From ad42390cdd3bc6b3328548e2ff35b0a914ceb6cc Mon Sep 17 00:00:00 2001
From: Brooke Vibber <bvibber@pobox.com>
Date: Sat, 31 Aug 2024 10:58:15 -0700
Subject: [PATCH] dither audio

---
 dither4.s   | 25 ++++++++++++++-----------
 pack-wav.js | 31 ++++++++++++++++++++++---------
 2 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/dither4.s b/dither4.s
index bbdb96c..c691b46 100644
--- a/dither4.s
+++ b/dither4.s
@@ -129,17 +129,9 @@ wait_loop:
     ldy VCOUNT    ; 4 cycles
     bne wait_loop ; 2 cycles
 
-    .macro audio_prep
-        ; Y is VCOUNT at entry
-        lda (sample_ptr),y ; 5/6 cyc
-        sta audiotemp      ; 3 cyc
-    .endmacro
-
     .macro inner_scanline frame_offset, line_offset
         ; Y should be VCOUNT at entry
         ; it'll fire on unused lines, but harmlessly
-        ;ldy scanline ; 3 cyc
-        ;inc scanline ; 5 cyc
 
         ; 23-26 cycles before break
         ; Leisurely memory fetches
@@ -149,6 +141,7 @@ wait_loop:
         lda frame1_palette3_even + frame_offset + line_offset - scanline_offset / 2,y ; 4/5
         tay                                                                           ; 2
         pla                                                                           ; 3
+
         ; Wait for horizontal blank
         sta WSYNC                                                                     ; 4
 
@@ -165,13 +158,21 @@ wait_loop:
         sta AUDC1          ; 4 cyc
     .endmacro
 
+    .macro audio_prep
+        ; Y is VCOUNT at entry
+        lda (sample_ptr),y ; 5/6 cyc
+        sta audiotemp      ; 3 cyc
+    .endmacro
+
+    ; call with A pre-loaded to audiotemp
     .macro audio_play_lo
-        lda audiotemp ; 3 cyc
+        ;lda audiotemp ; 3 cyc
         and #$0f ; 2 cyc
         ora #$10 ; 2 cyc
         sta AUDC1 ; 4 cyc
     .endmacro
 
+    ; clobbers Y
     .macro audio_play_hi ; 12 cycles
         ldy audiotemp ; 3 cyc
         lda audio_high_byte,y ; 5 cyc
@@ -212,11 +213,13 @@ wait_loop:
 
     each_scanline_pair:
         sty scanline ; 3 cycles
+        audio_prep
+        audio_play_lo
         inner_scanline frame_offset, 0 ; 23-26 cycles before break, 12 cycles after
 
-        ldy scanline   ; 3 cycles
-        audio_play_raw ; 8-10 cycles
+        audio_play_hi
 
+        ldy scanline   ; 3 cycles
         inner_scanline frame_offset, 128 ; 23-26 cycles before break, 12 cycles after
 
         ; pair cleanup: 6 cycles
diff --git a/pack-wav.js b/pack-wav.js
index f8ee37f..50379a0 100644
--- a/pack-wav.js
+++ b/pack-wav.js
@@ -14,31 +14,44 @@ function byte2byte(arr) {
     return lines.join('\n');
 }
 
-function to4bit(val8) {
-    let val = (val8 + 7) >> 4;
-    if (val > 15) {
-        return 15;
+class Dither {
+    constructor() {
+        this.err = 0;
+    }
+
+    to4bit(val8) {
+        let val = (val8 / 255) - this.err;
+        if (val < 0) {
+            val = 0;
+        }
+        if (val > 1) {
+            val = 1;
+        }
+        let val4 = Math.round(val * 15);
+        let dithered = (val4 / 15);
+        this.err = (dithered - val);
+        return val4;
     }
-    return val;
 }
 
 function pack(audio) {
     let packed = [];
-    /*
+    let dither = new Dither();
     for (let i = 0; i < audio.length; i += 2) {
         // little-endian 4-bit samples
-        let low = to4bit(audio[i]);
-        let high = to4bit(audio[i + 1]);
+        let low = dither.to4bit(audio[i]);
+        let high = dither.to4bit(audio[i + 1]);
         let byte = low | (high << 4);
         packed.push(byte);
     }
-    */
+    /*
     // raw push bytes
     for (let i = 0; i < audio.length; i += 2) {
         let val = to4bit(audio[i]);
         let byte = val | 0x10;
         packed.push(byte);
     }
+    */
     return packed;
 }