A social alarm is a short-range device.
It operates on 868.20-868.25 MHz.
First we get the data using a software-defined radio. The data itself is too large to upload here.
This will lead to data like this:
You see it just shifted in time a bit (too lazy to add a delay to match them), but that they match pretty well.
The data from decoded_message.bin can subsequently be processed by the C file in /decoder. Just install clang and run make.
The hackrf uses interleaved 8-bit data. Record through something like:
hackrf_transfer -r alarm869300000.wav -f 869300000
This can be replayed with:
hackrf_transfer -d 0000000000000000088869dc2532111b -t alarm869300000.raw -f 869300000 -x 20
The file is binary (s8) format.
# hd alarm869300000.raw | head -n2
00000000 02 02 01 02 01 02 01 02 02 02 01 02 01 02 02 02 |................|
00000010 01 02 02 02 01 02 01 02 01 02 01 02 01 02 01 02 |................|
The default sample rate of the hackrf is 10MHz.
To go from these signed bytes to something gnuradio can fathom, we first split this file in pieces of 5M.
split -b5m ../alarm869300000.raw s8alarm_
We can play a few, but the one we like to have is s8alarm_at.
Use sox to create floating point values of 32-bit for gnuradio.
sox -r 10000k -c 2 -t s8 s8alarm_at -t f32 -r 500k result.f32
The 500k_alarm_recorded file when parsed contains two times the same sequence.
This might very well have been recorded at 869.2 MHz because the frequencies are slightly different and I have forgotten...
# hd 500k_alarm_recorded | head -n2
00000000 00 00 00 3c 00 00 c0 bc 00 00 80 3c 00 00 c0 bc |...<.......<....|
00000010 00 00 80 3c 00 00 c0 bc 00 00 80 3c 00 00 c0 bc |...<.......<....|
To generate the 2-FSK signal, see the ./gnuradio directory.
The result is not pretty, but the alarm is going off! Yes!!
It first didn't work because the HackRF was generating just a single "line" on the spectrum, as if it had some kind of zero bandwidth setting somewhere. I got a second one and when using this to transmit and the other one to receive, voila!
Only start with this stuff, if you do have an SDR with which you can debug your SDR. :-)