fix AY freq-to-tone calculation

[ahihi]

fixes #348, and by extension https://codeberg.org/musikinformatik/SuperDirt/issues/281

AY.freqtotone is implemented with an "approximate empirical" formula:

sc3-plugins/source/AYUGens/sc/AY.sc

Lines 19 to 23 in 1c9bb52

	*freqtotone { |freq|
	// Approximate empirical...
	//^(109300 / (freq - 3.70727))
	^(110300 / (freq - 0.5))
	}

it is reasonably accurate at a sample rate of 44100 Hz, but at other rates it is severely out of tune.

i noticed that the ayemu code calculates a value ChipTacts_per_outcount, which depends on the chip frequency of the emulator as well as the sample rate:

sc3-plugins/source/AYUGens/AY_libayemu/src/ay8912.c

Line 369 in 1c9bb52

ay->ChipTacts_per_outcount = ay->ChipFreq / ay->sndfmt.freq / 8;

the AY_Ctor constructor does not set the chip frequency:

sc3-plugins/source/AYUGens/AY_UGen.cpp

Lines 49 to 51 in 1c9bb52

	ayemu_set_chip_type(ay, chiptype, NULL);
	//ayemu_set_chip_freq(ay, freq);
	ayemu_set_sound_format(ay, freq, chans, bits);

therefore we fall back to the default frequency of 1773400, defined here:

sc3-plugins/source/AYUGens/AY_libayemu/src/ay8912.c

Line 17 in 1c9bb52

AYEMU_DEFAULT_CHIP_FREQ = 1773400

so, some real-world values of ChipTacts_per_outcount are: (keeping in mind this is all integer division)

1773400/44100/8 = 5
1773400/48000/8 = 4
1773400/96000/8 = 2

now we can start to see where the magic number 110300 in freqtotone comes from; it is very close to 44100*5/2 = 110250. this hints at a general formula for the conversion constant:

sampleRate * chipTacts_per_outcount / 8

and indeed, this formula seems to work :) notes now sound in tune at both 44100 and 48000 Hz sample rates. (though due to the integer calculations, i guess there will always be some error.)

the PR just implements this calculation in AY.freqtotone.

[telephon]

Great! Thanks for looking into this ... Maybe it would be clearer to just pass the sample rate into the freqtotone method? Then we can also use the method in a synthDef (using the SampleRateugen). And if none is given, it can still be calculated as you do it.

Note that usually we call the argument server and there is no need to specify the default to nil, since the default is nil.