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Analog Computers

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Re: Analog Computers

#81

Post by kybernetikos »

JKCalhoun wrote: post #70
RachelF wrote: post #63

Earlier quoted context omitted.

Analog (or analogue) computers have their uses, however, in my experience, with analogue electronic computers, they have many problems: - limited dynamic range of perhaps 30dB (1000) - it is easy to saturate a signal (there is no overflow bit) - oscillations are easy to induce, but once again hard to detect, especially in a circuit in the middle of a calculation chain - noise gets amplified across the system

Reminded me of A.K. Dewdney's "Computer Recreations" column in "Scientific American" back in the day (I think a lot of people may have hated on his column since it followed in the shadow of Martin Gardener's infamous "Mathematical Recreations"). One of the "analog computers" he mentioned was using dried spaghetti to sort numbers — where the length of each spaghetti noodle represented the magnitude of a number. Of cou…

> One thing fascinating by "analog computers" is the way they seem to be practically instantaneous regardless of n. That is perhaps part of the efficiency reflected in the article.

If this were true, it would have profound implications. It's probably not true.

http://www.scottaaronson.com/papers/npcomplete.pdf

Re: Analog Computers

#82

Post by [deleted] »

alimw wrote: post #78
ramgorur wrote: post #13

It's strange that the article does not mention anything about hydraulic macroeconomics and MONIAC, they were once widely used to verify theories in economics. https://en.wikipedia.org/wiki/MONIAC https://en.wikipedia.org/wiki/Hydraulic_macroeconomics

So this looks interesting, but my first thought is how can a conserved quantity like water model something like money that is created and destroyed?

[deleted]

Re: Analog Computers

#83

Post by kybernetikos »

alimw wrote: post #78
ramgorur wrote: post #13

It's strange that the article does not mention anything about hydraulic macroeconomics and MONIAC, they were once widely used to verify theories in economics. https://en.wikipedia.org/wiki/MONIAC https://en.wikipedia.org/wiki/Hydraulic_macroeconomics

So this looks interesting, but my first thought is how can a conserved quantity like water model something like money that is created and destroyed?

I don't think there's any requirement that the amount of water in the model remains constant. In principle you could drain or open valves to add more.

Re: Analog Computers

#84

Post by pjc50 »

cr0sh wrote: post #16

A good summary of analog computers can be found on the wikipedia article: https://en.wikipedia.org/wiki/Analog_computer Takeaway: Analog computers are limited in precision and by "analog noise"; the precision of the components used determine the precision of the output. Usually no more than 3 or 4 decimal places are possible, at least with the tech that was used in their heydey. I would say that is still close to the…

The high point of analogue computing for control systems may have been Concorde. It normally operated in fly-by-wire through an analogue interconnection: the so-called synchro/resolver system, which is a AC servo control system. The flight computers (mostly but not entirely analogue) provided autothrottle and autostabilisation.

("Somewhere" in the Concorde megathread is a description of its analogue computers: http://www.pprune.org/tech-log/423988-concorde-question.html )

Re: Analog Computers

#85

Post by rhaps0dy »

kybernetikos wrote: post #81
JKCalhoun wrote: post #70

Earlier quoted context omitted.

Reminded me of A.K. Dewdney's "Computer Recreations" column in "Scientific American" back in the day (I think a lot of people may have hated on his column since it followed in the shadow of Martin Gardener's infamous "Mathematical Recreations"). One of the "analog computers" he mentioned was using dried spaghetti to sort numbers — where the length of each spaghetti noodle represented the magnitude of a number. Of cou…

> One thing fascinating by "analog computers" is the way they seem to be practically instantaneous regardless of n. That is perhaps part of the efficiency reflected in the article. If this were true, it would have profound implications. It's probably not true. http://www.scottaaronson.com/papers/npcomplete.pdf

It's "true". The catch is that the analog computer's circuit size (which is analog to the digital computer's time) has to scale with n.

Re: Analog Computers

#86

Post by pjc50 »

kurthr wrote: post #60
flavio81 wrote: post #33

Earlier quoted context omitted.

Good question. I guess precision is finally going to be fractions of the max voltage swing allowed by the computer. For example, if voltage goes from -5 to +5 volts, the voltage swing is 10v, and if noise allows 0.1mV of precision, then the precision is 1/100000 of the full voltage swing. This could be expressed, at the end, simply in decibels, though. Signal-to-noise, as in classic analog systems.

I think it usually refers to accuracy out of a range of 1. typically 3 decimal places means 1000ppm and 4dp means 100ppm. The typical problems with analog computers are many... precision of components (e.g. gain or attenuation) is limited to ~0.1% for resistors and ~1% for capacitors (inductors aren't typically used). You can try to tune things (ratiometrically) to get higher accuracy, but at the cost of increased no…

> ~0.1% for resistors and ~1% for capacitors

Don't forget the temperature compensation! Then there's irreducible noise like Johnson noise. As you say, the best use is in (properly stabilised) feedback systems which seek to minimise a difference.

