Expose internal audio RF filter option

src/drivers/libretro/libretro.c

@@ -1136,6 +1136,7 @@ static bool update_option_visibility(void)
          char options_list[][25] = {
             "fceumm_sndvolume",
             "fceumm_sndquality",
+            "fceumm_sndlowpass",
             "fceumm_swapduty",
             "fceumm_apu_1",
             "fceumm_apu_2",
@@ -1822,6 +1823,14 @@ static void check_variables(bool startup)
          FCEUI_SetSoundQuality(sndquality);
    }
 
+   var.key = "fceumm_sndlowpass";
+
+   if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)
+   {
+      int lowpass = (!strcmp(var.value, "enabled")) ? 1 : 0;
+      FCEUI_SetLowPass(lowpass);
+   }
+
    var.key = "fceumm_sndvolume";
 
    if (environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var) && var.value)

src/drivers/libretro/libretro_core_options.h

@@ -21,7 +21,7 @@ extern "C" {
  ********************************
 */
 
-#define MAX_CORE_OPTIONS 35
+#define MAX_CORE_OPTIONS 36
 
 /* RETRO_LANGUAGE_ENGLISH */
 
@@ -254,6 +254,20 @@ struct retro_core_option_v2_definition option_defs[] = {
       },
       "Low",
    },
+   {
+      "fceumm_sndlowpass",
+      "Audio RF Filter",
+      NULL,
+      "Apply a low pass audio filter to simulate the 'muted' sound of the NES when connected to a television via the RF modulator.",
+      NULL,
+      "audio",
+      {
+         { "disabled", NULL },
+         { "enabled",  NULL },
+         { NULL, NULL },
+      },
+      "disabled",
+   },
    {
       "fceumm_swapduty",
       "Swap Audio Duty Cycles",