

Gears of war: When mechanical analog computers ruled the waves - haomiao
http://arstechnica.com/information-technology/2014/03/gears-of-war-when-mechanical-analog-computers-ruled-the-waves/

======
Cogito
An interesting article, but it annoys me when the author talks of analog
computers having "an infinite level of resolution... because they use physical
rather than digital inputs and outputs".

Physical machines have varying levels of tolerance, and this impacts the
accuracy of calculations in just the same way as a truncated number in a
digital system reduces accuracy.

It was a lot more interesting to read about the pros and cons of digital vs
analog computers with respect to maintenance, reprogrammability, energy use,
and reliability. It's a pity there wasn't a similarly in-depth account of the
precision of each system.

~~~
jjoonathan
Yep, it's embarrassing that this got published. Did the author just forget
about mechanical tolerance? The random motion of molecules? Ambient sound
waves? Electromagnetic noise?

I suppose it's forgivable that the author didn't think of these things but
they should have run their technical article past a subject expert before
publishing it. This isn't a nitpick, they screwed up the core concept of their
article!

> there wasn't a similarly in-depth account of the precision of each system

That's because digital circuits won the "precision wars" so spectacularly that
there was no need for comparison. On a digital computer, you can just keep
adding bits to your datatype, doubling your precision with each bit, until you
are happy. Bits cost almost nothing today (and very little 30 years ago) so
you use as many as you need. If you underestimate how many you need, that's a
planning problem, not a limitation of the digital technique. Meanwhile, the
price of increasing the precision of an analog computer (mechanical,
electrical, or fluidic) shoots up dramatically after a few decimal places and
goes to infinity after a few more. That _is_ a limitation of the analog
technique.

Here's a fun demo of how the digital nature of a circuit grants it complete
noise immunity (within tolerances), unlike any analog circuit that could ever
be built:

[https://www.youtube.com/watch?v=4TCnYYpZxEc#t=2806](https://www.youtube.com/watch?v=4TCnYYpZxEc#t=2806)

~~~
agoetz
Make a circuit "digital" does not automagically make it more accurate, and you
cannot "add bits" to a datatype to make your measurement more accurate.

Before a digital circuit can process a signal, it must first be quantized, an
inherently analog process. Your digital data is only as good as your analog
signal, plus inherent quantization noise[1]. In addition, any mixed-signal IC
represents a design compromise between digital and analog constraints.

Any signal processing system is inherently limited by the SNR of its source
signal. There is no point in building a more accurate converter than the
noisiest component of your input data. In fact, when metrologists are trying
achieve the maximum accuracy in their measurements, it is not uncommon to
leave a test circuit on for days at a time, in order to reach a thermal
equilibrium to minimize the seebeck effect[2].

And when engineers to need to make the most accurate of measurements, what do
they turn to? Analog circuits. The world's most accurate voltage reference,
the Kelvin-Varley Divider, dates back to the late 19th century[3]. For a
modern example of a precision reference, see Jim William's excellent white
paper, "Quantifying Silence"[4].

[1]
[http://en.wikipedia.org/wiki/Quantization_(signal_processing...](http://en.wikipedia.org/wiki/Quantization_\(signal_processing\))

[2]
[http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/...](http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf)

[3]
[http://en.wikipedia.org/wiki/Kelvin%E2%80%93Varley_divider](http://en.wikipedia.org/wiki/Kelvin%E2%80%93Varley_divider)

[4] [http://cds.linear.com/docs/en/application-
note/an124f.pdf](http://cds.linear.com/docs/en/application-note/an124f.pdf)

~~~
aidenn0
Very inexpensive digital circuits can perform calculations on their inputs
with much higher precision than any analog calculator ever built. Weight,
volume, power, or dollars, pick your cost.

Yes, you are limited by your measurement, but you are so limited when
operating entirely within the analog domain as well. Therefore it makes sense
to make your measurements as accurate as possible, quantize once, and be done
with it. Keeping it in the analog domain merely allows more points for noise
to enter the picture.

------
NamTaf
Obligatory link to youtube video of a 1950s training video on the mechanical
fire control computers (which I stole from HN last time this sort of topic
came up):

[https://www.youtube.com/watch?v=s1i-dnAH9Y4](https://www.youtube.com/watch?v=s1i-dnAH9Y4)

edit: This video is the same footage featured throughout the article, in fact.

~~~
D9u
Ah... You beat me to it!

------
spingsprong
Some of these analogue computers are pretty amazing things.

I can kinda get my head around slow moving ships being tracked by a mechanical
computer, but the Russian ZSU-23-4 Shilka self propelled anti-aircraft
artillery vehicle used an electro mechanical gun aiming computer in the 60s
and 70s.

A mechanical computer that can track a supersonic rapidly manoeuvring jet
aircraft by radar, and adjust the aim of its guns quickly enough and
accurately enough to score hits. It's amazing what can be achieved.

------
D9u
Interestingly, several months ago I came across the following 1953 training
video dealing with this same subject.

[http://youtu.be/s1i-dnAH9Y4](http://youtu.be/s1i-dnAH9Y4)

------
nnq
off topic, but did anyone pay attention the Juniper deception force adds they
sport?

This line of one commercial just blew my mind: 'the less they know, the safer
they are' ...can a security oriented company actually get away with saying
that?!

------
mikelbring
No not about the video game.

