Re: Recording digital data to analog tape... revisited

From: CBFalconer (cbfalconer_at_yahoo.com)
Date: 10/18/04 

Date: Mon, 18 Oct 2004 10:44:08 GMT

Mark Borgerson wrote:
>
... snip ...
>
> My application was for an oceanographic cable, which can be quite
> different from a telephone cable. In particular, I had to cope
> with the fact that one of the wires might have about 300V DC
> to ground (or the other wire). I've also heard, but not been
> able to verify, that the cable characteristics change as the cable
> is reeled out into the ocean. I do know that we had be be able
> to cope with slip ring noise also--which was handled with ACK/NAK
> and packet repeats.

That sounds like moving power on the same copper. My approach
might be to use the common mode of a twisted pair to supply one of
the power potentials (including ground). Thus, if a 100 ohm line
is terminated by two 50 ohm resistors at each end, they look like
25 ohms per end to the common mode supply. The remote end can use
DC/DC converters to trade volts for amps. Redundant signal paths
mean redundant power paths with means of detecting partial
failures.

Changes in line characteristics look like taps on the line. The
system should be fairly immune to these when both ends are
terminated with a reasonable match, provided the taps are
reasonably high impedance. Obviously a short circuit doesn't let
anything past it.

You can even apply TDR (time domain reflectrometry) to the
installed line, and decide exactly what you have. You can then
compensate for its deficiencies or tell the maintenance people
where to look. 

-- 
 "I support the Red Sox and any team that beats the Yankees"
 
 "Any baby snookums can be a Yankee fan, it takes real moral
  fiber to be a Red Sox fan"

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