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I'm examining the signal sent by my car's fob. Using an RTL-STR and GNU Radio Companion, I've been able to capture and demodulate the signal.

Now I'm working on decoding the signal. What I'm not sure about is whether the signal is just FSK (frequency shift keying) or if it is FSK with Manchester coding.

Is there a way to tell if a signal is Manchester coded?

Image of the waveform captured after demodulation:

enter image description here

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    \$\begingroup\$ Not counting the very beginning, this could be a Manchester encoded signal. A Manchester encoded signal will be high or low for either 1/2 a data bit period or a full data bit period. This signal, ignoring the very beginning, meets that criteria. Note that Manchester encoding, by itself, does not specify any data framing. Sometimes a modified Manchester coding technique is used to help with framing. This could be what is going on at the very beginning, or maybe not. In any case, understanding the framing will be necessary. \$\endgroup\$
    – Mattman944
    Commented Jun 5, 2019 at 16:55
  • \$\begingroup\$ FOB is an abbreviation and stands for "free on board" and applies to having things sent by ship. "Fob" is a little dingus that hangs on your (pocket) watch chain or your key chain. \$\endgroup\$
    – JRE
    Commented Jun 5, 2019 at 17:09
  • \$\begingroup\$ @Mattman944 Which variation of Manchester coding would you suppose this is? G.E. Thomas or IEEE 802.3. \$\endgroup\$
    – vini_i
    Commented Jun 5, 2019 at 17:12

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Yes it's Bi-Phase (Mark, Space or Invert) aka Manchester code which has this typical 1T,2T pattern with a preamble of 1T for clock freq and phase sync, where T= 1/2 bit. The jitter tolerance depends on quality of clock stability and sampling or integration method.

FSK alternates f1,f2 for many cycles and the ratio of Δf/fbr (bitrate) gives a deviation ratio that improves conversion from SNR above CNR.

Microchip implementation

It certainly looks like "Manchester code"
enter image description here

This can be decoded with a 1.5T One-shot ( = 3/4 clock cycle time) and D FF and XOR gate IC on a breadboard or in software. I made my 1st one in '76.

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  • \$\begingroup\$ Did you want to make a breadboard decoder or use a software solution that is on the web? Or just try from paper i.sstatic.net/TxOnU.jpg or use a breadboard to decode Bi-ph Mark \$\endgroup\$
    – D.A.S.
    Commented Jun 5, 2019 at 17:32

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