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Routines |
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Used by the routines at keypress_tune_note and play_audio.
Looks up the note's three bytes in freq_tbl: two loop counts that set how long each half of a wave lasts, and how many waves make one unit. The length bits multiply the wave count by one to four. It then drives the speaker directly, off for one half-wave and on for the other, with the border held black.
The wave count in the table rises with the pitch, which keeps every unit close to 0.155 seconds whatever the note.
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| play_note | B2DA | AND $3F | index 0 is a rest | |||||||
| B2DC | JR Z,snd_delay | |||||||||
| B2DE | LD L,A | HL = freq_tbl + 3 * index | ||||||||
| B2DF | LD H,$00 | |||||||||
| B2E1 | ADD HL,HL | |||||||||
| B2E2 | CALL add_HL_A | |||||||||
| B2E5 | LD BC,freq_tbl | |||||||||
| B2E8 | ADD HL,BC | |||||||||
| B2E9 | LD B,(HL) | B and C time the half-wave; HL = the number of waves in one unit | ||||||||
| B2EA | INC HL | |||||||||
| B2EB | LD C,(HL) | |||||||||
| B2EC | INC HL | |||||||||
| B2ED | LD L,(HL) | |||||||||
| B2EE | LD H,$00 | |||||||||
| B2F0 | LD A,(DE) | the top two bits: the length, one to four units | ||||||||
| B2F1 | RLCA | |||||||||
| B2F2 | RLCA | |||||||||
| B2F3 | AND $03 | |||||||||
| B2F5 | INC A | |||||||||
| B2F6 | PUSH DE | DE = the waves in one unit, to add up | ||||||||
| B2F7 | LD E,L | |||||||||
| B2F8 | LD D,H | |||||||||
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