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Thread: I was thrown out of Best Buy

  1. #71
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    Dec 2006
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    Actually the twist makes a big difference, I had to scope those signals in college :0)

  2. #72
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    Jun 2005
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    Quote Originally Posted by NumberFive View Post
    This sort of thing really grinds my gears. Also CAT5 cable. I have several networks with tens of metres of burglar alarm cable and telephone wire (untwisted) linking them up and I still get file transfers running > 90 Mbps.
    In an 'ideal' world the twist does nothing.

    In a real world it cancels out a lot of noise induced by voltage spikes.

    If you can visualize a voltage gradient across the two wires, say, you could have a 5 volt differential between one wire and the other. Now image there was a twist in the middle of the gradient, both wires would be subject to the 5 volt differential and hence the net differential would be 0.

  3. #73
    Quote Originally Posted by carpeVideo View Post
    Actually the twist makes a big difference, I had to scope those signals in college :0)
    Yes & no.

    It depends on the cable.

    Unsheilded cat5 needs a twist.

    Sheilded phone cable doesn't.
    Alarm cable is sheilded (at least the stuff in my walls is)

  4. #74
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    Jun 2007
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    Yeah, maybe NumberFive is pretty good with electronics, you consider that?

    What most of you have eluded to is the balanced nature of the CAT5 signals, which is why the twist works, but not why the twist is there.

    CAT5 Ethernet uses balanced signals (i.e. one wire is push, the other is pull, in each pair). Any RF noise hitting the cable will ideally affect both wires in a pair equally.

    Consider that the signal is 'x' and the noise is 'n'.

    Transmitted is:

    Wire 1: + x
    Wire 2: - x

    Received is:

    Wire 1: + x + n
    Wire 2: - x - n

    Subtracting the signals cancels the noise, yielding '2x'.

    Now, this applies wheter the wires are twisted or not. Stage microphones use the same system and don't use twisted pairs.

    Twisted pairs are used so that the totally enclosed current of the pair is zero - and so the magnetic field outside the pair is also zero. This prevents crosstalk into adjacent wires. This really only matters if you have lots of ethernet cables side by side.

    Neither my telephone or burglar wire has any screen. They each have just a load of straight wires in a plain plastic coat. And it all works fine, so far to 50m.

    On another note, you can also use CAT5 cable to distribute DVB-T signals really effectively. You just need to stick a big 28dB mast-head-amplifier at the sending end. Baluns not required.

  5. #75
    Join Date
    Jan 2005
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    Gotta be careful at BestBuy these days, they're tasering their customers now.

    http://www.wftv.com/print/14898682/detail.html#
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  6. #76
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    Nov 2006
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    The police officer said she used the taser to calm the irate and upset woman down.
    Being tasered always makes me calm.

    Looking at the still frames though, it appears the officer drew the taser as she was approaching the woman and THEN she got irate. I believe I would too.

  7. #77
    Join Date
    Sep 2005
    Location
    Charlotte, NC USA
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    One time I heard a BestBuy employee tell a customer a dual layer dvd could be recorded on both sides.

  8. #78
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    Oct 2006
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    Hamilton New Zealand
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    like the old laser discs you had to turn over maybe he was getting confused

  9. #79
    Join Date
    Aug 2005
    Location
    Montreal Canada
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    I predict a class action lawsuit over cables sometime in 2008.

    During my last rush of research and shopping before I bought my TV, I overheard many instances of outright lying by big box salespeople to customers regarding cables.

    In some cases it's apparant that the salespeople really believe that there is a day and night picture difference between different quality HDMI cables. This gets to the point where salespeople outwardly express disgust with customers who hold their ground and refuse to buy high end cables.

    The Circuit City where I bought my TV had upconverting DVD players for $39.00 but carried no HDMI cable for less than $59.00. I bought one for $24.99 (still overpriced in my opinion) from Wal-Mart and it works fine.

    I actually saw a 6ft Monster HDMI cable at Future Shop for $179.00.

    This is all being run systematically from the top of levels of these big companies. It's just a matter of time before the lawyers get ahold of it.
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  10. #80
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    Quote Originally Posted by NumberFive View Post
    What most of you have eluded to is the balanced nature of the CAT5 signals, which is why the twist works, but not why the twist is there.
    Elude or allude?

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