Sunday, 17 March 2013

The Not So Obvious Telephone

Yep, after practising engineering for 18 odd years, I'm still (too) often dumbfounded about how simple things work. Like the telephone. Been around since 1876, and didn't require electronics as we know it to work - just batteries, wires and coils.

The Conundrum

So how do they work?

Thanks to the internets, the answer is right there for the looking. What I found amazing was that the 'hybrid' transformer allows the one pair of cables to carry bi-directional voice, and cancel out your voice when listening to some one on the other end. And Alexander Graham Bell worked this out oh so long ago.

But even thought I knew how to use a hybrid, I never really understood it. Until I read this article!

Circuit Cellar EQ 264

How mad is this – you can bidirectionally share signals, even analogue, on the one pair. So simple, I’m feeling pretty dumb for never realising this.

Figure 1:  Differential Line LTSpice Model

To help get the workings of this understood in my mind, I fired up LT Spice and gave it a quick simulation. The trick here is that to receive what is being sent, you need to look at the *difference* between what you are sending, and what the other guy is sending.

Adding Spice

Figure 2 below shows the two input signals I used, 1 at 5kHz (blue trace), the other at 10kHz (green trace).

Figure 2:  Input Singals

If you were to measure the signal on the 'line' (the wire connecting the two sides of the circuit) with respect to ground, you'd get what is shown in Figure 3 below.

Figure 3:  Mixed Signals

You see both wave-forms superimposed on each other - if they were voice signals and not sine waves like I'm using in my simulations, and you hooked these up to a speaker, you would here your voice (echo) and the other voice at the same time.

Not very useful.

But, if you subtract your signal from the combined signal on the line, the difference is the signal sent from the other side. Cool.

To do this in LT Spice, it's dead easy - just enter a mathematical equation, and you're done.

If you right click on the simulation plane and select 'Add Trace' you can select 'Add Traces to Plot' and you can select multiple signals and various mathematical operations (add, subtract, multiply, divide etc).

Figure 4:  Adding Traces

I've also added a new plot plane to my chart below.  The bottom plot shows the input signals and the mixed 'line' signal. The top plot shows the result of subtracting the line voltage (mixed signals) from the node I've labelled Rx2.

Figure 5:  Output!

If you look at the scales you will see that the Y axis of the top plot is only half as large as the bottom.  The output signal is only half of the input signal - this is a consequence of the resistor networks used.  But you now have all the *information* of the signal transmitted from the opposite side isolated from what you are trying to transmit.

In Bell's day they used a specially wound transformer, known as the hybrid, to do the subtracting for you (taking advantage of the fact that out of phase magnetic signals will cancel). Smart.  To do this today with electronics you just need an opamp, and this is what I've simulated below.  I've added a differential amplifier with nominal unity gain - but as it's not a ideal opamp in my simulation it';s not quite unity gain, but it proves the point.

Figure 6:  Modern Electric Magic

Figure 7: Voice Out!

If you want to have a play with this yourself, you can grab my LT Spice files from here:

Differential Line


So at the end of the day, it's not magic, but it is pretty amazing how you can use one pair of wires to convey multiple sets of information.

If you want to see how Bell wound his Hybrid, I think these links are excellent:
Old School Heathkit
Lundahl Hybrid

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