New Models - MotoTRBO
New RXC-3080-Trbo with Virtual DTMF signaling works with Motorola MotoTRBO portable and mobile radios in true digital mode.
DTMF Tx Level Adjustments
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.
Adjust Transmit Levels Correctly.
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.
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?
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.