[DSPrated] Modified EMD for swing trade

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Modified Ehlers Empirical Mode Decomposition indicator for swing trade based on Butterworth 2nd order IIR filter
Description
This script is inspired by John Ehlers' TECHNICAL PAPERS - Truncating Indicators and Empirical Mode Decomposition. But instead of detecting trend it applies to finding swing regions.
Also here is suggested canonical DSP approach for designing coefficients for Butterworth 2nd order IIR filters - bandpass and lowpass.
Besides, truncated IIR filter with configurable length parameter is used. It worth mentioning, that although truncated filter is more robust than original IIR, it losses specified properties (bandpass) the more, the less is length parameter.

Butterworth Bandpass Infinite Impulse Response (IIR) Filter
This is the 2nd order Butterworth Bandpass Infinite Impulse Response (IIR) Filter based on the transform from the 1st order lowpass
Based on the example 8.8 on p476 from book Digital Signal Processing: A Practical Approach 2nd Edition by Emmanuel C. Ifeachor (Author), Barrie W. Jervis (Author)
It differs from Ehlers BandPass Filter only in the way you initialize input parameters. Here you can define cutoff periods of region of interest. For example on a timeframe, where one bar equals 1 hour you can define periods 18 and 22, which mean you'll see the swing intensity of price movement components within specified range.

Parameters

• Source
  
• Period 1 - cutoff period of bandpass begining
  
• Period 2 - cutoff period of the end of bandpass
  
• length - IIR truncation length
  

Concept of usage
Within specified bandpass this indicator eliminates the Trend line according to Ehlers EMD. The bandpass periods is recommended to choose accordingly to personal comfortable trading style and timeframe.
The trendline painted with 3 colors depending of the next modes:

• up tend - green
  
• cycling - black
  
• downtrend - red
  

So the buy signal is generated when trend line in cycling mode and filtered component reaches it local minimum.
And the sell signal is generated when trend line in cycling mode and filtered component reaches it local maximum.

Secure long and short zones marked with color.
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// TO DO
// - compare truncated and full version using signal generators
// - apply zero lag filter modification fordetectig ternd and swing peroids
// - implement strategy scripts
// - implement somewhat "true" EMD with sevral IMFs(intrinsic mode function)
// - better description?
// - parameter optimization
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Please, feel free to report any issues and improvement suggestions.