This is Bandpass filter Indicator.
I looked for Mq4 code but could not find it before.
Therefore, I made MT4 version with reference to EasyLanguage ,PineScript, and Mladen's Bandpass filter code.

Cycle Analytics for Trader John F. Ehlers p56-57

Several types of Bandpass filter exist. The characteristics of this filter are Auto Gain Control, Leading Trigger line, and Roofing Filter.

This is rough description. If you want to understand it properly, read the citation.

Roofing Filter improves the performance of Bandpass filter. No set value.

Leading Trigger line predicts the movement of the waveform, not a signal line. No set value.

Bandwidth(Q) adjusts the waveform. If it is made smaller, the waveform will be better, but less likely to reflect price movements. If it is made larger, it will be more responsive to price movements, but the waveform will collapse. The default setting is 30% (0,3).
Example1 ( Period20 Upper : 0.3 Lower :0.01)

Auto Gain Control displays the waveform as a relative value based on the most recent maximum or minimum value. The default setting is 99,1% (0,991).
This function stretches the small waveform to make it easier to see.
Example2 ( Period40 Upper : 0.991 Lower :0.900)
Example3 ( Period40 Upper : MTF Lower : Current)

When AutoGain is switched on, Gain is calculated automatically from the MaxCycle and MinCycle settings.


If you like, I would like to hear your ideas.
Yoake