Snes9x2005 Non-Plus: Add optional low pass audio filter

4 files changed, 202 insertions, 20 deletions

diff --git a/libretro.c b/libretro.c
index 2b04506..f64b71c 100644
--- a/libretro.c
+++ b/libretro.c
@@ -62,6 +62,11 @@ static float audio_samples_per_frame   = 0.0f;
 static float audio_samples_accumulator = 0.0f;
 #endif
 
+#ifndef USE_BLARGG_APU
+static bool audio_low_pass_enabled  = false;
+static int32_t audio_low_pass_range = (60 * 65536) / 100;
+#endif
+
 static unsigned frameskip_type             = 0;
 static unsigned frameskip_threshold        = 0;
 static uint16_t frameskip_counter          = 0;
@@ -412,7 +417,12 @@ static void audio_upload_samples(void)
       audio_samples_accumulator -= 1.0f;
    }
 
-   S9xMixSamples(audio_out_buffer, available_frames << 1);
+   if (audio_low_pass_enabled)
+      S9xMixSamplesLowPass(audio_out_buffer,
+            available_frames << 1, audio_low_pass_range);
+   else
+      S9xMixSamples(audio_out_buffer, available_frames << 1);
+
    audio_batch_cb(audio_out_buffer, available_frames);
 #endif
 }
@@ -474,6 +484,10 @@ void retro_deinit(void)
    frameskip_type             = 0;
    frameskip_threshold        = 0;
    frameskip_counter          = 0;
+#ifndef USE_BLARGG_APU
+   audio_low_pass_enabled     = false;
+   audio_low_pass_range       = (60 * 65536) / 100;
+#endif
    retro_audio_buff_active    = false;
    retro_audio_buff_occupancy = 0;
    retro_audio_buff_underrun  = false;
@@ -528,6 +542,9 @@ static void check_variables(bool first_run)
       var.key = "snes9x_2005_region";
       var.value = NULL;
 
+      Settings.ForceNTSC = 0;
+      Settings.ForcePAL  = 0;
+
       if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
       {
          Settings.ForceNTSC = !strcmp(var.value, "NTSC");
@@ -557,9 +574,30 @@ static void check_variables(bool first_run)
    if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
       frameskip_threshold = strtol(var.value, NULL, 10);
 
+#ifndef USE_BLARGG_APU
+   var.key = "snes9x_2005_low_pass_filter";
+   var.value = NULL;
+
+   audio_low_pass_enabled = false;
+
+   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+      if (strcmp(var.value, "enabled") == 0)
+         audio_low_pass_enabled = true;
+
+   var.key = "snes9x_2005_low_pass_range";
+   var.value = NULL;
+
+   audio_low_pass_range = (60 * 0x10000) / 100;
+
+	if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+      audio_low_pass_range = (strtol(var.value, NULL, 10) * 0x10000) / 100;
+#endif
+
    var.key = "snes9x_2005_overclock_cycles";
    var.value = NULL;
 
+   overclock_cycles = false;
+
    if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
    {
       if (strcmp(var.value, "compatible") == 0)
@@ -576,20 +614,16 @@ static void check_variables(bool first_run)
          slow_one_c = 3;
          two_c = 3;
       }
-      else
-         overclock_cycles = false;
    }
 
    var.key = "snes9x_2005_reduce_sprite_flicker";
    var.value = NULL;
 
+   reduce_sprite_flicker = false;
+
    if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
-   {
       if (strcmp(var.value, "enabled") == 0)
          reduce_sprite_flicker = true;
-      else
-         reduce_sprite_flicker = false;
-   }
 
