5 ms·

Analog computation, Part 1: What and why

75 points by hk__2 8y ago

adamnemecek 8y ago
They will come back in the form of continuous variable quantum computers.
- jacobush 8y ago
  Distinct quanta ... variable?
mariefred 8y ago
Maybe I'm old but that's common knowledge, the reason more and more computations are moving to the digital world is weirdly the simplicity of implementation and not accuracy, speed, price, reliability or anything else super sophisticated. It's much easier, faster, maintainable and usually cheaper to add a cheap controller and peripherals than to find an electrical engineer to do the math and design a circuit.
- Zenst 8y ago
  Same reason why the majority of CPU's are synchronous instead of asynchronous, cheaper to design by far.
- setquk 8y ago
  Actually it's usually better in the digital domain for all of those reasons. Most systems these days have minimal transducer processing stuff in the analogue domain and get it into the digital one as quickly as possible. Analogue systems have a lot of non-ideal characteristics such as drift, thermal sensitivity, parasitic capacitance and inductance, impedance matching problems, insane cumulative errors and manual tuning requirements that don't exist in the digital domain. They are also more expensive to modify, and the price goes up significantly with complexity and speed. Not to mention small and fast is really difficult in the analogue domain thanks to everything being a transmission line.
  - jacquesm 8y ago
    > insane cumulative errors Any sufficiently advanced analog computer is indistinguishable from a noise generator.
- hopler 8y ago
  TFA explains the nuances of the trade-off. For certain domains (simple calculations at extremely high frequency with tolerance for approximation), analog is a good choice.
- SomeHacker44 8y ago
  I fly a plane with an old analog computer autopilot/flight director. It works perfectly well. Its failure modes are well understood and easy to diagnose. The boards and components can be easily troubleshot and repaired by competent electricians. Modern autopilots are all sensors, DACs, ADCs, and various digital computing resources. Cheaper, faster, more featured, and mystifying in their failure modes. Although I am a big digital person, even for photography, audio and other seemingly analog things - I admire and appreciate the one major analog computer in my life. Alas, after 7 years, it finally needs a repair. Sigh.
  - darkmighty 8y ago
    Does it have some kind of built-in redundancy? And how are you aware that it needs repair?
    - SomeHacker44 8y ago
      (It is broken because:) When engaged, the altitude hold function commands a 10 degree pitch up attitude in all cases. This is a reasonably extreme pitch. Even the built in “take off/go around” mode usually commands only about 7-8 degrees nose up. The altitude is “held” using a “static” pressure sensor and pitch is adjusted in a control and feedback loop to attempt to keep that signal constant. When it works, it is very smooth. The system is not as a whole redundant, but there are many failsafes. You can put it into a mode where the system does not fly the plane but tells you what it thinks you should do. This is called a flight director. You fly it almost like a video game, get the pointers aligned. Each component has some redundancy. It has a self test, an annunciator panel saying what mode it is in, a mode control panel, several controls on the yoke including a kill switch, several circuit breakers (one, the electric trim, which has a big red flange on it - see the recent 737 crash artickes), etc. The workflow of using the autopilot is always command a change, check that the change is reflected on the annunciators and that the plane is adjusting attitude and such appropriately, then resume the normal scan. If at any time anything looks off my instant reaction is always to hit the big red button and take the plane’s control back. When you do this you need to be wary for an out of trim condition and other sudden forces on the yoke and rudder.
Tade0 8y ago
I still have a few operational transconductance amplifiers lying around which I tried to use for my guitar effects projects(none of them ever reached completion of course). Not obvious to work with, but a great solution if you want to multiply two signals.
- atoav 8y ago
  Other ways to do this: - Vactrols (a LED/LDR combination) - Blackmer VCA Chips (e.g THAT2181 or SSM2164) - Analog Multiplier ICs (these are actually four-quadrant multipliers), (e.g. AD 633) - PWM switching - ...
bogomipz 8y ago
The FAQ states: >"Q: What are some of the issues associated with analog computation? A: They are similar to the ones that are of concern whenever analog functions (op amps and more) are used for signal processing or signal conditioning. These include linearity, offset, temperature-related drift, aging, and bandwidth." I understand bandwidth but could someone elaborate in lay terms what these other "concerns" are in analog circuits? Specifically what are linearity and drift?
- detaro 8y ago
  Linearity: you expect a circuit to e.g. always multiply an input with 2x. Turns out it doesn't, it does so close to e.g. 0, but multiplies by less or more at higher values. Somewhat generalized, for other components with other curves of behavior, they also don't stick perfectly to the mathematical model, especially when the environment changes. Which leads us to Drift: temperature changes, behavior of component changes (e.g. resistors change slightly in resistance, circuits to generate precise voltages produce slightly different voltages, oscillators change in frequency). E.g. precision frequency sources often have a heating element to keep their core oscillator at a stable frequency, audio devices thermally couple two components of which the ratio is important, so the changes cancel out as much as possible, ...
- pjc50 8y ago
  Linearity: if a system is linear you can draw a nice straight line on an X-Y plot of input vs output. Nonlinearity is a measure of how much it deviates from that line. Drift: component values are not fixed! Most vary slightly with temperature, and capacitors change value over time too. Can be seen in any kind of old equipment which needs to "warm up".
  - bogomipz 8y ago
    Thanks for the wonderful and detailed responses. I really appreciate it. One follow up - is it accurate to say that linearity and drift are always related then?
    - 8y ago
audiometry 8y ago
Omega Tau podcast has quite a long episode on analog computers.
- adyavanapalli 8y ago
  Episode link: http://omegataupodcast.net/159-analog-computers/ http://omegataupodcast.net/159-analog-computers/

75 points by hk__2 8y ago

adamnemecek 8y ago

They will come back in the form of continuous variable quantum computers.

jacobush 8y ago
Distinct quanta ... variable?

mariefred 8y ago

Maybe I'm old but that's common knowledge, the reason more and more computations are moving to the digital world is weirdly the simplicity of implementation and not accuracy, speed, price, reliability or anything else super sophisticated. It's much easier, faster, maintainable and usually cheaper to add a cheap controller and peripherals than to find an electrical engineer to do the math and design a circuit.

