39 lines
1.5 KiB
Plaintext
39 lines
1.5 KiB
Plaintext
this code was widely shared for playing tones on Apple II
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by POKEing the machine language and CALLing from BASIC
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it's originally by Paul Lutus, from the Apple II Red Book, p45
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this code is just toggling the speaker ($C030) on and off with cycle-counted
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frequncies.
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There are fancier ways to get music out of the speaker, see Electric Duet
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(also by Paul Lutus), dac522 (by Michael Mahon), and play-btc and
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a2stream by Oliver Schmidt.
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using regular load/store/bit of $C030 is safe. Some of the more
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advanced addressing modes can double-toggle due to how some 6502
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implementations run the address bus
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these was a table of notes floating around, but it seems the note values
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were off a bit and also seems to have been calculated assuming
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a 1MHz clock but the Apple II actually runs at roughly 1.023MHz
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; the inner freq loop is roughly FREQ*10cycles
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; so the square wave generated has a period of
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; freq*20*1.023us
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; or a frequency of 1/(freq*20.46e-6)
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; more exactly, it is (4+10F)+(13+10F) = 20F+17
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Note this probably will be a bit off on PAL Apple IIs
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The code was originally in the form of a machine language program that could
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be POKEd to address $00 of the zero page, and called from Integer Basic.
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Later this was adapted for Applesoft and loaded to $300 instead, although
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when doing this Y was never initialized properly (Integer BASIC had it at $00
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but Applesoft it would be $02).
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see ../../utils/asoft_sound for some routines that are related but
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have slightly different sounds to them
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