    /* Reinitialise frameskipping, if required */
    if (!first_run &&
diff --git a/libretro_core_options.h b/libretro_core_options.h
index 6f46d16..28bb210 100644
--- a/libretro_core_options.h
+++ b/libretro_core_options.h
@@ -122,6 +122,53 @@ struct retro_core_option_v2_definition option_defs_us[] = {
       },
       "33"
    },
+#ifndef USE_BLARGG_APU
+   {
+      "snes9x_2005_low_pass_filter",
+      "Audio Filter",
+      NULL,
+      "Enable a low pass audio filter to soften treble artifacts and mimic the 'bassy' sound of original SNES hardware.",
+      NULL,
+      NULL,
+      {
+         { "disabled", NULL },
+         { "enabled",  NULL },
+         { NULL, NULL },
+      },
+      "disabled"
+   },
+   {
+      "snes9x_2005_low_pass_range",
+      "Audio Filter Level",
+      NULL,
+      "Specify the cut-off frequency of the low pass audio filter. A higher value increases the perceived 'strength' of the filter, since a wider range of the high frequency spectrum is attenuated.",
+      NULL,
+      NULL,
+      {
+         { "5",  "5%" },
+         { "10", "10%" },
+         { "15", "15%" },
+         { "20", "20%" },
+         { "25", "25%" },
+         { "30", "30%" },
+         { "35", "35%" },
+         { "40", "40%" },
+         { "45", "45%" },
+         { "50", "50%" },
+         { "55", "55%" },
+         { "60", "60%" },
+         { "65", "65%" },
+         { "70", "70%" },
+         { "75", "75%" },
+         { "80", "80%" },
+         { "85", "85%" },
+         { "90", "90%" },
+         { "95", "95%" },
+         { NULL, NULL },
+      },
+      "60"
+   },
+#endif
    {
       "snes9x_2005_overclock_cycles",
       "Reduce Slowdown (Hack, Unsafe, Restart)",
diff --git a/source/soundux.c b/source/soundux.c
index 66b546b..1c59ff0 100644
--- a/source/soundux.c
+++ b/source/soundux.c
@@ -8,25 +8,18 @@
 
 #include <retro_inline.h>
 
-#define CLIP16(v) \
-if ((v) < -32768) \
-    (v) = -32768; \
-else \
-if ((v) > 32767) \
-    (v) = 32767
-
-#define CLIP8(v) \
-if ((v) < -128) \
-    (v) = -128; \
-else if ((v) > 127) \
-         (v) = 127
-
 #include "snes9x.h"
 #include "soundux.h"
 #include "apu.h"
 #include "memmap.h"
 #include "cpuexec.h"
 
+#define CLIP16(v) \
+(v) = (((v) <= -32768) ? -32768 : (((v) >= 32767) ? 32767 : (v)))
+
+#define CLIP8(v) \
+(v) = (((v) <= -128) ? -128 : (((v) >= 127) ? 127 : (v)))
+
 extern int32_t Echo [24000];
 extern int32_t MixBuffer [SOUND_BUFFER_SIZE];
 extern int32_t EchoBuffer [SOUND_BUFFER_SIZE];
@@ -89,6 +82,15 @@ uint32_t KeyOffERate         [10];
 #define LAST_SAMPLE 0xffffff
 #define JUST_PLAYED_LAST_SAMPLE(c) ((c)->sample_pointer >= LAST_SAMPLE)
 
+/* Used by S9xMixSamplesLowPass() to store the
+ * last output samples for the next iteration
+ * of the low pass filter. This should go in
+ * the SSoundData struct, but doing so would
+ * break compatibility with existing save
+ * states (with little in the way of tangible
+ * benefits) */
+static int32_t MixOutputPrev[2];
+
 static INLINE uint8_t* S9xGetSampleAddress(int32_t sample_number)
 {
    uint32_t addr = (((APU.DSP[APU_DIR] << 8) + (sample_number << 2)) & 0xffff);
@@ -807,6 +809,102 @@ void S9xMixSamples(int16_t* buffer, int32_t sample_count)
    }
 }
 
