dc_filter.jsfx

desc:       DC Filter
version:    1.8.3
author:     chokehold
tags:       utility dc offset filter blocker
link:       https://github.com/chkhld/jsfx/
screenshot: https://github.com/chkhld/jsfx/blob/main/assets/screenshots/dc_filter.png
about:
 # DC Filter
 
 Multi-channel capable and extremely narrow DC offset removal filter.
 
 Why would you need another DC filter plugin, if Reaper already comes with one?
 Simple: the DC Filter JSFX that comes with Reaper has a wider filter (meaning
 it removes more sub low-end) and it only operates on two channels, so it's of
 little use when processing multi-channel material.
 
 This DC filter is as narrow as makes sense, and it keeps sub-bass frequencies
 as untouched as possible. The processing automatically adapts to the count of
 channels of the track it operates on, even if the channel count should change.

// ----------------------------------------------------------------------------
// Metering is not really required here
options:no_meter

// By not having in_pin and out_pin assignments, this plugin is able to adapt to
// the number of channels on its track automatically.

@init // -----------------------------------------------------------------------
  
  // Stop Reaper from re-initialising the plugin every time playback is reset as
  // it could mess up the state buffers and cause DC to pass during operation.
  ext_noinit = 1;
  
  // DC REMOVAL FILTER ---------------------------------------------------------
  //
  // Removes 0 Hz content that would offset the signal to one polarity.
  //
  // The previousAddress argument needs to point to the memory address where the
  // previous state value is buffered. Needed for simple multi-channel operation.
  //
  // Implemented after Sam Koblenski:
  // https://sam-koblenski.blogspot.com/2015/11/everyday-dsp-for-programmers-dc-and.html
  // Modified "alpha" value from 0.9 for narrower filtering.
  // 
  function dcFilter (sample, previousAddress) local (previous, filtered)
  (
    previous = previousAddress[0]; // Bracket addressing is slow, saving time
    sample  += previous * 0.99988487;
    filtered = sample - previous;
    previousAddress[0] = sample; // Can't avoid bracket addressing here
    filtered;
  );
  
  // Memory buffer to store previous states of the DC filters
  dcBuffer = 1000;

@sample // ---------------------------------------------------------------------
  
  // Start at channel 0
  channel = 0;
  
  // Cycle through all channels on this track
  while (channel < num_ch)
  (
    // Replace this channel's sample by a DC filtered version of itself
    spl(channel) = dcFilter(spl(channel), dcBuffer[channel]);
    
    // Advance to the sample on the next channel
    channel += 1;
  );