Biquad filters

[DiplacusisAlt]

Hello,

A couple of weeks ago, I asked about tweaking an N3DS XL's hardware, in order to slightly pitch-shift it (in my case, I'd like to lower it by half a semitone to counteract my perfect pitch's drift). I was given the following advice by the head dev, TuxSH:

It depends. What software are you using to mitigate this on PC/Mac/etc.? (Peace+EqAPO, SoundSource... ?)

If this can be fixed by one, or two biquad filters then I think this could be done (even w/o needing to make changes to luma itself); see https://www.earlevel.com/main/2021/09/02/biquad-calculator-v3/ - is that the case for you?

Unfortunately, I don't know how to apply a biquad filter to my N3DS XL (and moreover, what to actually do with the calculator itself). On my backup laptop, I did get EqAPO and Peace running properly -- is it as simple as finding and installing an equalizer on my device? My attempts to find one have been unsuccessful, though; I was previously told by one sakumikimiko that,

All sounds are linked to the actual game code, or to console hardware, and in order to change the sounds by half a semitone, which is what you are implying, you would need to re-make the sounds for a game and make it a custom mod, or alter the sound used in the hardware of the n3ds.

Any advice in this matter would be greatly appreciated. Thank you for your time!

[TuxSH]

All sounds are linked to the actual game code, or to console hardware, and in order to change the sounds by half a semitone, which is what you are implying, you would need to re-make the sounds for a game and make it a custom mod, or alter the sound used in the hardware of the n3ds.

That's wrong, there is equalization done by the hardware itself, and there are two free biquad slots

Have you been able to use and configure EqApo/Peace correctly with the setting you wanted?

[DiplacusisAlt]

Ahhhh, that's good to know -- thank you so much. Also, yes, I was able to set it up on my backup laptop a couple of weeks ago - it seems to be working very smoothly!

[TuxSH]

Could you export the eq presets you are using on that PC?

[DiplacusisAlt]

I'll look into it, but assuming I'm able to, what did you have in mind? Are you saying I could copy the EQ file over to my 3DS and put it into one of the two biquad slots?

[DiplacusisAlt]

@TuxSH I hope you don't mind me pinging you -- after spending far too long trying to look it up, it turns out that there's an export button built into Peace itself. It allows me to export it as a .txt or a .xml -- it has another choice too, for either an Audacity Filter curve EQ or an Audacity Graphic EQ. I'm not sure what the difference is between these. Finally, it allows me to choose the number of frequencies. Please see the below screenshot.

With this in mind, how might the exported equalization be applied to one of the two free biquad slots? Thank you again for your time!

[DiplacusisAlt]

I should also ask: is there a way to revert this, should my sense of pitch somehow fix itself (or get worse)?

[TuxSH]

With this in mind, how might the exported equalization be applied to one of the two free biquad slots? Thank you again for your time!

I'll try to write some homebrew code that I will share; I haven't even confirmed this works.
Could you please send me the preset over (via .zip, Gist or otherwise)?

Also sorry for not having answered sooner - I'm a bit busy these days.

[profi200]

Keep in mind CODEC expects the filters in a very special format. See here:
https://github.com/profi200/libn3ds/blob/master/include/arm11/drivers/hw_cal.h#L257

Pay special attention to the comments in the relevant structs. Some coefficients are halved, inverted or both.

I have an unfinished tool that can convert the coefficients from standard biquad to fixedpoint format in the way CODEC expects them. Have not figured out the exact way Nintendo rounds them yet. Please note that this is old code and i was in the middle of rewriting this so not everything may be right. See the TSC2117 datasheet.

...
#define FIXED_RANGE  (32767) // (2¹⁵) - 1.
...
			a0 = strtod(argv[1], NULL);
			a1 = strtod(argv[2], NULL) / (-2 * a0);
			a2 = strtod(argv[3], NULL) / -a0;
			b0 = strtod(argv[4], NULL) / a0;
			b1 = strtod(argv[5], NULL) / (2 * a0);
			b2 = strtod(argv[6], NULL) / a0;
...
			a1 = floor(a1 * FIXED_RANGE);
			a2 = floor(a2 * FIXED_RANGE);
			b0 = floor(b0 * FIXED_RANGE);
			b1 = floor(b1 * FIXED_RANGE);
			b2 = floor(b2 * FIXED_RANGE);
...
			printf("N0 = %.17g\nN1 = %.17g\nN2 = %.17g\n"
			       "D1 = %.17g\nD2 = %.17g\n",
			       b0, b1, b2, a1, a2);
...

[DiplacusisAlt]

With this in mind, how might the exported equalization be applied to one of the two free biquad slots? Thank you again for your time!

I'll try to write some homebrew code that I will share; I haven't even confirmed this works. Could you please send me the preset over (via .zip, Gist or otherwise)?

Also sorry for not having answered sooner - I'm a bit busy these days.

No worries at all -- thank you so much for your time!

Please see the attached zipped folder -- I've included both the graphic and filter curves, and each as both a .txt and a .xml. I left the number of frequencies at 50.

presets.zip

[DiplacusisAlt]

@TuxSH Sorry to ping you again -- I wanted to check in quickly to see whether you've made any headway!

[TuxSH]

I haven't -- I'm really busy right now; I didn't forget about you

The idea was to overwrite calibration stored in blk 0x00070000 of CFG savegame using CFG commands; I'll try to write an hb app at some point in the future

[DiplacusisAlt]

Very glad to hear from you again -- sorry to intrude. Thank you so much for letting me know!