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Re: [Discuss-gnuradio] Detecting unipolar and repeating preamble on wire

From: Sylvain Munaut
Subject: Re: [Discuss-gnuradio] Detecting unipolar and repeating preamble on wired communications?
Date: Tue, 5 May 2015 09:27:11 +0200


> (Bipolar: a signal swings between +A volts (or amperes) and -A volts;
> Ethernet, USB, Wi-Fi, ... are using bipolar signals,
> Unipolar: a signal swings between +A volts and 0 volts)

There is no difference ... "0 volt" is just an arbitrary reference point.

0 <-> +A swing is the same as
-A/2 <-> A/2 swing if you shift your reference ...

Ethernet and USB are _differential_ but that's just to improve signal
integrity / noise immunity and such. At the receiver end, the
differential input buffer essentially compares both and see which one
is higher. (a bit simplified, but that's the gist of it).

> Is there anyone already implemented unipolar preamble detection
> succesffully, or could anyone give me some hints on mathematical approach or
> something?

If 'A' is known, just remove -A/2 from your input signal.
If it is unknown,you can use a very long time constant low pass to
find it. (assuming your signal doesn't have long runs of 0 or 1).



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