From 9d839a0328de52dabe4459cb0c9aef6fe9a9a212 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Fri, 21 Oct 2022 05:12:45 -0300
Subject: [PATCH] fix: remove DC offset from audio output

---
 src/apu.zig  | 5 ++++-
 src/main.zig | 7 +++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/apu.zig b/src/apu.zig
index 5a202fb..fd3b2ea 100644
--- a/src/apu.zig
+++ b/src/apu.zig
@@ -200,7 +200,10 @@ pub const Apu = struct {
             self.stream = SDL.SDL_NewAudioStream(SDL.AUDIO_U16, 2, @intCast(c_int, self.sampleRate()), SDL.AUDIO_U16, 2, host_sample_rate) orelse unreachable;
         }
 
-        _ = SDL.SDL_AudioStreamPut(self.stream, &[2]u16{ final_left, final_right }, 2 * @sizeOf(u16));
+        // If we add 0x4040 to each sample the DC Offset is removed
+        // note: found this through guess and check
+        // FIXME: Pretty sure this is a dirty hack and should be fixed
+        _ = SDL.SDL_AudioStreamPut(self.stream, &[2]u16{ final_left + 0x4040, final_right + 0x4040 }, 2 * @sizeOf(u16));
         self.sched.push(.SampleAudio, self.sampleTicks() -| late);
     }
 
diff --git a/src/main.zig b/src/main.zig
index 58dd840..85f41d7 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -239,10 +239,13 @@ fn initAudio(apu: *Apu) SDL.SDL_AudioDeviceID {
     return dev;
 }
 
-// FIXME: Sometimes, we hear garbage upon program start. Why?
 export fn audioCallback(userdata: ?*anyopaque, stream: [*c]u8, len: c_int) void {
     const apu = @ptrCast(*Apu, @alignCast(8, userdata));
-    _ = SDL.SDL_AudioStreamGet(apu.stream, stream, len);
+    const written = SDL.SDL_AudioStreamGet(apu.stream, stream, len);
+
+    // If we don't write anything, play silence otherwise garbage will be played
+    // FIXME: I don't think this hack to remove DC Offset is acceptable :thinking:
+    if (written == 0) std.mem.set(u8, stream[0..@intCast(usize, len)], 0x80);
 }
 
 fn getSavePath(alloc: std.mem.Allocator) !?[]const u8 {