1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
|
.segment "CODE"
;; The sound for this game is extremely simple, coming from a system that only
;; allowed for 1-bit beeps. Here the sound is a bit different due to a vastly
;; different audio hardware, and we make use of three channels:
;;
;; 1. Square 1: used on level entry and bullets.
;; 2. Square 2: used for item collection or dropping.
;; 3. Noise: enemy explosion and rocket launch.
.scope Sound
;; Bullets can go super fast, and if we delivered the sound effect for each
;; bullet it could potentially be annoying. Moreover, because of the
;; limitations from the ZX Spectrum, the original game didn't spit a sound
;; effect for each bullet either. Hence, wait for some frames before
;; producing a sound. This is why we call Sound::tick() on NMI code, and why
;; the sound effect for bullet is called via Sound::play_bullet_maybe().
;;
;; NOTE: the maximum count value is supposed to be bigger than the timer for
;; bullets creation. This is guaranteed in 'jetpac.s'.
zp_frame_count = $DA
BULLET_SFX_FRAME_COUNT = HZ / 10
;; Period values for square channels.
.ifdef PAL
BULLET_SFX_LOW = $29
BULLET_SFX_HIGH = $00
ENTER_SFX_LOW = $4D
ENTER_SFX_HIGH = $01
PICKUP_SFX_LOW = $61
PICKUP_SFX_HIGH = $01
DROP_SFX_LOW = $3A
DROP_SFX_HIGH = $01
.else
BULLET_SFX_LOW = $2C
BULLET_SFX_HIGH = $00
ENTER_SFX_LOW = $67
ENTER_SFX_HIGH = $01
PICKUP_SFX_LOW = $7C
PICKUP_SFX_HIGH = $01
DROP_SFX_LOW = $52
DROP_SFX_HIGH = $01
.endif
;; Initialize all the sound channels which are needed and reset some
;; register values.
.proc init
;; Enable square 1, 2; and noise.
lda #%00001011
sta APU::m_status
;; Reset sweep registers and frame count.
lda #0
sta APU::m_square_1_sweep
sta APU::m_square_2_sweep
sta Sound::zp_frame_count
;; Silence channels.
lda #0
sta APU::m_noise_envelope
lda #$30
sta APU::m_square_1_envelope
sta APU::m_square_2_envelope
rts
.endproc
;; Tick the internal frame count for sound effects.
;;
;; NOTE: expected to only be called at the end of NMI code.
.proc tick
;; If there is no bullet sound effect to be delivered, don't even sweat
;; it.
lda Sound::zp_frame_count
beq @end
;; Increase the frame counter and check the limit. If we reached that
;; limit, reset it so we don't tick until the next bullet sfx request
;; comes in.
clc
adc #1
cmp #Sound::BULLET_SFX_FRAME_COUNT
beq @reset
sta Sound::zp_frame_count
rts
@reset:
lda #0
sta Sound::zp_frame_count
@end:
rts
.endproc
;; Play the bullet sound effect if we can (i.e. the frame count allows us to
;; do it).
.proc play_bullet_maybe
;; If we cannot play the sound yet, skip this altogether.
lda Sound::zp_frame_count
bne @end
inc Sound::zp_frame_count
lda #$01
sta APU::m_square_1_envelope
lda #%10000001
sta APU::m_square_1_sweep
lda #BULLET_SFX_LOW
sta APU::m_square_1_low
lda #BULLET_SFX_HIGH
sta APU::m_square_1_high
@end:
rts
.endproc
.endscope
;; Make an explosion sound via the noise channel.
.macro SOUND_EXPLOSION
lda #$03
sta APU::m_noise_envelope
lda #$8F
sta APU::m_noise_mode
lda #$F8
sta APU::m_noise_counter
.endmacro
;; Make a small beep, suitable for level entry.
.macro SOUND_ENTER_LEVEL
lda #%10000100
sta APU::m_square_1_envelope
lda #Sound::ENTER_SFX_LOW
sta APU::m_square_1_low
lda #Sound::ENTER_SFX_HIGH
sta APU::m_square_1_high
.endmacro
;; Make a small beep for item pickup.
.macro SOUND_ITEM_PICKUP
lda #%10000100
sta APU::m_square_2_envelope
lda #Sound::PICKUP_SFX_LOW
sta APU::m_square_2_low
lda #Sound::PICKUP_SFX_HIGH
sta APU::m_square_2_high
.endmacro
;; Make a small beep for item collection in the droppping zone (i.e. fuel tanks
;; and shuttle parts making into the shuttle).
.macro SOUND_ITEM_DROP
lda #%10000100
sta APU::m_square_2_envelope
lda #Sound::DROP_SFX_LOW
sta APU::m_square_2_low
lda #Sound::DROP_SFX_HIGH
sta APU::m_square_2_high
.endmacro
;; Start the sound effect for the rocket take off animation.
.macro START_TAKE_OFF_SOUND
lda #$38
sta APU::m_noise_envelope
lda #$0F
sta APU::m_noise_mode
lda #0
sta APU::m_noise_counter
.endmacro
;; Stop the sound effect for the rocket take off animation.
.macro STOP_TAKE_OFF_SOUND
lda #0
sta APU::m_noise_envelope
.endmacro
|
