From 89311623faea2da01481be7f96704a4c2b7ccc16 Mon Sep 17 00:00:00 2001
From: "B. Watson" <yalhcru@gmail.com>
Date: Tue, 6 Sep 2016 15:22:06 -0400
Subject: bit more work on the sound engine

---
 sound.txt | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 sound.txt

(limited to 'sound.txt')

diff --git a/sound.txt b/sound.txt
new file mode 100644
index 0000000..c9754e9
--- /dev/null
+++ b/sound.txt
@@ -0,0 +1,56 @@
+Most of the sounds in the game are played by a sound engine that
+runs 'in the background' (called every vblank). There are some
+exceptions:
+
+- The descending tones played as the letters materialize, on the
+  title screen (but, the music after that uses the engine).
+- The descending tone you hear as Jumpman materializes, at the
+  start of a level.
+- The sound you hear as the screen crumbles, after you lose your
+  last life.
+
+Everything else uses the engine, and is stored as a series of
+engine opcodes. I've labelled these sfx_* in the source.
+
+The engine has 2 entry points: cue_sfx and cue_sfx_lowprior. The
+difference is that the _lowprior entry point will abort, if another call
+to cue_sfx is in progress. Also the arguments are passed differently.
+Also there are _jv (jump vector) versions of both entry points, which
+are the ones most of the rest of the code uses.
+
+Example sound effect:
+
+sfx_jump:
+        .byte   $01,$A5,$00,$79,$04,$60,$04,$51 ; BFBE 01 A5 00 79 04 60 04 51  ...y.`.Q
+        .byte   $04,$3C,$04,$51,$04,$60,$04,$79 ; BFC6 04 3C 04 51 04 60 04 79  .<.Q.`.y
+        .byte   $04,$00                         ; BFCE 04 00                    ..
+
+Sound opcodes:
+
+00          snd_end     end of sound
+01 xx yy    snd_audc    set distortion and volume (AUDCx value) to xx,
+                        play previous freq for yy frames.
+                        yy looks to always be zero in actual use,
+                        except the title screen music where it's $02.
+02 xx yy    snd_jump    jump to address yyxx. this is an unconditional
+                        jump, meaning the sfx plays continuously until
+                        preempted or silenced.
+03 xx       snd_rest    play a rest, xx frames (sets AUDCx to 0)
+[04-FF] xx  snd_note    play [opcode] frequency for xx frames
+
+so sfx_jump might look like this:
+
+sfx_jump:
+  snd_audc $A5,$00
+  snd_note $79,$04
+  snd_note $60,$04
+  snd_note $51,$04
+  snd_note $3C,$04
+  snd_note $51,$04
+  snd_note $60,$04
+  snd_note $79,$04
+  snd_end
+
+or in english: distortion $A (pure tone), volume 5. for 4 frames each,
+play 3 ascending notes, a higher note, then the same 3 notes in reverse
+order. the whole thing takes 28 frames to complete, or about 1/2 sec.
-- 
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