On 1999-05-31 Boanne said:
>I have always heard that "analog computers are impossible" and
>that they would be HUGE... Could you explain what the difference
>between an analog and a digital computer is, and why they would
>be impossible?
Definitely not impossible. There were many made and used when digital
computers were still less powerful than today's calculators.
Really it all depends on your definition of the word "computer" and
what you expect of the machine.
In a nutshell, digital computers tend to be programmable, i.e. they
can be loaded with software to do almost anything.
Analogue (English english :) computers are only programmable via patch
cords and switches, and even then tend to be designed for a specific
task only. In fact an analogue computer is using the word "computer"
in it's literal sense whereas a digital "computer" is a type of machine
with an agreed capability; "computer" there is just it's name.
Technically, the difference is in how the machine represents data. The
digital machine performs *very* simple math on binary numbers. An analogue
computer uses a variable voltage to represent a value, e.g. 7.635 volts
might indicate an angle of 76.35 degrees of an aileron. All the values
required to perform a particular calculation would be fed into electronic
circuits as accurate voltage levels that would then be multiplied,
divided, converted to a log or antilog, square or square-root or simple
addition or subtraction. The electronic circuit modules could perform
all these functions *directly* and the answer came out as a voltage
level again. Compared to digital machines these analogue computers were
thousands or millions of times faster, which was pretty impressive in the
50s and 60s which was their heyday. Analogue computers were normally
used to control equipment rather than provide numerical data, though
they were capable of both tasks.
My example of an aileron angle wasn't a random one either: the major
use for analogue computers was for flight simulators. All the early
flight simulators used analogue computers; digital machines didn't
exist that would be even remotely fast enough to perform all the
calculations necessary to accurately model a real aircraft in flight
and control the simulator realistically.
You've no doubt heard the term Op-Amp mentioned many times. Operational
Amplifiers were (are) one of the major building blocks of the analogue
computer; that's what they were designed for.
To sum up: whoever told you that analogue computers were impossible, in
some ways couldn't have been more wrong; to equal the capabilities of
an analogue computer the digital machine would be impossibly huge.
Maybe not true nowadays but definitely true in it's time. :)
Mike
Electron Mobility Systems
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