Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!emory!hubcap!dawill
From: dawill@hubcap.clemson.edu (david williams)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Apple SCSI not compatible with standard SCSI?
Message-ID: <7418@hubcap.clemson.edu>
Date: 13 Dec 89 19:00:53 GMT
References: <24105@cup.portal.com> <15450001@hpdml93.HP.COM> <593@bmers58.UUCP> <8910@cbmvax.UUCP>
Organization: Clemson University, Clemson, SC
Lines: 41

In article <8910@cbmvax.UUCP>, valentin@cbmvax.UUCP (Valentin Pepelea) writes:
> In article <7388@hubcap.clemson.edu> dawill@hubcap.clemson.edu (david williams) writes:
> >
> >    Since I seem to have started this, I'll answer your question:  Differential
> >drive is a technique of sending signals to eliminate noise.  You use two wires
> >for each signal.  One wire carries the normal signal, and the other wire carries
> >the signal after being inverted.  At the recieving end, you invert the signal
> >that was inverted at the transmiter, and add them together.  Any noise on the
> >line cancels out (since each line gets the same amount of noise placed on it
> >during the traversal of the cable) and you end up with a clean signal.
> 
> Not quite. Adding the inversion of the inverted signal to the positive signal
> will result in the doubling of the amplitude of the signal. However, the noise
> is in no way eliminated. It too in fact increseas in magnitude, but does not
> double because it is random by nature. That means that some times you get
> spikes which are as high as double in usual magnitude, and sometimes you get
> lucky ang get a cancel out.
> 
> Visualise this as adding two sinusoidal waves of different magnitude together.
> Do they cancel each other out? What is the magnitude of the resultant wave?
> 

   Um.  You seem to be missing one vital point here:  most differential
cables have the inverted and non-inverted wire pairs twisted.  According
to my physics book, this means both wires get AN EQUAL AMOUNT OF NOISE
ON THEM.  So, when we invert the - signal, the noise is inverted and cancels.
It dosen't matter how random it is, as long as the amount of noise on the
+ and - lines are the same, it goes away at the reciever.  Don't believe me?
Come down to WSBF in Clemson, and I'll talk to you over a 1200 foot piece of
twisted pair cable sitting next to a large 60Hz transforer and a HAM radio
transmitter.  Funny, I don't hear any noise.  (Each end of my cable is 
driven by a SHURE remote broadcasting mixer.)
    Most stuff in audio differential uses a signal that is 1/2 of the desired
signal on each wire, resulting in a unit signal at the reciever.

         -------------------------------------------------------
 /'''''\      Fake sig file - at a CTS terminal
| o   o |         Dave Williams      dawill@hubcap.clemson.edu
|   m   |             "Huh?  What?  Could you repeat the question?"
| \___/ |
 \__U__/