I had a hunch that this value is treated as Signed instead of Unsigned in the code somewhere, mainly because 0.4 is around half the full value and that’s where it stopped working.
The values that matches the JAP Model 1 VA1 console is still 40.
Later Genesis/MegaDrive consoles have more filtering than the original JAP Model1, therefor 40 is a good default start, but please add 0.05 intervals in future build for even more accurate adjustment.
If the brickwall filtering at 15kHz could be fixed it would be ideal because it may have some effect on the overall curve.
Maybe it’s because GenPlusGX still runs in 44.1kHz while Retroarch is 48kHz?
EDIT: “Audio Resampler Driver” has huge effect on how the Lowpass filter behaves.
All measurement taken with Sinc, 48000, xaudio.
Yep, it is the 44.1kHz to 48kHz conversion that causes the steep down-slope at 15kHz.
I vote for removing the EQ function altogether, it’s unnecessary and buggy.
JAP Model1 VA1, USA Model1 VA3 and PAL Model1 VA4 all have absolutely identical measured frequency response.
Big thanks to all the people who helped solve/implement this.