SX1276 and DTMF tones

[D-Alexandr]

Dear RadioLib Team,

I am using your library to work with the SX1276 radio module. I wanted to share my interest and inquire about the capabilities of your library regarding working with DTMF tones through the SX1276 module.

My current project involves receiving DTMF signals through the radio module, and I am curious about the possibility of working with these signals within the context of your library. Specifically, I would like to know if there is a way to receive any tone signal and then work with them.

Any information you can provide about the capabilities or recommendations regarding this matter would be extremely helpful for my work.

I appreciate your work on this library and the time you dedicate to responding to user inquiries. If there is any additional information or details that would be helpful to address this question, please let me know.

[jgromes]

Hi, RadioLib team here.

I don't think it's going to be very easy. Digital transceivers like SX127x are just not meant to be used like this. In the past, I was able to get them to transmit simple tones to be used by RTTY or SSTV. However, so far it has all been single tones. That's because the "tones" are generated by feeding a square wave to the SX127x. The transceiver can only take a digital input, that's why it needs to be a square wave. As you can see, that approach will not work once we need to transmit two tones at the same time.

There might still be a way to achieve this, though I have no idea if this would actually work. I was able to playback a WAV file via SX1278 and my SDR was able to receive it, so maybe DTMF would also work ...? I used the TMRpcm library (https://github.com/TMRh20/TMRpcm/tree/master), though unfortunately I don't have the source code anymore. I do have a video though: https://www.youtube.com/watch?v=FAT3iFEyLvE

While transmission could maybe perhaps be possible, I don't think reception is possible even in theory. I have no idea how you would get the tones from the digital/square wave output. There's an active issue #569 for APRS reception, which is order of magnitude easier in my opinion (because you are only dealing with single tone at a time), and I still wasn't able to do it.

[D-Alexandr]

God mode is the next one in the plans😏 Can I somehow see what preamble code comes to me and then set it?

[jgromes]

Preamble has no "code", it's just a 101010... sequence. In the example I linked above, the program waits for a 16-bit long preamble and then a 2-byte sync word because that's the expected packet structure. In your case, you can wait for just the preamble e.g. by calling radio.setDirectSyncWord(0x5555, 16); or whatever lenbgth is appropriate for your bit rate and expected tone length.

[D-Alexandr]

Hi, I finally achieved a great result using your example for channel activity detection. I'm using the preamble as a trigger for the tone burst frequency used by many ham radio. At the moment, I believe this is the maximum achievable for tone reception, so my issue is probably resolved. Thank you for your help!

[jgromes]

CAD is intended to detect LoRa preambles, if it works then that's likely an accident, probably just because the signal is very strong, and you won't be able to differentiate between different tone frequencies. Seems it would be more reliable to use direct reception ...

[D-Alexandr]

I tested direct reception at different bit rate and different bandwidths, but the module was still unable to detect the tone(1, 1.75, 2 kHz)