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Cuki79 algorithm

Wednesday 1 April 2020, by Cuki79

UPDATE: New algorithm makes IRs that don’t need EQ. Those files are denoted "Match"

Hi everyone,

Most of the IRs of the website are made with my algorithm or the sliglty modded Jon Fields flavored variant. Unlike Jon, I don’t disclose my algorithm.

It does not mean that you can’t have a custom IR for your guitar with this algorithm. It just means you will have to send me (or Jon) the recordings using dropbox or google drive and that we will then send you the IR files and publish them on the free database.

One has to understand that making IR for acoustic guitar is a fight against noise. The highest frequency note on the fretboard is around 1000 Hz.

It means that all frequency content above 1000 Hz are harmonics with very weak power. When the power of the harmonics reach the noise floor, the problems start. You get all kind of digital ringing artefacts that eventually polute your IR file.

As a consequence, the hardest part of the job is to make the response of the guitar pickup match the microphone at high frequencies.

Jon’s algorithm: To put it simply, Jon’s algorithm considers that when the signal is not strong enough it is not worth making the tone correction. So for those high frequencies, more pickup signal is present and less processed signal is used.

My algorithm is different from Jon’s. It tries to use all the information it can get from the signal and produce the most accurate high end possible without making any assumption. This is very important for me. Tonedexter sorts information based on known properties of "string instruments". For years, I interpreted that as an assumption, however James May wrote on the AGF that Tonedexter did not make any assumption.

Anyway, back in 2016-2017, my main motivation was to make an algorithm that had those properties:

  • Limit the digital ringings
  • Produce the most accurate high end
  • Extract the maximum information from the microphone and pickup recordings without any assumption.

Please use the "Match" files first.

The following only applies to older IR files:

The original algorithm (non "match" files) produces giant low-end boost.

This due to the fact that the soundhole of a guitar acts as the bass-reflex port of a loudspeaker. It adds a resonance due to the air in the box flowing back and forth through the soundhole. This is the very resonance that usually feedbacks when the guitar is amplfied, making a big "Wwoooo" rambling sound. Pickups are usually less sensitive to this resonance (also to prevent feedback). SInce it is present in the external microphone recording, the IR will try to recreate it from the pickup signal. Therefore the IR usuallys add a huge boost around 100-200 Hz.

With my original algorithm, this is recommended to tame that resonance with a Parametric EQ cut and a high-pass filter. You can see an illustration in the videos.

All IR files come with a png spectrum image file that shows which frequency has to be tamed.

Note that the problem is handled automatically by a pedal like Tonedexter and that Baggs Soundscape propose a pre tuned parametric filter to handle it.