Multiband Compressor or Octave-Band Filter plugin

Question

I am looking for an octave-band filter which will allow me to adjust the individual gains of each octave band in the range 63 Hz - 8 kHz (8 bands) with as close to a brickwall response as possible (i.e. clearly defined bands with little or no overlap). Failing this, I imagine a multiband compressor would achieve the same effect but I'm struggling to find one which goes above 5 bands (same with EQs, also very few EQs offer the kind of approximate brickwall response I am looking for).

Crucially though, the output of the filter must have no perceptual difference to the input in the case where its parameters are set to leave the input signal unchanged. By this I mean if I run a signal through the compressor with a 1:1 ratio and 0 dB makeup gain, there would be no perceptible change due to phase differences or other spectral artefacts.

This is for an industrial auralisation project and not music production. I understand that this might be too much to ask from a plugin but could anyone point me in the direction of a plugin that will achieve this effect? I'm working in REAPER.

Many thanks!

Answer 1

What you're looking for is incredibly hard to implement. Filters always have a slope measured in dB/Octave or dB/Decade and it's a measure of the attenuation of frequencies beyond the cutoff frequency. This slope can never be vertical so frequencies next to each other can never be completely isolated. To increase the slope, we normally stack multiple filters.

But there is another side effect of filtering that you might not be very happy with. Filters introduce phase difference between input and output. Usually 90 or 180 degrees on the cutoff point depending on design.

You would find the same issues with a multiband compressor because that too utilizes filters to achieve band separation.

So you would need to specify what amount of overlap and phase shift is acceptable for your project.

There is a digital signal processing stack exchange and I would expect the people there to be able to give you much more knowledgeable answers about filtering. They would be able to tell you which kind of filter design is the closest to your specifications but I'm not sure if they could advise on a plugin. I would ask on their meta first.

Finally, what you're describing sounds somewhat similar to the input and filtering section of a vocoder.

The vocoder (voice encoder) was initially designed as an experiment by telephone companies to narrow the bandwidth of the information that would need to be carried by telephone lines. It works by splitting a signal into many frequency bands, looking at the amplitude in each band and sending that information to the synthesizing part that recreates (quite badly for their purposes) the original signal. So I would expect a vocoder to already have the facilities for what you need but some moding might be required. Vocoders exist in analogue as well as digital implementations so if you could find an open source vocoder plugin, you could probably tweak the code to set the bands to octaves.

I hope some of the above helps.