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#1 2012-01-16 20:17:23

adam@algroup.co.uk
Contributor
From: UK
Registered: 2009-05-01
Posts: 203
Website

Mystery modulation scheme

Hi all...

I'm playing with a system that's using  a modulation scheme that I don't recognise and wondered if anyone's seen it before... It looks almost like manchester, but when you dig a bit deeper it's clear the timings aren't right for that. In manchester you should be able to measure only two pulse widths, one being exactly double the size of the other, but in this case we see long pulses of 1.5 times...  The grid here is set to 40 and the large pulse is measuring 60...

Mystery modulation scheme

If you want to load and play with the data direct, it's here:

http://www.rfidiot.org/proxmark-test.pm3

Cheers!
Adam

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#2 2012-01-16 21:05:54

rule
Administrator
Registered: 2008-05-21
Posts: 416

Re: Mystery modulation scheme

Hey Adam,

Can you give some more info, I assume you are looking into LF samples?

You are sure it is not the Binary puls length modulation (BPLM) used on the reader side of the hitag protocols?
This one has a 100% ASK modulation where the 'one' is 1.5 times the length of a 'zero'.

If you want more details about this, you know how too contact me wink

Cheers,

  Roel

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#3 2012-01-16 21:24:15

adam@algroup.co.uk
Contributor
From: UK
Registered: 2009-05-01
Posts: 203
Website

Re: Mystery modulation scheme

OK, cards on the table... It's an RF system, not RFID/NFC, but I loaded the trace into PM3 because it's a nice environment to play with such things... smile

I've tried decoding it as per your suggestion, but no joy... Any other ideas?

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#4 2012-01-16 21:46:19

rule
Administrator
Registered: 2008-05-21
Posts: 416

Re: Mystery modulation scheme

So you could be looking at a BPSK modulation maybe? More info also available on wiki.

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#5 2012-01-16 22:48:43

adam@algroup.co.uk
Contributor
From: UK
Registered: 2009-05-01
Posts: 203
Website

Re: Mystery modulation scheme

Hmmm... Yep, I thought it might be some kind of PSK, but haven't tried that yet... Trying it now, thanks!

Last edited by adam@algroup.co.uk (2012-01-16 23:14:35)

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#6 2012-01-17 10:06:30

adam@algroup.co.uk
Contributor
From: UK
Registered: 2009-05-01
Posts: 203
Website

Re: Mystery modulation scheme

OK, for those that are interested, it turned out it *was* manchester, just with strange timings!

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#7 2012-01-17 11:51:27

moebius
Contributor
Registered: 2011-03-10
Posts: 206

Re: Mystery modulation scheme

Sielox tags?

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