# Capture and analyze devices

# TL;DR

Capture sample data with -S unknown. Note down the expected measurement values from a read-out or head unit. Check the spectrogram by dropping samples on https://triq.org/iqs/ (opens new window) (it should look "busy" like this (opens new window)) Try analyzing each sample with rtl_433 -A gfile.cu8 to see if there is some real data. Use the analyzer hints to create a plausible -X decoder and demod the data codes. Then upload some zipped samples to an issue and post a description and tabled codes and values per sample file.

# Verify a transmission

rtl_433 processes radio data in multiple stages. You can follow the stages and verify the data at each point.

First a radio data packet is found and framed.

Get on overview of the band. Check if the transmission is visible and in the expected frequency range. Use CubicSDR, Gqrx, SigDigger, SDR#, SDRangel or similar SDR UIs to verify you receice a signal. If you have the SDR reciever on a headless machine try rtl_tcp to transport data to a GUI.

TIP

A quick substitute for an SDR UI is to record a sample, e.g. -w file_433.92M_250k.cu -T 60 (adjust for the actual frequency and sample rate). Now drop that .cu8 sample file on https://triq.org/pdv/ (opens new window) to visually inspect the spectrogram (a sideways view of the common SDR waterfalls).

WARNING

Do not plug the receiver directly in a USB port, avoid noise and use a short usb cable.

# Grab a sample

Note the frequency, pick a frequency a little off, e.g 50k above or below. Then grab the signal with rtl_433, e.g. rtl_433 -f 433.92M -S unknown Visually verify the samples in https://triq.net/iqs (opens new window)

TIP

The modes for the sample grabber are

  • -S all: grab all frames found
  • -S unknown: grab frames that are not decoded by any decoder
  • -S known: grab frames successfully decoded by some decoder

The band covered is equal to the sample rate. At the default 433.92M and 250k sample rate that's 433.67 MHz to 434.17 MHz. For the 868M default sample rate of 1024k that's 867.5 MHz to 868.5 MHz. For the 868M it's like good to pick 868.3M for a band of 867.8 MHz to 868.8 MHz.

To get a clean signal remove the receiver antenna and place the device at 10cm to the receiver, that mostly isolates the transmissions.

# Analyze the data packet

Then next stage is demodulation of OOK or FSK data. A run of pulse/gap (OOK) or mark/space (FSK) timings is generated by the demod. Run rtl_433 -A SAMPLE.cu8 to get an overview of the timings, or rtl_433 -w OOK:- SAMPLE.cu8 to see the raw data. Write the pulses to a file with rtl_433 -w SAMPLE.ook SAMPLE.cu8 and visualize the file with https://triq.net/pdv (opens new window)

WARNING

You need to give the sample rate if it's not 250k, look at the file name, e.g. use rtl_433 -s 1000k -A SAMPLE_1000k.cu8

For advanced analysis you can also try out SigRok's Pulseview with rtl_433 -W out.sr SAMPLE.cu8.

Be sure to also try with higher sensitivity: -Y autolevel -Y magest -M noise -M level

Try different sample rates, for 433M try -s 1024k, for 868M try -s 250k or -s 2048k.

Try different demods, for 433M try -Y minmax, for 868M try -Y classic.

# Build a flex decoder

Now build a flex decoder to slice the data into bits. Use the suggestion or make a guess based on the analyzed pulse data on the coding.

# Document data codes

The last stage is the protocol decoding from the bit data. Build a table of codes and the expected sensor values to identify where the bytes are and what is contained. Preferably put the codes and annotations in a BitBench (opens new window).

# Example commands

  • capture samples not decoded by rtl_433 rtl_433 -S unknown
  • capture samples of every received frame rtl_433 -S all
  • anaylze a capture to get an overview of the timings rtl_433 -A SAMPLE.cu8
  • show the raw data pulse data from a captured sample rtl_433 -w OOK:- SAMPLE.cu8
  • convert pulse data from a capture to OOK file rtl_433 -w SAMPLE.ook SAMPLE.cu8
  • try to read codes from a captured sample rtl_433 -X '...' SAMPLE.cu8
  • open a captured sample in SigRok Pulseview rtl_433 -W SAMPLE.sr SAMPLE.cu8