Nov

8

One has found that there is an electronics circuit that almost always retrospectively provides a great description of price action in markets. I wonder if there is an electronics circuit that compresses the voltage output keeping it in a range, sort of like the finger in the dike, but then after the compression is over on the negative side, e.g after the negative feedback is taken away, the voltage doesn't immediately lead to tremendous negative voltage. I seem to remember such a circuit with op amps.

Jon Longtin writes:

There are a variety of electronic circuits that perform such a role, depending on the application. One common application is a voltage regulator, which provides a (nearly) constant voltage, regardless of the load applied to it. The circuit monitors the actual voltage currently being provided and compares to a pre-set reference value. The difference between the actual and desired (setpoint) values is called the error, and is used to adjust the current provided to the circuit to bring the voltage back to the setpoint value. For example if the load increases (more electricity demand) the load voltage will drop and the voltage regulator will provide more current to bring the voltage back up. Same goes for a decrease in load.

There are some limitations and compromises in such a circuit. First is there is a finite amount of current that the power supply/voltage regulator can be provided, and if the error signal requests more than this amount, the output will not be maintained. Also of importance is the time response: a circuit with a very fast time response will respond more quickly to fluctuations in the load, but can also result in so-called parasitic oscillations, in which the output oscillates after a fast change in load is made. By contrast a longer time response provides a slower response to a variation, but tends to damp oscillations. This same behavior, of course, is seen in countless financial indicators, and is part of the art in deciding, e.g., how many prior data points to include in a signal.

A somewhat more complex version of the above, and perhaps more closely aligned with the behavior of a market signal, is an audio "compressor/limiter". This is a device that constantly monitors the volume (magnitude or voltage) of an audio signal and makes adjustments as needed. A limiter is the simpler of the two and simply sets a threshold above which a loud signal will be attenuated. The attenuation is not (usually) a brick-wall however; rather a signal that exceeds the threshold value is gently attenuated to preserve fidelity without overloading the audio or amplifier circuitry. A compressor is a more complicated animal and provides both attenuation for loud signals AND amplification for quieter ones. In essence a HI/LO range or window is established on the unit, and signals exceeding the HI limit are attenuated, while signals below the LO limit are amplified. This resulting output then (generally) falls within the HI/LO range. This is used extensively (too much!) in commercial music. Humans naturally pay attention to louder sounds (ever notice how the volume universally jumps when commercials come on TV? They are trying to grab your attention with the louder volume). Pop music attempts to achieve the same by using aggressive compression to provide the loudest average volume for program material without exceeding the maximum values set by broadcast stations or audio equipment. The result, however is that the music sounds "squished" and doesn't "breath" because the dynamic range of the content has been reduced considerably. With such devices there are a variety of adjustments to determine the thresholds, time before taking action (the attack time) and how gradually or strongly to attenuate (amplify) signals that exceed the envelop range.

Here' s a fairly decent article that describes this in more detail.

Incidentally both of the above are examples of a large branch of engineering called Controls Engineering. The idea, as Vic stated, is to monitor the output by using feedback and make adjustments accordingly. There are countless different algorithms and approaches, as well as very sophisticated mathematical models (people build careers on this) to best do the job. Like most complex things, there is no single approach that works best for every problem, but rather involves a balance of performance, cost, and reliability.

I highly suspect such algorithms have already found their way into many trading strategies, one way or another.

If interested, I can suggest some references for further reading (though I am not a Controls person myself).

Bill Rafter writes: 

 Think of your voltage regulator as a mean-reversion device. If a lot of this is being done, then your trading strategy must morph into simply following the mean.

In light of recent changes in the investment climate we suggest that one should tighten up controls in which one is long a given market. Perhaps that might also or alternatively mean (a la Ralph) tightening the size of the positions. The result will be taking less risk and incurring less return, but taking additional risk would seemingly not be rewarded in the current milieu.

Jim Sogi writes: 

Dr. Longtin's description of compressors and limiters was
fascinating.  A compressor on my guitar signal chain prolongs the
sustain on a signal in addition to smoothing out the volume spikes and has less fade as the signal weakens.  With added volume, one gets a
nice controlled feedback.

Sometimes in the markets one sees a sustained range with the spikes being attenuated reminiscent of a nice guitar sustain.

On a different note, one curious thing is that people cannot  discern differences in absolute volume.  It's very hard to hear the differences
in volume between two signals unless they are placed side by side.


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3 Comments so far

  1. Murali on November 9, 2012 10:47 am

    Hi Jon,

    Could you please suggest other reading material that you alluded to in your post? Thanks.

  2. Vikram Krishnan on November 13, 2012 6:45 am

    Couldn’t clippers accomplish something similar? Not compression per se but the reduction of the voltage by a fixed amount (0.7 V per Si diode in series)

  3. Jon Longtin on November 13, 2012 10:50 am

    Here are a few more references on feedback control that I think are particularly well done:

    For starters, Wiki (not surprisingly) does a good job of the very basics:
    http://en.wikipedia.org/wiki/Controller_(control_theory)

    Here is an article that provides the key elements of industry-standard PID control. PID stands for Proportional, Integral, and Derivative, and basically includes all of the ways that an error signal can be processed to make adjustments:
    http://www.eetimes.com/design/industrial-control/4206216/The-basics-of-doing-PID-design-Part-1-Classical-control-theory

    Here is a very detailed description that is surprisingly easy to understand. It’s written for an industrial user of control systems/algorithms, but is very descriptive. Probably far more than you will ever need.
    http://www.pacontrol.com/download/Process%20Control%20Fundamentals.pdf

    Finally, an article closer to the finance side of things: A Powerpoint presentation on developing trading algorithms, with a nod to machine control as a predecessor. I don’t really understand the finance side…
    www.infotech.northwestern.edu/KumiegaNorthwestern.ppt

    Surprisingly I did not find thousands of articles discussing control theory as applied to trading systems. I’m sure it has been considered (extensively); I may just not have the right words to describe it.

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