External radio, direct modulation "module"

[ManoDaSilva]

It would be interesting to create a "module" that would use the direct transmission mode (for any 2-FSK mode) to feed it into an external analog FM radio (such as what is done for most of high power POCSAG transmitters, or for the MMDVM) :)

[jgromes]

I'm not sure what exactly you're referring to. Direct mode in the context of sub-GHz transceivers like SX1278 takes clock and data as digital signals on two pins and transmits that without any packetization. I don't think that's something an analog FM radio can deal with. In addition, direct mode is used for some hacks in RadioLib (to transmit AFSK signals by applying square wave tone to the data input).

Do you mean to output just the tone from AFSK on some pin?

[ManoDaSilva]

I got the idea when dissecting the inner workings of your POCSAG implementation:
If you look into unipager, which is a software that turns a Rpi + Analog radio into a POCSAG transmitter, it uses the audio card to generate a square wave tone directly fed into the FM modulator input to create the required 2-FSK (some adjustment on the audio level is required to get the appropriate deviation).

That's why I think it could be interesting to see if we could do the same with your library, and create a module that would just use the packetization you implemented. I'll play around with it next week to see if I can send the DIO0(?) data signal straight into an FM radio.

(AFSK should also be doable I guess)

[jgromes]

Actually, AFSK is the only one that can be implemented in this way. If you take a carefull look at the implementation of transmitDirect for various modules, you will find out it actually accesses the device registers to enable unmodulated carrier output on some radio frequency - you will find nothing on the DIO pins. So what happens is that this frequency is changed by RadioLib during transmission, and so it's the library, not the transceiver that does the modulation in this case.

AFSK is different, since it actually outputs signal to the DIO2 pin, and the carrier is modulated by the transceiver. It is going to work with everything that implements AFSK (Morse, RTTY, SSTV etc.), but not for Pagers, since those only use 2-FSK.

[jgromes]

@ManoDaSilva FYI I added support for external radios, see https://github.com/jgromes/RadioLib/tree/master/examples/AFSK/AFSK_External_Radio

[ManoDaSilva]

Hi Sorry for the delay! That's really nice! Now we can use the full library with a nice 25W radio :)

Regarding 2-FSK, isn't there a way to toggle a pin instead of changing registers, and redirect that to whatever external transmitter we're using? Would that imply some timing issues?

[jgromes]

How exactly would you determine the frequency at which to toggle the pin?

An example: POCSAG specifies frequency deviation of +- 4.5 kHz between the 1s and 0s. What should be the freequencies the pin toggles at?

[ManoDaSilva]

The FSK deviation (when driving an FM radio via a packet/unfiltered input) is adjusted by varying the amplitude of the input signal, not its frequency... The toggling "speed" will dictate the baud rate :)

[jgromes]

Varying the amplitude is somewhat challenging on a digital output pin. That would effectively limit this to Arudino boards with DAC. Effectively, the only thing that can be changed here is the frequency, hence why I think this will remain limited to AFSK. If your radio can take audio input, that is.

[ManoDaSilva]

I was seeing something like the digital output being fed to either a simple amp or a resistor divider (depending on whether it needs amplification or attenuation) to bring it to the appropriate level for the desired deviation, it does require some additional components, but I believe it's still quite simple to set up. Once I've got some free time, I'll give it a go to see how it does.

[ManoDaSilva]

Okay, 7 months later, I came back to this issue as I'm still very interested in using this lib for directly driving my radios for 2-FSK :)

Granted I still need to adjust the level after the output pin via a simple potentiometer.

I tried to implement it using the ExternalRadio Protocol, and modding Pager.cpp to handle driving a pin directly instead of changing the registers. Most of the tweaks were about making sure it made no calls to the Physical Layer for frequency settings, deviation, etc. I also added the pin toggling function where the registry access was.

My implementation is very hacky, but I managed to get a square signal that looks exactly like the signal I usually feed into my radio using Unipager, when transmitting a simple "Tone" message.
Weirdly enough, when transmitting a BCD or ASCII message, my ATmeg328 resets. I wonder if it has anything to do with memory running out, I'll try it on an ESP32 to confirm my doubts.

[jgromes]

Seems like that should be plenty to run the program, so I'm guessing there's sometihing wrong with memory mangement. Can you post your Arduino sketch?

[ManoDaSilva]

RadioLib_DummyTx.txt
(Had to rename my Arduino Sketch)

I also modified Pager.cpp and .h to implement handling I/Os. Tried to implement the External Radio management by checking the _RADIOLIB_EXTERNAL_RADIO_H define, but it doesn't seem to be handled by the preprocessor when declared in ExternalRadio.h, so I had to force it in Pager.cpp for now

Pager.h.txt
Pager.cpp.txt

[jgromes]

I don't see anything immediately obvious that could be causing a crash, so I'll investigate a bit more.

Regarding the external radio, a better approach would be to implement the transmitDirect method in the ExternalRadio class. I'll do that and let you know.

[jgromes]

@ManoDaSilva I found and fixed a memory leak in the BCH encoder, everything is now stable on ATmega328P. In addition, I have also implemented transmitDirect, so now you can do the following:

#include <RadioLib.h>

// output on pin 4
ExternalRadio radio(4);
PagerClient pager(&radio);

void setup() {
  pager.begin(434.0, 1200);
}

I examined the output with an oscilloscope and can clearly see the preamble followed by the frame synchronization word. One thing I noticed is that the waveform is inverted compared to the binary data, so if it doesn't work the first time I suggest checking the inversion setting.

[ManoDaSilva]

Good copy! I will try it this week :)

Thank you for the hard work!