From f4007769c2f821f8b78e6927c30fdce81172cf2a Mon Sep 17 00:00:00 2001
From: jdgleaver <james@leaver.myzen.co.uk>
Date: Thu, 22 Nov 2018 14:58:34 +0000
Subject: [PATCH] Add optional low pass audio filter

---
 pico/pico.h                  |  2 ++
 pico/sound/sound.c           | 41 ++++++++++++++++++++++++++++++++++++
 platform/libretro/libretro.c | 17 +++++++++++++++
 3 files changed, 60 insertions(+)

diff --git a/pico/pico.h b/pico/pico.h
index ac1550d4..a669215d 100644
--- a/pico/pico.h
+++ b/pico/pico.h
@@ -99,6 +99,8 @@ typedef struct
 	unsigned short overclockM68k;  // overclock the emulated 68k, in %
 
 	int sndRate;                   // rate in Hz
+	unsigned short sndFilter;      // Set low pass sound filter 0: off, 1: on (use integer in case we want to add other filter types later)
+	int32_t sndFilterRange;        // Low pass sound filter range [0, 65536]
 	short *sndOut;                 // PCM output buffer
 	void (*writeSound)(int len);   // write .sndOut callback, called once per frame
 
diff --git a/pico/sound/sound.c b/pico/sound/sound.c
index 95aac128..c828b30e 100644
--- a/pico/sound/sound.c
+++ b/pico/sound/sound.c
@@ -28,6 +28,9 @@ short cdda_out_buffer[2*1152];
 // sn76496
 extern int *sn76496_regs;
 
+// Low pass filter 'previous' samples
+static int32_t lpf_lp;
+static int32_t lpf_rp;
 
 static void dac_recalculate(void)
 {
@@ -86,6 +89,10 @@ PICO_INTERNAL void PsndReset(void)
   // PsndRerate calls YM2612Init, which also resets
   PsndRerate(0);
   timers_reset();
+
+  // Reset low pass filter
+  lpf_lp = 0;
+  lpf_rp = 0;
 }
 
 
@@ -318,6 +325,40 @@ static int PsndRender(int offset, int length)
   if ((PicoIn.AHW & PAHW_32X) && (PicoIn.opt & POPT_EN_PWM))
     p32x_pwm_update(buf32, length, stereo);
 
+  // Apply low pass filter, if required
+  if (PicoIn.sndFilter == 1)
+  {
+    int samples = length;
+    int *out32 = buf32;
+    // Restore previous samples
+    int32_t lpf_l = lpf_lp;
+    int32_t lpf_r = lpf_rp;
+
+    // Single-pole low-pass filter (6 dB/octave)
+    int32_t factor_a = PicoIn.sndFilterRange;
+    int32_t factor_b = 0x10000 - factor_a;
+
+    do
+    {
+      // Apply low-pass filter
+      lpf_l = (lpf_l * factor_a) + (out32[0] * factor_b);
+      lpf_r = (lpf_r * factor_a) + (out32[1] * factor_b);
+
+      // 16.16 fixed point
+      lpf_l >>= 16;
+      lpf_r >>= 16;
+
+      // Update sound buffer
+      *out32++ = lpf_l;
+      *out32++ = lpf_r;
+    }
+    while (--samples);
+
+    // Save last samples for next frame
+    lpf_lp = lpf_l;
+    lpf_rp = lpf_r;
+  }
+
   // convert + limit to normal 16bit output
   PsndMix_32_to_16l(PicoIn.sndOut+offset, buf32, length);
 
diff --git a/platform/libretro/libretro.c b/platform/libretro/libretro.c
index 601f4d36..7eb55637 100644
--- a/platform/libretro/libretro.c
+++ b/platform/libretro/libretro.c
@@ -541,6 +541,8 @@ void retro_set_environment(retro_environment_t cb)
 #ifdef DRC_SH2
       { "picodrive_drc", "Dynamic recompilers; enabled|disabled" },
 #endif
+      { "picodrive_audio_filter", "Audio filter; disabled|low-pass" },
+      { "picodrive_lowpass_range", "Low-pass filter %; 60|65|70|75|80|85|90|95|5|10|15|20|25|30|35|40|45|50|55"},
       { NULL, NULL },
    };
 
@@ -1337,6 +1339,21 @@ static void update_variables(void)
    if(!ctr_svchack_successful)
       PicoIn.opt &= ~POPT_EN_DRC;
 #endif
+
+   var.value = NULL;
+   var.key = "picodrive_audio_filter";
+   PicoIn.sndFilter = 0;
+   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value) {
+      if (strcmp(var.value, "low-pass") == 0)
+         PicoIn.sndFilter = 1;
+   }
+
+   var.value = NULL;
+   var.key = "picodrive_lowpass_range";
+   PicoIn.sndFilterRange = (60 * 65536) / 100;
+   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value) {
+      PicoIn.sndFilterRange = (atoi(var.value) * 65536) / 100;
+   }
 }
 
 void retro_run(void)