I'm not sure what limited world you live in,
but clearly it's a different one than many
of the rest of us.


Cripes! Actually you could be right...since I realised there are no
square waves in my world I have lusted after a 20MHz dual storage
scope, but that's another story.

Let's say you use your 4-channel scope to conveniently display
input, output, and two intermediate stages simultaneously. I
contend that when you look at your display, you compare traces in
pairs. This is not just a limitation of your eyes or your brain:
it is because there is no actual sense in comparing more than two
things. The question "What is the difference between these X
things?" only makes sense where X = 2.

There are (X-1)*X/2 differences between X things. You can sensibly
ask "What are the differences?" and these you may evaluate by
considering each possible pair. Hence, whatever comparison you
carry out with your 4-channel scope can be done with a maximum of 6
probe swaps with a dual-channel scope.

If you remember all your early maths, like long division and
multiplication, and even adding several numbers, that was all about
breaking the process down until you could deal with numbers in
pairs.

That is how our single dialectic universe is.

Many things
can be compared only with reference to a single other thing, to
which they are compared one at a time.

And many can not.

sorry, ambiguous. Replace my "Many things" with "A plurality of
things"

Is it *necessary*? Usually not. But it
can still make life a lot easier - and
it's very educational.

Never, actually. But yes, it may be worth the money to save those 6
probe swaps if you often need to compare more than 2 traces.
Whether it is educational or not is a tricky question. If I had a
student who claimed to understand a 4-trace display, and yet could
not map this claimed understanding onto several 2-trace displays,
then I would suspect a narrow concentration span or a lack of
primary education, or both. These are by far the hardest students
to deal with, and I would start by restricting them to 2-trace
displays until they grasped the plot.

Comms network technicians seem to need dozens of traces...I suppose
the avoidance of *extreme* inconvenience almost qualifies as
necessity in the commercial world.

cheers, Ian