Add sound effects

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>

7 files changed, 212 insertions, 0 deletions

diff --git a/src/bullets.s b/src/bullets.s
index 1072a8f..9e40546 100644
--- a/src/bullets.s
+++ b/src/bullets.s
@@ -133,6 +133,9 @@
         beq @find_free_bullet_bucket
 
     @initialize_bucket:
+        ;; Play the bullet sound if appropiate.
+        jsr Sound::play_bullet_maybe
+
         ;; We found a free bucket. Initialize the first byte to 0 since it has
         ;; not moved yet. The heading is taken from the player's state.
         lda Player::zp_state
diff --git a/src/driver.s b/src/driver.s
index 2bc78ca..4e54e8c 100644
--- a/src/driver.s
+++ b/src/driver.s
@@ -240,6 +240,9 @@
         sta zp_next_bullet_cycle
         sta zp_next_enemy_cycle
 
+        ;; And make the sound for entering into a level.
+        SOUND_ENTER_LEVEL
+
     @game:
         ;; Has the player died?
         lda Globals::zp_flags
@@ -818,6 +821,9 @@
         ora #%01100000
         sta Globals::zp_flags
 
+        ;; Make some noise!
+        START_TAKE_OFF_SOUND
+
         rts
     .endproc
 
@@ -928,6 +934,9 @@
         ora #%01100000
         sta Globals::zp_flags
 
+        ;; Stop the sound effect for take off.
+        STOP_TAKE_OFF_SOUND
+
         rts
 
     @set:
diff --git a/src/enemies.s b/src/enemies.s
index 2208758..76f3ddf 100644
--- a/src/enemies.s
+++ b/src/enemies.s
@@ -695,6 +695,9 @@
         ldx Enemies::zp_pool_index
         sta Enemies::zp_pool_base + 3, x
 
+        ;; Play an explosion effect.
+        SOUND_EXPLOSION
+
         ;; Restore back the value for the 'y' register.
         pla
         tay
diff --git a/src/interrupts.s b/src/interrupts.s
index 7026223..658f6a3 100644
--- a/src/interrupts.s
+++ b/src/interrupts.s
@@ -161,6 +161,11 @@
     sta PPU::m_scroll
     sta PPU::m_scroll
 
+    ;; The sound effect from bullets follow a frame rule. Tick the frame count
+    ;; from sound as the very last thing to be done before unblocking the main
+    ;; code.
+    jsr Sound::tick
+
     ;; Unblock the main code.
     lda #%01111111
     and Globals::zp_flags
diff --git a/src/items.s b/src/items.s
index d8d1391..1e8a111 100644
--- a/src/items.s
+++ b/src/items.s
@@ -591,6 +591,9 @@
         ;; Increase the number of collected items.
         inc Items::zp_collected
 
+        ;; Make the "item drop" sound
+        SOUND_ITEM_DROP
+
         ;; Now we unset the 'S' bit, which is unconditionally true regardless of
         ;; the collection state. That being said, if we still need to collect
         ;; more fuel tanks (the rocket has all its parts and we have not filled
@@ -679,6 +682,9 @@
     @set_modes:
         sta Items::zp_pool_base, x
 
+        ;; Make the sound for grabbing an item.
+        SOUND_ITEM_PICKUP
+
         ;; Mark the player's to be already grabbing an item.
         lda Items::zp_state
         ora #$80
@@ -988,6 +994,9 @@
         ADD_ITEM_SCORE
         ldx Globals::zp_tmp2
 
+        ;; Make the sound for item pickup.
+        SOUND_ITEM_PICKUP
+
         rts
     .endproc
 
diff --git a/src/jetpac.s b/src/jetpac.s
index f2e28b2..a011a45 100644
--- a/src/jetpac.s
+++ b/src/jetpac.s
@@ -42,6 +42,7 @@
 .include "assets.s"
 .include "background.s"
 .include "prng.s"
+.include "sound.s"
 .include "explosions.s"
 .include "score.s"
 .include "items.s"
@@ -53,6 +54,14 @@
 .include "driver.s"
 .include "interrupts.s"
 
+;; Sanity check for some constant values. 'cl65' fails at this, so I'm only
+;; doing the check on 'nasm'.
+.ifdef __NASM__
+    .if Bullets::BULLET_TIMER_VALUE > Sound::BULLET_SFX_FRAME_COUNT
+        .error "Sound::BULLET_SFX_FRAME_COUNT must be smaller than Bullets::BULLET_TIMER_VALUE"
+    .endif
+.endif
+
 ;; Pretty standard reset function, nothing crazy.
 .proc reset
     ;; Disable interrupts and decimal mode.
@@ -73,6 +82,9 @@
     stx PPU::m_mask
     stx APU::m_dmc
 
+    ;; Initialize the APU.
+    jsr Sound::init
+
     ;; First PPU wait.
     bit PPU::m_status
 @vblankwait1:
diff --git a/src/sound.s b/src/sound.s
new file mode 100644
index 0000000..a824898
--- /dev/null
+++ b/src/sound.s
@@ -0,0 +1,171 @@
+.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