Topic: Cable length / delay

[boojum]

One of those useless facts that stuck with me from when I was a kid was a story on 60 Minutes. Must have been in the late sixties when I was first getting interested in astronomy. The story was about an instructor at the US Naval Academy. In one of her physics lectures a teaching aid she used was a piece of wire just under a foot long. She explained that was the distance light travelled in free space in one nano-second. Given resistence from the wire, dielectric and some things I no longer recall, she said a good approximation of electrons in a wire was 11 inches travel per nano-second. So using that rule of thumb, a ten foot difference in length would be roughly 11 nano-seconds in time. Can any of the usual editing programs deal in time on that small a scale?

<edit> Try googling "nanosecond in feet" for more info; a quick read of results, #3 seems to have the most exact answer and brings up several other points. Looks like you'll experience more delay from the other circuits in the chain than from the difference in cable lengths. </edit>

The woman you speak of was Admiral Grace Murray Hopper, the grandmother of COBOL.  Also the highest ranking woman in the history of the Navy and first Admiral.  She and her husband were math Ph. D.'s and were instrumental in building and running the Navy's GENIAC computer during WW II.  The story is that she actually used the piece of wire, I heard it was 13", to explain to other admirals why it took so long for a signal to get to a satelite.  She was a wonderful woman I learned from a few folks I knew who met her and had dealings with her.  And by inventing COBOL she made it possible for lots of Lib Arts guys like me to earn a living.   

Cheers

PS - a footnote: the length of that wire also explained why the old IBM 360-20 had trouble with some cycles being stalled while processing.  They  had to retrofit all of the 360-20's with a shorter wire in one circuit for the signal to get to it's destination and back in time for the cycle.  Otherwise there was a delay.  Obscure, yes, interesting, well, to me yes. 

[flipp]

Thanks for the name. I couldn't recall it if my life depended on it. Time to visit google and see if I can find if that wire was just under or over a foot. Like I said, it's one of those useless little factoids that sticks with you for some unknown reason.

+T

<edit a quick search didn't verify the length of the wire but it did yield a date of the 60 Minutes broadcast - March 1983, though I seem to recall seeing it as a kid, not at the age of thirty; strange how the mind stores memories>

<edit2 further searching yielded: "Grace Murray Hopper was a great thinker. One anecdote says that she had a clock in her office that ran counterclockwise; this reminded her that there was always more than one solution to a problem. As a teacher, Hopper often used a physical demonstration to lecture future computer scientists about not wasting time. Pulling out a piece of foot-long wire, Hopper explained the wire represented a nanosecond, which was "the maximum distance electricity could travel in wire in one billionth of a second" . Then she would display a coil of wire about one thousand feet long and explain that it represented a microsecond -- certainly something no programmer would want to waste." 

as well as this little gem: "After the war, Hopper continued at Harvard as faculty in the Computation Laboratory. She worked on the Mark II and III, and it was during her work there that "She traced an error in the Mark II to a moth trapped in a relay, coining the term bug"  /edit>

[BobW]

OK, OK....I've got one for you.     

How many Volkswagon Microbuses containing 2,000 2 hour DAT tapes each, and traveling at an average speed of 35 MPH are required to equal the bandwidth of a T1 data pipe?
(fractional answer to two decimal places)
(This was a final question at school for me)


Absolutely right on wire delay, delay not a factor.
(or when it becomes one, it is huge...usually in digital and all about clocking)
Think about pre-fibre analog phone calls from NY to California and the very, very small perceivable delay, if any.

[Petrus]

If the cable lengths differ something like a half a mile between a stereo pair you could start getting comb effects in the highest frequences. Which nobody would notice.

One bottleneck in supercomputers is the physical size of the units & processors. With the highest clock rates the electrons can not make it to their destination in time.

In cellular phone systems the maximun distance between phone and cell transmitter is not governed by the signal strength, but sychronizing problems. With the Europian GSM system this distance is 36 km, about 18 miles.