Saturday, October 4, 2014

Studying the TPMS signals from Subaru a.k.a. Schrader signals

1) Observing the car at standstill produced no signals. Even after driving.

Data collection: So far windows and HDSDR have been used. SDR# is not available to me, Neither a portable coputer with gnuradio. Recordings produced files that ended like this: 433789kHz_RF.wav Stereosignals in Audacity with something that looked like quadrature detection. Is this the I/Q?

2) While driving, an some signals were registered, approx one every 14 seconds.

The first part of the signal is constant, the second part varies.  48 bits. 8 first bits a short pulse. The second part is closer to the resonance frequency. Driving on, I never manage to observe this signal again. Maybe a local, exterior signal?

3) After some fiddeling with parameters, some other signal are observed.

I first processed these data with a low pass filter, then a normalization. Not wise, a repeating signal is also observed without the initial low pass filter:

Eight groups of packets, separated by 100 ms.

The packets are not yet analyzed, could this signal shape indicate FSK? Seems to be three different pulse lengths in the packet.  Like Jared Boon is taking about here.

Screendump from HDSDR:


  1. Hi,

    Have you managed to decode the bits yet? Or found a way to receive the signals with standalone RX?

    I've just aquired one of those sensors from ebay, and received it's data with SDR#. Next step is to try receiving data with RFM69 module, but it looks like I'm the only one trying this route.

  2. Hi,

    No, I have not looked at the problem since then. I will try gnuradio next summer, when the tires with the sensors are on again.

    I think I concluded that I need a FSK transmitter to recreate the signals. That RFM69 looks like a really versatile reciever.

  3. Hi Dephiox,

    I have some progress, thanks to the guys behind the rtl_433 software.

    Check out the thread I started here!topic/rtl_433/MIW40yGrUiA

    My sensor is Schraeder FCC ID: MRX C4W4MA4 433.92MHz

    1. Thanks for the info. I just got a TX and RX of that RFM69 modules you mentioned. I will surely check your thread.