Re: Analog Computers

#87

Post by pjc50 »

JKCalhoun wrote: post #73
adrianN wrote: post #72

Earlier quoted context omitted.

If you're for example an archer you can "calculate" angles and velocities to a pretty high precision.

I agree, but I don't think it is "calculated" in decimal places (if the makes sense). Sort of like how slide rules didn't give you "decimal precision".

You're right that the normal measure of precision for analogue systems is either a percentage error or a signal-to-noise ratio in dB.

Re: Analog Computers

#88

Post by VLM »

kybernetikos wrote: post #81
JKCalhoun wrote: post #70

Earlier quoted context omitted.

Reminded me of A.K. Dewdney's "Computer Recreations" column in "Scientific American" back in the day (I think a lot of people may have hated on his column since it followed in the shadow of Martin Gardener's infamous "Mathematical Recreations"). One of the "analog computers" he mentioned was using dried spaghetti to sort numbers — where the length of each spaghetti noodle represented the magnitude of a number. Of cou…

> One thing fascinating by "analog computers" is the way they seem to be practically instantaneous regardless of n. That is perhaps part of the efficiency reflected in the article. If this were true, it would have profound implications. It's probably not true. http://www.scottaaronson.com/papers/npcomplete.pdf

His soap bubble analogy is pretty cool. There is a direct analogy for filtering where its impossible to get instantaneous perfect response out of a filter, either DSP simulated or hardware. It would seem that merely filtering a signal is much simpler than simulating the airflow across a wing or whatever.

There are also conceptual issues that the airflow across a wing is ideally under stable low angles of attack, constant, but during "fun times" which is precisely when you'd want a model instead of a test pilot, the air flow will vary over time (to the general detriment of flying ability...) so what it means to instantly solve a wing is unclear in itself.

Re: Analog Computers

#89

Post by pmontra »

JKCalhoun wrote: post #70
RachelF wrote: post #63

Earlier quoted context omitted.

Analog (or analogue) computers have their uses, however, in my experience, with analogue electronic computers, they have many problems: - limited dynamic range of perhaps 30dB (1000) - it is easy to saturate a signal (there is no overflow bit) - oscillations are easy to induce, but once again hard to detect, especially in a circuit in the middle of a calculation chain - noise gets amplified across the system

Reminded me of A.K. Dewdney's "Computer Recreations" column in "Scientific American" back in the day (I think a lot of people may have hated on his column since it followed in the shadow of Martin Gardener's infamous "Mathematical Recreations"). One of the "analog computers" he mentioned was using dried spaghetti to sort numbers — where the length of each spaghetti noodle represented the magnitude of a number. Of cou…

I remember that article. This is Spaghetti Sort from Wikipedia https://en.m.wikipedia.org/wiki/Spaghetti_sort

Re: Analog Computers

#90

Post by VLM »

kurthr wrote: post #60
flavio81 wrote: post #33

Earlier quoted context omitted.

Good question. I guess precision is finally going to be fractions of the max voltage swing allowed by the computer. For example, if voltage goes from -5 to +5 volts, the voltage swing is 10v, and if noise allows 0.1mV of precision, then the precision is 1/100000 of the full voltage swing. This could be expressed, at the end, simply in decibels, though. Signal-to-noise, as in classic analog systems.

I think it usually refers to accuracy out of a range of 1. typically 3 decimal places means 1000ppm and 4dp means 100ppm. The typical problems with analog computers are many... precision of components (e.g. gain or attenuation) is limited to ~0.1% for resistors and ~1% for capacitors (inductors aren't typically used). You can try to tune things (ratiometrically) to get higher accuracy, but at the cost of increased no…

A big technical EE problem for analog computers is interconnects and their EMI/EMC interference issues and impedance issues. The analog specs for on-chip digital circuitry are much more relaxing to develop around. You can work around the interconnect issues on analog computers by dumping lots of power into the driver and input circuits but eventually some joker is going to point out that it would be electrically cheaper (in terms of current/power draw, etc) to transmit that 0 to 5 volt signal using something like I2C or SPI and then you're on a fast slippery slope to turning your analog computer into an exercise in DSP programming. At some point of complexity the interconnect cable driver circuitry is going to be power hungry enough that its cheaper to emulate the whole thing in floating point on a digital computer.

If you make a graph of PITA vs bit resolution, we're all pretty comfortable emulating digital computers on analog real world circuits using binary ones and zeros. Surely the gain is very little and the PITA increases very much by implementing digital computers on trinary + - 0 analog computers. Some think the graph is U shaped and at some resolution level, the PITA of analog high resolution falls beneath performance so it makes sense. Many like me think that graph never U shapes such that anything is "better" at emulating digital computers than using analog physical computers based on binary 0/1. AFAIK no one has built a modern floating point accelerator using opamps and A/D and D/A converters, so I find it unlikely its useful.

A two transistor NAND gate is after all just a analog computer using simple binary signals. All computers are analog its just the popular digital ones are only defined and well behaved when using binary analog signals.

There is some audiophile effect going on. Surely a mp3 codec running on a vacuum tube opamp would sound more mellow and all that.

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