+void S9xMixSamplesLowPass(int16_t* buffer, int32_t sample_count, int32_t low_pass_range)
+{
+   int32_t J;
+   int32_t I;
+
+   /* Single-pole low-pass filter (6 dB/octave) */
+   int32_t low_pass_factor_a = low_pass_range;
+   int32_t low_pass_factor_b = 0x10000 - low_pass_factor_a;
+
+   if (SoundData.echo_enable)
+      memset(EchoBuffer, 0, sample_count * sizeof(EchoBuffer [0]));
+   memset(MixBuffer, 0, sample_count * sizeof(MixBuffer [0]));
+   MixStereo(sample_count);
+
+   /* Mix and convert waveforms */
+   if (SoundData.echo_enable && SoundData.echo_buffer_size)
+   {
+      /* 16-bit stereo sound with echo enabled ... */
+      if (FilterTapDefinitionBitfield == 0)
+      {
+         /* ... but no filter defined. */
+         for (J = 0; J < sample_count; J++)
+         {
+            int32_t *low_pass_sample = &MixOutputPrev[J & 0x1];
+            int32_t E = Echo [SoundData.echo_ptr];
+            Echo[SoundData.echo_ptr++] = (E * SoundData.echo_feedback) / 128 + EchoBuffer [J];
+
+            if (SoundData.echo_ptr >= SoundData.echo_buffer_size)
+               SoundData.echo_ptr = 0;
+
+            I = (MixBuffer[J] * SoundData.master_volume [J & 1] + E * SoundData.echo_volume [J & 1]) / VOL_DIV16;
+            CLIP16(I);
+
+            /* Apply low-pass filter */
+            (*low_pass_sample) = ((*low_pass_sample) * low_pass_factor_a) + (I * low_pass_factor_b);
+            /* 16.16 fixed point */
+            (*low_pass_sample) >>= 16;
+
+            buffer[J] = (int16_t)(*low_pass_sample);
+         }
+      }
+      else
+      {
+         /* ... with filter defined. */
+         for (J = 0; J < sample_count; J++)
+         {
+            int32_t *low_pass_sample = &MixOutputPrev[J & 0x1];
+            int32_t E;
+            Loop [(Z - 0) & 15] = Echo [SoundData.echo_ptr];
+            E =  Loop [(Z -  0) & 15] * FilterTaps [0];
+            if (FilterTapDefinitionBitfield & 0x02) E += Loop [(Z -  2) & 15] * FilterTaps [1];
+            if (FilterTapDefinitionBitfield & 0x04) E += Loop [(Z -  4) & 15] * FilterTaps [2];
+            if (FilterTapDefinitionBitfield & 0x08) E += Loop [(Z -  6) & 15] * FilterTaps [3];
+            if (FilterTapDefinitionBitfield & 0x10) E += Loop [(Z -  8) & 15] * FilterTaps [4];
+            if (FilterTapDefinitionBitfield & 0x20) E += Loop [(Z - 10) & 15] * FilterTaps [5];
+            if (FilterTapDefinitionBitfield & 0x40) E += Loop [(Z - 12) & 15] * FilterTaps [6];
+            if (FilterTapDefinitionBitfield & 0x80) E += Loop [(Z - 14) & 15] * FilterTaps [7];
+            E /= 128;
+            Z++;
+
+            Echo[SoundData.echo_ptr++] = (E * SoundData.echo_feedback) / 128 + EchoBuffer[J];
+
+            if (SoundData.echo_ptr >= SoundData.echo_buffer_size)
+               SoundData.echo_ptr = 0;
+
+            I = (MixBuffer[J] * SoundData.master_volume [J & 1] + E * SoundData.echo_volume [J & 1]) / VOL_DIV16;
+            CLIP16(I);
+
+            /* Apply low-pass filter */
+            (*low_pass_sample) = ((*low_pass_sample) * low_pass_factor_a) + (I * low_pass_factor_b);
+            /* 16.16 fixed point */
+            (*low_pass_sample) >>= 16;
+
+            buffer[J] = (int16_t)(*low_pass_sample);
+         }
+      }
+   }
+   else
+   {
+      /* 16-bit mono or stereo sound, no echo */
+      for (J = 0; J < sample_count; J++)
+      {
+         int32_t *low_pass_sample = &MixOutputPrev[J & 0x1];
+         I = (MixBuffer[J] * SoundData.master_volume [J & 1]) / VOL_DIV16;
+         CLIP16(I);
+
+         /* Apply low-pass filter */
+         (*low_pass_sample) = ((*low_pass_sample) * low_pass_factor_a) + (I * low_pass_factor_b);
+         /* 16.16 fixed point */
+         (*low_pass_sample) >>= 16;
+
+         buffer[J] = (int16_t)(*low_pass_sample);
+      }
+   }
+}
+
 void S9xResetSound(bool full)
 {
    int32_t i;
@@ -866,6 +964,8 @@ void S9xResetSound(bool full)
 
    SoundData.master_volume [0] = SoundData.master_volume [1] = 127;
    so.mute_sound = true;
+
+   memset(MixOutputPrev, 0, sizeof(MixOutputPrev));
 }
 
 void S9xSetPlaybackRate(uint32_t playback_rate)
diff --git a/source/soundux.h b/source/soundux.h
index 5cb4218..8a0e02e 100644
--- a/source/soundux.h
+++ b/source/soundux.h
@@ -135,6 +135,7 @@ void S9xFixEnvelope(int32_t channel, uint8_t gain, uint8_t adsr1, uint8_t adsr2)
 void S9xStartSample(int32_t channel);
 
 void S9xMixSamples(int16_t* buffer, int32_t sample_count);
+void S9xMixSamplesLowPass(int16_t* buffer, int32_t sample_count, int32_t low_pass_range);
 void S9xSetPlaybackRate(uint32_t rate);
 #endif
 #endif