio in is just like speaking further from a microphone. It is less loud, and is less likely to exceed the maximum channel output.
The DTMF “2/3” Rule of Thumb.
An easy way to remember the best settings for sending DTMF signals is to remember that the signal should never exceed “2/3” of the maximum channel deviation.
If your using a 5 KHz wideband channel, this would mean you would set the DTMF output to 3.3 KHz or less.
On a 2.5 KHz narrowband channel, the deviation would be set to 1.65 KHz or less.
Pre-Emphesis or Not?
Encoders and transmitters can each have a circuit called “Pre-emphesis”. The purpose of pre-emphesis is to increase the level of higher frequency audio tones to overcome a natural tendency of wires and amplifiers to degrade higher tones more than lower tones. In the industry, this imbalance is referred to as “Roll-Off” since the higher audio tones are rolled-off faster than the lower tones.
The whole purpose of pre-emphesis is to have all the tones, no matter what their frequency, be exactly equal in loudness.
Pre-emphesis may be necessary on your radio system, or it may not. It just depends upon the equipment and the audio signal pathway. If you experience decoding issues, be sure to check to make sure if you need (or don’t need) pre-emphesis turned on.