Mike Millen
Tue, 1 Jun 1999 00:27:12 -0700
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 To unsubscribe from SURVPC send a message to [EMAIL PROTECTED] with unsubscribe SURVPC in the body of the message. Also, trim this footer from any quoted replies.