Definition of an Analog(ue) Computer

From: <(>
Date: Wed May 6 12:03:39 1998

Allison quoted me as having written:

> <What's that got to do with it? Diodes are analogue parts - the output
> <(current) is a continuous function of the input (voltage), not a
> <discrete one (to me the difference between an analogue and a digital
> Yes, but they don't (generally) amplify.

I think you may be confusing my remark with Tony's. I remarked that
"all circuits are amplifiers" meaning that the general circuit can be
modelled as taking an input, applying some sort of gain, and producing
an output. A gross oversimplification, but I wasn't talking about the
active/passive issue (gain>1 => active, etc.) Tony made a slightly
different remark, "Digital circuits are built with analogue parts".
Unlike mine, this is not an oversimplification, it is absolutely true.
And has nothing to do with amplifiers. You can build a digital circuit
element with diodes; they are analogue parts, and they don't amplify.
Clear now?

> <component). In fact, Allison, you were saying only a few days ago that
> <you don't need any amplification to make an analogue _computer_ (with
> <which I agree - although some of your examples I wouldn't call
> <computers).
> I still hold that amplification is a factor in the equation that an analog
> function may contain but it is not required.

I never (intensionally) disputed this!

> This is an analogue function, take a shot at the equation it solves.

I couldn't quite read your diagram, I'm afraid. Was it series capacitor
followed by shunt diode? Looks like it refers the input to the lowest
value it (the input) ever takes, rather than to ground or a fixed

> <For non-electronic digital computers, where do Facit mechanical
> <calculators lie? I have one (which is driven by an electric motor but
> Computers, mechanical, fixed program.

Roughly what I thought. I wasn't sure whether people would class it as
a computer, but I think it is no less of one than a 4-function pocket


> You've not seen a modern production line that uses air logic. I've worked

No. I've seen some of the components in catalogues, though, and
wondered if they'd be of use in organ building, though!

> on one that was used to produce pharaceuticals that were in flamable bases
> (ethanol). There was some fairly complex logic in that system. Working

Sounds fun!

> with it is like designing with relays.

I can imagine.

Received on Wed May 06 1998 - 12:03:39 BST

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