From 3818ecf193c6950ae0034e89baca7fd2726aa42a Mon Sep 17 00:00:00 2001
From: alyosha-tas <alexei.f.k@gmail.com>
Date: Mon, 4 Dec 2017 14:39:41 -0500
Subject: [PATCH] GBHawk: Implement obscure audio effects

---
 .../Consoles/Nintendo/GBHawk/Audio.cs         | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Audio.cs b/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Audio.cs
index 28b9c2edf2..6af2ade6ce 100644
--- a/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Audio.cs
+++ b/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Audio.cs
@@ -188,11 +188,20 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 						SQ1_len_cntr = SQ1_length;
 						break;
 					case 0xFF12:                                        // NR12 (envelope)
-						Audio_Regs[NR12] = value;
 						SQ1_st_vol = (byte)((value & 0xF0) >> 4);
 						SQ1_env_add = (value & 8) > 0;
 						SQ1_per = (byte)(value & 7);
+
+						// several glitchy effects happen when writing to NRx2 during audio playing
+						if (((Audio_Regs[NR12] & 7) == 0) && !SQ1_vol_done) { SQ1_vol_state++; }
+						else if ((Audio_Regs[NR12] & 8) == 0) { SQ1_vol_state += 2; }
+
+						if (((Audio_Regs[NR12] ^ value) & 8) > 0) { SQ1_vol_state = (byte)(0x10 - SQ1_vol_state); }
+
+						SQ1_vol_state &= 0xF;
+
 						if ((value & 0xF8) == 0) { SQ1_enable = SQ1_swp_enable = false; SQ1_output = 0; }
+						Audio_Regs[NR12] = value;
 						break;
 					case 0xFF13:                                        // NR13 (freq low)
 						Audio_Regs[NR13] = value;
@@ -269,11 +278,19 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 						SQ2_len_cntr = SQ2_length;
 						break;
 					case 0xFF17:                                        // NR22 (envelope)
-						Audio_Regs[NR22] = value;
 						SQ2_st_vol = (byte)((value & 0xF0) >> 4);
 						SQ2_env_add = (value & 8) > 0;
 						SQ2_per = (byte)(value & 7);
+
+						// several glitchy effects happen when writing to NRx2 during audio playing
+						if (((Audio_Regs[NR22] & 7) == 0) && !SQ2_vol_done) { SQ2_vol_state++; }						
+						else if ((Audio_Regs[NR22] & 8) == 0) { SQ2_vol_state += 2; }
+							
+						if (((Audio_Regs[NR22] ^ value) & 8) > 0) { SQ2_vol_state = (byte)(0x10 - SQ2_vol_state); }
+							
+						SQ2_vol_state &= 0xF;
 						if ((value & 0xF8) == 0) { SQ2_enable = false; SQ2_output = 0; }
+						Audio_Regs[NR22] = value;
 						break;
 					case 0xFF18:                                        // NR23 (freq low)
 						Audio_Regs[NR23] = value;