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From: Jay Miller (jnmiller, cryptofreak dot org)
Date: 2001.03.08 - 22.16 MST


X-Mailer: Mutt 1.2.5i

- Words by Michael Dwyer <mdwyer, timestreamtech dot com> [010308 21:21]:
> Anyway, I made a mistake -- you sample every /other/ zero crossing. 

Zis is what I have been saying!

> The first pixel is the zero-crossing between ...ahem... tits.  It is
> rising. (+) The next time it touches zero is where the phase shifts
> (actually, the whole polarity, but I digress).  The next zero crossing
> is when the signal is falling. (-).  Next is a little tit, then a
> rising signal (+).  
> 
> +-+  or 101  or 010.  

Yes, I believe this is right, but do you guys see how there are two
possibilities in the counting?  We know we're supposed to mark a 1 or a
0 when the wave *crosses zero*.  That does not include the tits
"touching zero", as it were.  Here.. here's the first four bits as it
shows up on the sheet:

     1     2     3     4
     |     |     |     |
/\ /\|     |/\ /\|   /\|
     |\/ \/|     |\/   |\/

The pipes indicate the points at which we record a bit.  Now, according
to http://wheelie.tees.ac.uk/users/a.clements/Async/async.htm, each
phase change indicates a *HIGH* signal.  See figure 4.  So that would
result in a '1110' reading of the above bits.

But, looking over at figure 5 in
http://www.rad.com/networks/1994/modems/modem.htm, we see that each
phase change indicates a reversal, ie. if signal was low, go high - and
vice versa.  On that reading, the result becomes '1010'.

See the dilemma?  Any thoughts?

-- 
Jay Miller

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