[blackcat] L2 Ehlers Two Pole Butterworth Filter

Level: 2

Background

John F. Ehlers introuced Two Pole Butterworth Filter in his "Cybernetic Analysis for Stocks and Futures" chapter 13 on 2004.


Function

Dr. Ehlers translated analog Butterworth filters to their digital approximations. The transfer response is characterized by a single variable—the cutoff frequency. The cutoff frequency is that frequency where the input is attenuated by 3 dB. Below the cutoff frequency, the input frequency components are passed to the output; above the cutoff frequency, the input frequency components are rejected to the extent possible by the filter characteristics. Since traders are more comfortable with period, which is the reciprocal of frequency, the equations for the Butterworth digital filters are characterized in terms of the cutoff period.

As opposed to the Regularized filter, the order of Butterworth filters can be increased indefinitely to increase the sharpness of the filter rejection. For traders, this quickly reaches the point of diminishing returns because increasing the number of poles in the filter means the lag of the filter is also increased. A three-pole filter gives just about the limit of tolerable lag for a selected cutoff period.


Key Signal

Butter ---> Two Pole Butterworth Filter fast line
Trigger ---> Two Pole Butterworth Filter slow line


Pros and Cons

100% John F. Ehlers definition translation of original work, even variable names are the same. This help readers who would like to use pine to read his book. If you had read his works, then you will be quite familiar with my code style.

Remarks

The 29th script for Blackcat1402 John F. Ehlers Week publication.

Readme

In real life, I am a prolific inventor. I have successfully applied for more than 60 international and regional patents in the past 12 years. But in the past two years or so, I have tried to transfer my creativity to the development of trading strategies. Tradingview is the ideal platform for me. I am selecting and contributing some of the hundreds of scripts to publish in Tradingview community. Welcome everyone to interact with me to discuss these interesting pine scripts.

The scripts posted are categorized into 5 levels according to my efforts or manhours put into these works.

Level 1 : interesting script snippets or distinctive improvement from classic indicators or strategy. Level 1 scripts can usually appear in more complex indicators as a function module or element.

Level 2 : composite indicator/strategy. By selecting or combining several independent or dependent functions or sub indicators in proper way, the composite script exhibits a resonance phenomenon which can filter out noise or fake trading signal to enhance trading confidence level.

Level 3 : comprehensive indicator/strategy. They are simple trading systems based on my strategies. They are commonly containing several or all of entry signal, close signal, stop loss, take profit, re-entry, risk management, and position sizing techniques. Even some interesting fundamental and mass psychological aspects are incorporated.

Level 4 : script snippets or functions that do not disclose source code. Interesting element that can reveal market laws and work as raw material for indicators and strategies. If you find Level 1~2 scripts are helpful, Level 4 is a private version that took me far more efforts to develop.

Level 5 : indicator/strategy that do not disclose source code. private version of Level 3 script with my accumulated script processing skills or a large number of custom functions. I had a private function library built in past two years. Level 5 scripts use many of them to achieve private trading strategy.

릴리즈 노트:

thanks to contribution of @sal157011, i corrected the wrong parameter in script, i will reference rocket book 2001:
rocket book page 155:

a = ExpValue(-1.414*3.14159/P);
b = 2*a*Cos (1.414*180/P);
y=b*y(1)-a*a*y(2)+((1-b+a*a)/4)*(x+2*x(1)+x(2));