DTMF: Open squelch via repeater?

I am learning about DTMF. My question is, can DTMF be employed to squelch all traffic on a repeater unless the DTMF tone is heard?

For example, say a rig is set to a local repeater frequency, but I only want to hear from my friend "Carl". Can the rig be set to only only the squelch if the specified DTMF tone is transmitted?

It's possible but I don't think any current amateur or cheap Chinese radios have the capability.

You would have to buy or build an external DTMF decoder. They used to be popular with some ARES/RACES goups decades ago for member alerting without having to listen to normal repeater traffic all the time.

A few Chinese radios have 2-tone or 5-tone paging capability which can perform the same function.

oguruma said:

I am learning about DTMF. My question is, can DTMF be employed to squelch all traffic on a repeater unless the DTMF tone is heard?

For example, say a rig is set to a local repeater frequency, but I only want to hear from my friend "Carl". Can the rig be set to only only the squelch if the specified DTMF tone is transmitted?

Click to expand...

Are your confusing "DTMF" with "CTCSS" in analog FM radios?

It may be possible to configure both yours and Carl's radios to respond to DTMF. Know that not all repeaters will pass DTMF tones, and since they can be used for repeater control, you may inadvertently interfere with repeater operations. You'll probably also annoy everyone else on the channel.

What you want to do would be much easier using one of the digital modes like DMR.

The bigger problem is that many repeater controllers don't pass DTMF tones from the receiver to the transmitter. Since DTMF tones are often used to control repeaters, it's generally not a good idea to broadcast your DTMF control tones to the entire listening population. As a result, many controllers mute the transmit audio when they detect DTMF tones on the input.

Some repeater controllers can be configured with a prefix that will temporarily unmute the DTMF-mute function to allow selcal and remote control of radios or devices on the repeater output.

I have a few older Kenwood TM-261A radios that do exactly what you are describing. I see I am a bit late to the party, but the TM-261A (and I believe the 241A) have a feature called DTSS (dual-tone squelch system). It will mute all audio on the frequency until the proper set of DTMF tones are transmitted. Once the radio hears and decodes the tones, the receiver opens up to the radio traffic. The only limitation to this is that with these particular models, they are only 3 digit codes utilizing digits 0-9 and no special characters such as star or pound or A, B, C, D. You can use those special characters as a group code. If you can set one of those radios up as a receiver on your repeater system, you would be all set. There are other ways of accomplishing this as well including writing your own repeater controller software and having the repeater interfaced to the computer running your program.