Homebrew Tone Decoder?

[aaknitt]

This post stems from a PM conversation a couple of us have been having. We're discussing what might be a good way to make a fairly inexpensive, versatile two-tone decoder from scratch. I know there are two-tone decode boards out there that aren't very expensive to start with (from Midian, for example), but I've been tossing around the idea of building my own. What are some ways that this could be done?

The cheapest way would probably be a single chip solution using a microcontoller that's powerful enough to do some DSP (FFT, DFT, etc.). That would be extremely versatile, and that's probably what the commercially available boards do.

I'm only so-so with software, though, so that would take me some time to figure out. I was thinking that I could use an LM567 tone decoder IC interfaced to a small microcontroller. A digital potentiometer could be used to change the decode frequency of the LM567 on the fly. That would be a three-chip solution (LM567, digital pot, micro), but should be pretty versatile and easy from a software perspective.

Another option would be to convert the input signal to a square wave and count pulses, but that might be susceptible to falsing. It would take quite a bit of testing and tweaking to get that to work.

Any other ideas?

Andy

[spareparts]

This post stems from a PM conversation a couple of us have been having. We're discussing what might be a good way to make a fairly inexpensive, versatile two-tone decoder from scratch. I know there are two-tone decode boards out there that aren't very expensive to start with (from Midian, for example), but I've been tossing around the idea of building my own. What are some ways that this could be done?

If all you are doing it 2 tone, take a look at http://www.national.com/pf/LM/LMC567.html. Have to ask, is this for a life safety application, IE alerting firemen or setting off a siren?

[aaknitt]

Have to ask, is this for a life safety application, IE alerting firemen or setting off a siren?

No, just for scanner listening, similar to the functionality that's in the new Uniden scanners. Something for people that already have a scanner but don't want to spend the money on a true alert receiver like an Informer or Minitor.

Andy

[spareparts]

No, just for scanner listening, similar to the functionality that's in the new Uniden scanners.

OK - Just wanted to make sure. I'm a big fan of the folks to build it themselves, but sometimes it's just not good enough where life, limb, and property are at risk.

[aaknitt]

Here's what I ended up trying:

http://andyknitt.googlepages.com/twotonepagingdecoder

It works pretty well on the bench...I haven't tried it with a radio and probably won't anytime soon. I've got a spare Minitor III laying around so I don't have a whole lot of use for this circuit right now, but it might come in handy in the future.

Andy

[Asst17-1]

That looks like good stuff.

One application I can think of is we have a lot of old MII pagers that some members have at home to use as a 1-channel scanner. They work great except they do not have PL tone capibility. It looks like we could use the lm567 to add PL tones.

Not exactly two tone but its an idea.

[aaknitt]

Yeah that should work. I know LM567s have been used to make PL encoders (http://www.rason.org/Projects/impdecod/impdecod.htm), and would think that they could be used as decoders also, as long as you're careful about the filter bandwidth.

Andy

[EEnerd]

What about a microcontroller doing the whole thing? If you want to change the decoded frequency map you can add a DIP switch or some kind of power-up button-press and LED flash mode or something. This seems like it could all be done in a single uP.

[aaknitt]

I agree. I'm just not good enough with uP signal processing to do it in my spare time.

Andy

[Jonathan KC8RYW]

What about the Mitel 8870?