DTMF is something we use regularly ...

Foundations of Amateur Radio

Ingenuity is the name of the game in Amateur Radio, building, inventing, solving and helping are all part and parcel of this hobby. We like to lay claim to being the source of all that is good in the world, all that was invented came from Amateur Radio first, right?

Seriously though, sometimes we pick up a technology along the way from other places.

If you've ever picked up your microphone and pushed one or more buttons on it whilst the push to talk button was down, you've likely used this technology that's set out in an ITU recommendation called Q.23. It has the quaint title of: "Technical Features of Push-Button Telephone Sets". It's a brief document as such, all of four pages, two title pages and one mostly dealing with why this Push-Button idea is a great one and how it relates to international phone calls etc.

The meat is in the final page, showing eight frequencies and how you combine them to generate voice frequency signals.

If you've been paying attention, you might recognise this as DTMF or Dual Tone Multiple Frequency signalling.

It's pretty nifty. Send two discrete frequencies at the same time across some link and decode it at the other end. It's nifty because these frequencies might happen during a normal conversation, but not at the same time for a particular duration.

As Amateurs we use this to communicate with our repeaters, to send signals to it, to activate links, to power on and off stuff and all manner of other interesting things.

So, how does it actually work?

Well, you have two sets of four non-overlapping frequencies which you can combine into 16 different combinations, enough for 10 digits, four letters and two symbols. To make this work, the frequencies must be pretty stable, the ITU recommends less than 1.8% off the nominal frequency and distortion must be 20 dB below the fundamental frequencies.

Today producing such a thing is trivial, a chip for a dollar will do the job and another one at the other end to decode it. Four bucks and you're good to go for two-way DTMF at both ends. Bargain.

Being the curios type I went looking to find out what a DTMF circuit might look like before we could buy such products. The closest I came was a build-your-own voice mail system in BYTE! magazine of April 1982 using LM567 tone decoders, but a quick look at the box shows that these are also something that we'd call an Integrated Circuit.

I'm going out on a limb here, since DTMF has been around since it was first supplied to customers in 1963 and suggest that the original DTMF decoders were not quite as trivial as a dollar chip. They likely contained many discrete components including eight separate filters and ways to combine them so signals could be added to each other to detect the existence or absence of a specific tone, but I've yet to actually lay eyes on anything more fundamental than the tone decoders.

That being said, you can connect your all-in-one dollar chip, the CM-8870, to something like an Arduino and do your own decoding of DTMF signals.

Seems that the 1982 BYTE! magazine article was just the beginning of the revitalisation of DTMF, robot controllers, home automation, in-vehicle signalling and more, not to mention, using it to activate IRLP and other wonderful radio services.

Before you start sending me email about this non-Amateur invention, I'll point out that Amateurs also didn't invent copper wire, that was two Scots, or was it Dutch, I forget, fighting over a coin.

I'm Onno VK6FLAB