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

From: Mark Borgerson (m-a-r-k_at_oes.to)
Date: 10/17/04 

Date: Sun, 17 Oct 2004 21:12:20 GMT

In article <41717ED0.3ECEEB97@yahoo.com>, cbfalconer@yahoo.com says...
> Mark Borgerson wrote:
> >
> ... snip ...
> >
> > I've managed to get 230KB async from a UART through a kilometer
> > of intercom wire (simulating an oceanographic cable) by putting
> > out full cycles of pseudo sine waves for 1 bits and nothing at all
> > for zero bits. The signals were transformer coupled for impedance
> > matching and DC isolated so the cable could be used for HV power.
>
> You should be able to go a lot further and faster with balanced
> pairs, provided you avoid DC effects. The basic driver is a
> differential pair, with a current source 2I in the emitter
> circuit. The normal current mirroring methods can be used to
> generate currents I from the positive rail into the collector
> circuits of that pair, so that each output line now switches a net
> +-I. There is only one switch, avoiding nasty crossover
> distortions, which can make the receiver get slowly varying signals
> just at its most sensitive point. The DC component of injected
> signal is zero over the differential line pair. Other methods can
> ensure the average on each half of the line is also at zero net DC.

I'll look into that idea if I work with long cables again and have
actual twisted pairs to work with. I was looking at some of
the techniques used for DSL communications. I would love to
find a pair of DSL modems that accepted serial from the microcontroller
and produced DSL signals for coupling to the cable. Didn't seem to
be anything workable when I was looking about three years ago.
>
> The drive is then high impedance, so the line conditions depend
> solely on the line termination. In particular any biases set for
> receivers are set for the overall line, in just one place, with
> current injection. Most cables have well defined impedances, in
> particular almost all twisted pair telephone cable looks like 100
> ohms or so at 1 Mhz up. With the above configuration the single
> line is a buss, and you can fairly freely add transmitters and
> receivers, provided you can compensate for the varying point to
> point propagation times. There are ways to handle this. One is by
> defining a transmission direction and adding a master clock line.
> Now all you have to do is put all transmitters to the left of all
> receivers, assuming direction is left to right. No tuning.

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.

Mark Borgerson

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