Zenst 8y ago
Same reason why the majority of CPU's are synchronous instead of asynchronous, cheaper to design by far.
setquk 8y ago
Actually it's usually better in the digital domain for all of those reasons. Most systems these days have minimal transducer processing stuff in the analogue domain and get it into the digital one as quickly as possible. Analogue systems have a lot of non-ideal characteristics such as drift, thermal sensitivity, parasitic capacitance and inductance, impedance matching problems, insane cumulative errors and manual tuning requirements that don't exist in the digital domain. They are also more expensive to modify, and the price goes up significantly with complexity and speed. Not to mention small and fast is really difficult in the analogue domain thanks to everything being a transmission line.
- jacquesm 8y ago
  > insane cumulative errors Any sufficiently advanced analog computer is indistinguishable from a noise generator.
hopler 8y ago
TFA explains the nuances of the trade-off. For certain domains (simple calculations at extremely high frequency with tolerance for approximation), analog is a good choice.
SomeHacker44 8y ago
I fly a plane with an old analog computer autopilot/flight director. It works perfectly well. Its failure modes are well understood and easy to diagnose. The boards and components can be easily troubleshot and repaired by competent electricians. Modern autopilots are all sensors, DACs, ADCs, and various digital computing resources. Cheaper, faster, more featured, and mystifying in their failure modes. Although I am a big digital person, even for photography, audio and other seemingly analog things - I admire and appreciate the one major analog computer in my life. Alas, after 7 years, it finally needs a repair. Sigh.
- darkmighty 8y ago
  Does it have some kind of built-in redundancy? And how are you aware that it needs repair?
  - SomeHacker44 8y ago
    (It is broken because:) When engaged, the altitude hold function commands a 10 degree pitch up attitude in all cases. This is a reasonably extreme pitch. Even the built in “take off/go around” mode usually commands only about 7-8 degrees nose up. The altitude is “held” using a “static” pressure sensor and pitch is adjusted in a control and feedback loop to attempt to keep that signal constant. When it works, it is very smooth. The system is not as a whole redundant, but there are many failsafes. You can put it into a mode where the system does not fly the plane but tells you what it thinks you should do. This is called a flight director. You fly it almost like a video game, get the pointers aligned. Each component has some redundancy. It has a self test, an annunciator panel saying what mode it is in, a mode control panel, several controls on the yoke including a kill switch, several circuit breakers (one, the electric trim, which has a big red flange on it - see the recent 737 crash artickes), etc. The workflow of using the autopilot is always command a change, check that the change is reflected on the annunciators and that the plane is adjusting attitude and such appropriately, then resume the normal scan. If at any time anything looks off my instant reaction is always to hit the big red button and take the plane’s control back. When you do this you need to be wary for an out of trim condition and other sudden forces on the yoke and rudder.

Tade0 8y ago

I still have a few operational transconductance amplifiers lying around which I tried to use for my guitar effects projects(none of them ever reached completion of course). Not obvious to work with, but a great solution if you want to multiply two signals.

atoav 8y ago
Other ways to do this: - Vactrols (a LED/LDR combination) - Blackmer VCA Chips (e.g THAT2181 or SSM2164) - Analog Multiplier ICs (these are actually four-quadrant multipliers), (e.g. AD 633) - PWM switching - ...

bogomipz 8y ago

The FAQ states: >"Q: What are some of the issues associated with analog computation? A: They are similar to the ones that are of concern whenever analog functions (op amps and more) are used for signal processing or signal conditioning. These include linearity, offset, temperature-related drift, aging, and bandwidth." I understand bandwidth but could someone elaborate in lay terms what these other "concerns" are in analog circuits? Specifically what are linearity and drift?

detaro 8y ago
Linearity: you expect a circuit to e.g. always multiply an input with 2x. Turns out it doesn't, it does so close to e.g. 0, but multiplies by less or more at higher values. Somewhat generalized, for other components with other curves of behavior, they also don't stick perfectly to the mathematical model, especially when the environment changes. Which leads us to Drift: temperature changes, behavior of component changes (e.g. resistors change slightly in resistance, circuits to generate precise voltages produce slightly different voltages, oscillators change in frequency). E.g. precision frequency sources often have a heating element to keep their core oscillator at a stable frequency, audio devices thermally couple two components of which the ratio is important, so the changes cancel out as much as possible, ...
pjc50 8y ago
Linearity: if a system is linear you can draw a nice straight line on an X-Y plot of input vs output. Nonlinearity is a measure of how much it deviates from that line. Drift: component values are not fixed! Most vary slightly with temperature, and capacitors change value over time too. Can be seen in any kind of old equipment which needs to "warm up".
- bogomipz 8y ago
  Thanks for the wonderful and detailed responses. I really appreciate it. One follow up - is it accurate to say that linearity and drift are always related then?
  - 8y ago

audiometry 8y ago

Omega Tau podcast has quite a long episode on analog computers.

adyavanapalli 8y ago
Episode link: http://omegataupodcast.net/159-analog-computers/ http://omegataupodcast.net/159-analog-computers/