[blackcat] L2 Ehlers Truncated BP Filter

Level: 2


John F. Ehlers introuced Truncated BandPass ( BP ) Filter in Jul, 2020.


In Dr . Ehlers' article “Truncated Indicators” in Jul, 2020, he introduces a method that can be used to modify some indicators, improving how accurately they are able to track and respond to price action. By limiting the data range, that is, truncating the data, indicators may be able to better handle extreme price events. A reasonable goal, especially during times of high volatility . John Ehlers shows how to improve a bandpass filter’s ability to reflect price by limiting the data range. Filtering out the temporary spikes and price extremes should positively affect the indicator stability. Enter a new indicator ——— the Truncated BandPass ( BP ) filter.

Cumulative indicators, such as the EMA or MACD , are affected not only by previous candles, but by a theoretically infinite history of candles. Although this effect is often assumed to be negligible, John Ehlers demonstrates in his article that it is not so. Or at least not for a narrow-band bandpass filter.

Bandpass filters are normally used for detecting cycles in price curves. But they do not work well with steep edges in the price curve. Sudden price jumps cause a narrow-band filter to “ring like a bell” and generate artificial cycles that can cause false triggers. As a solution, Ehlers proposes to truncate the candle history of the filter. Limiting the history to 10 bars effectively dampened the filter output and produced a better representation of the cycles in the price curve. For limiting the history of a cumulative indicator, John Ehlers proposes “Truncated Indicators,” John Ehlers takes us aside to look at the impact of sharp price movements on two fundamentally different types of filters: finite impulse response, and infinite impulse response filters. Given recent market conditions, this is a very well timed subject.

As demostrated in this script, Ehlers suggests “truncation” as an approach to the way the trader calculates filters. He explains why truncation is not appropriate for finite impulse response filters but why truncation can be beneficial to infinite impulse response filters. He then explains how to apply truncation to infinite impulse response filters using his bandpass filter as an example.

Key Signal

BPT --> Truncated BandPass ( BP ) Filter fast line
Trigger --> Truncated BandPass ( BP ) Filter slow line

Pros and Cons

100% John F. Ehlers definition translation, even variable names are the same. This help readers who would like to use pine to read his book.


The 98th script for Blackcat1402 John F. Ehlers Week publication.


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