Subject:
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Re: A Generic Idea
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Newsgroups:
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lugnet.robotics
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Date:
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Sun, 5 Jan 2003 21:58:13 GMT
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Original-From:
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Steve Baker <[sjbaker1@airmail.net]AntiSpam[]>
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Viewed:
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954 times
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Jim Choate wrote:
> There's a caveat you miss, it is limited using the forms of modulation
> that modems use (ie FSK derived). For example ISDN uses a different sort
> of modulation (ie Q-modulation) and it does 128k over the same phone
> lines.
Yes - but when they install ISDN or DSL on your line, they switch out some
electronics at the telephone exchange - remove some line filtering, pull
out the digital part of the stream from the analog part, etc, etc.
If you just have the audio part of the phone line to play with, you can't
do better than 56Kbaud. That's why there are no modems rated above 56K
out there. Modem speeds increased every year or two for about 10 years
as the electronics got more sophisticated - but for the last six of seven
years, there has been no speed improvement over 56Kbaud - that's because
that is the absolute limit of what an analog phone line can carry.
Getting higher speeds over the same copper wires from your home/office to
the telephone exchange is possible (by switching out the electronics
at the telephone exchange) - but it's not possible to do better over an
end-to-end link because the switching electronics in the telephone system
don't permit it.
So, DSL/ISDN speeds are possible over the same wires - but NOT without
help from the phone company.
> The -real- caveat with all these is the -distance- to the switch where it
> all gets turned into digital data. -That- is what limits the modulation
> speeds - noise.
The distance does limit what DSL and ISDN can do - but for the analog voice
line, the most stringent limit by far is the filters in the telephone
exchange.
(I used to design telephone exchanges by the way)
> W/ regard to this whole discussion of 2-way wireless and Lego; use either
> 900MHz (AMD makes a chip that will do 1.45 Mb/s here, or 802.11x.
Yes - that would work well too.
In the end, this discussion boils down to these two alternatives:
___________ _______ _______ ______
RCX <=====>| | | | | | | |
| Interface |<==>| Radio |....| Radio |<==>| PC |<==> Human
Camera ===>|___________| |_______| |_______| |______|
(Brain)
...or...
___________ _______ _______
RCX <=====>| | | | | |
| Brain |<==>| Radio |....| Radio |<==> Human
Camera ===>|___________| |_______| |_______|
Putting the brain on the left of the radio link means that the bandwidth
over radio can be *small*. The robot can be truly autonomous and respond
rapidly to events.
Putting the brain to the right of the radio link mandates that full motion
video goes across the radio - which is horribly difficult and results in a
LOT of electronics in the 'interface' box.
Therefore, my contention is that the box marked 'interface' in the top
picture is hardly any simpler than the box marked 'brain' in the bottom
picture. In fact, IMHO, it would be a lot MORE complicated.
If you want to use off-the-shelf parts and have the radio link be truly
'global' then:
* To implement the second option, a PDA with an integrated camera and
cell phone would make a lot of sense...but it's not cheap.
* To implement the first option with a reasonable amount of work seems to
require a cell phone with integrated camera. It seems pretty doubtful
that you could extract the picture it captures and get it into your
PC 'brain'...but if you could, that approach might work cheaply.
---------------------------- Steve Baker -------------------------
HomeEmail: <sjbaker1@airmail.net> WorkEmail: <sjbaker@link.com>
HomePage : http://www.sjbaker.org
Projects : http://plib.sf.net http://tuxaqfh.sf.net
http://tuxkart.sf.net http://prettypoly.sf.net
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Message has 2 Replies:
 | | RE: A Generic Idea
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| Steve Baker summarized neatly: The big design issue is whether to have the brain near or far. If far, you need more communications bandwidth, but you can have a mongo brain. If near, you can minimize your investment in communications overhead, but (...) (22 years ago, 6-Jan-03, to lugnet.robotics)
| | | Re: A Generic Idea
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| (...) It's actually worse than that since 56k is only 33.6k upstream unless you have a special line. Most ISP's have these special lines giving their customers 53k/56k downstream. The 53k limit is an FCC thing imposed on older modems that was lifted (...) (22 years ago, 6-Jan-03, to lugnet.robotics)
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Message is in Reply To:
| | Re: A Generic Idea
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| (...) There's a caveat you miss, it is limited using the forms of modulation that modems use (ie FSK derived). For example ISDN uses a different sort of modulation (ie Q-modulation) and it does 128k over the same phone lines. I use ISDN to connect (...) (22 years ago, 5-Jan-03, to lugnet.robotics)
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