Signal Preprocessing

GNU Radio Companion

GNU Radio can be used to preprocess and display the signal sent to rtl_433. This can be helpful when process IQ samples from a HackRF. The HackRF has a minimum sample rate (bandwidth) of 2 Msps and has a significant DC offset spike, which seems to cause rtl_433 to fail to detect weak signals. In this case, it is beneficial to preprocess the signal before sending it into rtl_433. NU Radio can be used for this. The trick is to create a named pipe and set GNU Radio to output to that pipe. Then, use the -r option in rtl_433 to read from that named pipe. Because of the behavior of a named pipe, rtl_433 won't read off the end of the pipe and exit when finished like it would when reading from a file. Instead, it will block as it waits for more samples from GNU radio. Moreover, no data is saved to disk with this method. The details for how to achieve this are as follows:

1. Create a named pipe as follows. Note that the name is important. The rtl_433 program will use this name to detect the sample rate and bandwidth. See "File names" for details on how this works. In this example, we use a sample rate of 500 ksps and a center frequency of 433.95 Mhz, with 16-bit signed interleaved IQ samples (.cs16).

   mkfifo 001_433.95Mhz_500.0ksps.cs16

2. Start rtl_433 with the required options. It seems that -Y autolevel is necessary for this method as the noise floor can change significantly depending on how the gain is configured in the GNU Radio. In this example, we also add -M time:iso to get an absolute timestamp (instead of a relative count of the number of seconds since the program started).

   rtl_433 -r 001_433.95Mhz_500.0ksps.cs16 -Y autolevel -M time:iso

3. Add a Complex To IShort block and set the scale factor to 32767 (as the docs suggest). This will interleave the IQ samples as 16-bit signed integers, so that we match the file format we specified above. Now, add a File Sink. Set the file name to match the name of the pipe created with mkfifo above and set the input type to "short".

4. Last, start the flowgraph and rtl_433 should begin displaying output (showing that the noise level is being adjusted). If a signal is received, the output data will be displayed in the terminal window running rtl_433.

Note: This method has only been tested on Linux.

Example Flowgraph

Setting all of this up every time you want to use GNU Radio can be a pain, especially if you adjust the center frequency or bandwidth often - both of which require changing the file name. The example GNU Radio companion flowgraph below automatically creates the correctly-named FIFO (named pipe) in the /tmp/ directory and launches rtl_433 with the parameters needed to pull from that pipe. On exit, it cleans up the named pipe. This means you can change the center frequency or bandwidth and just click the "run" button in GNU Radio companion to start the flowgraph. The flowgraph is available here: rtl_433_connect.grc

A screenshot of this flowgraph running from the terminal is shown below.