From fa2398109edad38f50f0af3f1c4cc223099a0eb0 Mon Sep 17 00:00:00 2001
From: Roman Fomin <rfomin@gmail.com>
Date: Fri, 9 Dec 2022 17:11:09 +0700
Subject: [PATCH] OPL output gain (#829)

* crude OPL volume gain

* replace SDL_MixAudioFormat with MixAudioFormat with added gain

* remove unnecessary code path, set default gain = 2

* fix gain calculation

* apply gain only to `src1`

* cosmetic changes
---
 opl/opl_sdl.c | 36 ++++++++++++++++++++++++++++++++++--
 src/m_misc.c  |  8 ++++++++
 2 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/opl/opl_sdl.c b/opl/opl_sdl.c
index ad90b80e..2e017861 100644
--- a/opl/opl_sdl.c
+++ b/opl/opl_sdl.c
@@ -22,6 +22,7 @@
 #include <string.h>
 #include <errno.h>
 #include <assert.h>
+#include <limits.h>
 
 #include "SDL.h"
 #include "SDL_mixer.h"
@@ -154,6 +155,38 @@ static void AdvanceTime(unsigned int nsamples)
     SDL_UnlockMutex(callback_queue_mutex);
 }
 
+#if SDL_BYTEORDER == SDL_LIL_ENDIAN
+  #define OPL_SHORT SDL_SwapLE16
+#else
+  #define OPL_SHORT SDL_SwapBE16
+#endif
+
+int opl_gain = 200;
+
+static void MixAudioFormat(Uint8 *dst, const Uint8 *src, Uint32 len)
+{
+    Sint16 src1, src2;
+    int dst_sample;
+
+    while (len--)
+    {
+        src1 = OPL_SHORT(*(Sint16 *)src);
+        src2 = OPL_SHORT(*(Sint16 *)dst);
+        src += 2;
+        dst_sample = src1 * opl_gain / 100 + src2;
+        if (dst_sample > SHRT_MAX)
+        {
+            dst_sample = SHRT_MAX;
+        }
+        else if (dst_sample < SHRT_MIN)
+        {
+            dst_sample = SHRT_MIN;
+        }
+        *(Sint16 *)dst = OPL_SHORT(dst_sample);
+        dst += 2;
+    }
+}
+
 // Call the OPL emulator code to fill the specified buffer.
 
 static void FillBuffer(uint8_t *buffer, unsigned int nsamples)
@@ -166,8 +199,7 @@ static void FillBuffer(uint8_t *buffer, unsigned int nsamples)
     // OPL output is generated into temporary buffer and then mixed
     // (to avoid overflows etc.)
     OPL3_GenerateStream(&opl_chip, (Bit16s *) mix_buffer, nsamples);
-    SDL_MixAudioFormat(buffer, mix_buffer, AUDIO_S16SYS, nsamples * 4,
-                       SDL_MIX_MAXVOLUME);
+    MixAudioFormat(buffer, mix_buffer, nsamples * 2);
 }
 
 // Callback function to fill a new sound buffer:
diff --git a/src/m_misc.c b/src/m_misc.c
index f69c8588..9d90370c 100644
--- a/src/m_misc.c
+++ b/src/m_misc.c
@@ -100,6 +100,7 @@ extern int winmm_reset_delay;
 extern int winmm_reverb_level;
 extern int winmm_chorus_level;
 #endif
+extern int opl_gain;
 extern boolean demobar;
 extern boolean smoothlight;
 extern boolean brightmaps;
@@ -2321,6 +2322,13 @@ default_t defaults[] = {
   },
 #endif
 
+  {
+    "opl_gain",
+    (config_t *) &opl_gain, NULL,
+    {200}, {100, 1000}, number, ss_none, wad_no,
+    "fine tune OPL emulation output level (default 200%)"
+  },
+
 #if defined(_WIN32)
   {
     "winmm_reset_type",