Re: History Lesson: 600 ohm balanced line


"Soundhaspriority" <nowhere@xxxxxxxxxxx> wrote in message
news:ELWdnbAlZoKUU4TXnZ2dnUVZ_uqdnZ2d@xxxxxxxxxxxxxxx

"Chris Hornbeck" <chrishornbeckremovethis@xxxxxxx> wrote in message
news:dqhj159p5975eotavhcve83m9a53742ifs@xxxxxxxxxx
On Sun, 24 May 2009 17:56:34 -0400, "Soundhaspriority"
<nowhere@xxxxxxxxxxx> wrote:

Good point, it's "ladder line", http://en.wikipedia.org/wiki/Ladder_line
.

But since telephony audio was never transmitted over ladder line, it
seems
they picked 600 ohms as a standard audio impedance simply because it was
a
number they knew.

Two conductors in free air with an occasional spacer at a coupla
inches is about 600 Ohms.

It wasn't so much arbitrarily chosen as mandated by fundamentals.
Kinda like the World is 73 Ohms, but different!


Much thanks, as always,
Chris Hornbeck
----------------------------------------------------------------------------
OK, correct me on this. Way back when, did they actually transmit long
distance audio telephony over ladder line? Perhaps they did, before they
developed effective insulation.

Bob Morein
(310) 237-6511

Yes I have a gray beard, Nobody here remembers when telephone poles had
cross arms with green or clear glass insulators, yes open wire pairs, multi
pair cable hadn't been perfected yet, the last few places that used open
wire were the railroads for signals and com. the phone company started
frequency multiplexing on open wire lines 12 channels on 2 pairs just in
time for WWII. we have come a long way in a short time, not everybody had
telephones back then usually the rich and the Doctor and the sheriff, had 8
party lines too. Very few trunk lines between cities, it would take hours
to set up a long distance call, some of the little companies didn't talk to
each other at all. It wasn't exactly ladder line but the wires were
stretched tight enough they never crossed and stayed equidistant,
occasionally there would be a mid span cross-over device to minimize
crosstalk on the multiplex. I started with Bell Tel in 1970, my observation,
new technology is built on old, you have a system of standards in place that
works and everyone has in common. All of our program audio ckts were 600
ohms, all of the test sets were 600, 900, or 1200 ohm impedance, for 19 or
22 or 24 or 26 gauge wire. When I was in the Navy I noticed the military had
an affinity for 500 ohm ckts ?? another convention. For Bell it was all
defined in the Bell System Practice, It said what and how and how often, if
you didn't follow the practice you could get a day off or fired. 0 dbm at 1
kHz into 600 ohms = 1 milliwat, I'm not going to complain it works for me.
Best regards,
David____________


.

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