add lowpass filter from StSound

src/chips/ay3891.zig

@@ -4,7 +4,7 @@
 const bitutils = @import("common").bitutils;
 const mask = bitutils.mask;
 const maskm = bitutils.maskm;
-const dcadjust = @import("common").dcadjust;
+const filter = @import("common").filter;
 
 /// map chip pin names to bit positions
 ///
@@ -46,6 +46,8 @@ pub const Config = struct {
     model: Model = .AY38910,
     pins: Pins,
     bus: type,
+    dcadjust_buf_len: u32 = 128,
+    enable_lowpass_filter: bool = true,
 };
 
 pub fn Type(comptime cfg: Config) type {
@@ -177,7 +179,11 @@ pub fn Type(comptime cfg: Config) type {
             volume: f32 = 0.0,
             value: f32 = 0.0,
             ready: bool = false, // true if a new sample value is ready
-            dcadj: dcadjust.Type(.{ .buf_len = 128 }) = .{},
+            filter: filter.Type(.{
+                .enable_dcadjust = true,
+                .enable_lowpass_filter = cfg.enable_lowpass_filter,
+                .dcadjust_buf_len = cfg.dcadjust_buf_len,
+            }) = .{},
         };
 
         tick_count: u38 = 0, // tick counter for internal clock division
@@ -240,7 +246,7 @@ pub fn Type(comptime cfg: Config) type {
 
         pub fn reset(self: *Self) void {
             self.active = false;
-            self.sample.dcadjust.reset();
+            self.sample.filter.reset();
             self.addr = 0;
             self.tick_count = 0;
             for (&self.regs) |r| {
@@ -342,7 +348,7 @@ pub fn Type(comptime cfg: Config) type {
                         }
                     }
                 }
-                self.sample.value = self.sample.dcadj.put(sm) * self.sample.volume;
+                self.sample.value = self.sample.filter.put(sm) * self.sample.volume;
                 self.sample.ready = true;
             } else {
                 self.sample.ready = false;

src/common/common.zig

@@ -2,4 +2,4 @@ pub const bitutils = @import("bitutils.zig");
 pub const memory = @import("memory.zig");
 pub const glue = @import("glue.zig");
 pub const clock = @import("clock.zig");
-pub const dcadjust = @import("dcadjust.zig");
+pub const filter = @import("filter.zig");

src/common/filter.zig

@@ -0,0 +1,57 @@
+//! audio filter (dc adjustment and lowpass filter)
+//! taken from: https://github.com/arnaud-carre/StSound
+const std = @import("std");
+const assert = std.debug.assert;
+const isPowerOfTwo = std.math.isPowerOfTwo;
+
+pub const Config = struct {
+    enable_dcadjust: bool,
+    enable_lowpass_filter: bool,
+    dcadjust_buf_len: u32,
+};
+
+pub fn Type(cfg: Config) type {
+    assert(isPowerOfTwo(cfg.dcadjust_buf_len));
+    return struct {
+        const Self = @This();
+
+        lopass: [2]f32 = [_]f32{0} ** 2,
+        dcadj: struct {
+            sum: f32 = 0,
+            pos: u32 = 0,
+            buf: [cfg.dcadjust_buf_len]f32 = [_]f32{0} ** cfg.dcadjust_buf_len,
+        } = .{},
+
+        pub fn reset(self: *Self) void {
+            self.* = .{};
+        }
+
+        pub fn put(self: *Self, in: f32) f32 {
+            var s = in;
+            if (cfg.enable_dcadjust) {
+                s = self.dcAdjust(s);
+            }
+            if (cfg.enable_lowpass_filter) {
+                s = self.lowPassFilter(s);
+            }
+            return s;
+        }
+
+        fn dcAdjust(self: *Self, in: f32) f32 {
+            const pos = self.dcadj.pos;
+            self.dcadj.sum -= self.dcadj.buf[pos];
+            self.dcadj.sum += in;
+            self.dcadj.buf[pos] = in;
+            self.dcadj.pos = (pos + 1) & (cfg.dcadjust_buf_len - 1);
+            const div: f32 = @floatFromInt(cfg.dcadjust_buf_len);
+            return in - (self.dcadj.sum / div);
+        }
+
+        fn lowPassFilter(self: *Self, in: f32) f32 {
+            const out = self.lopass[0] * 0.25 + self.lopass[1] * 0.5 + in * 0.25;
+            self.lopass[0] = self.lopass[1];
+            self.lopass[1] = in;
+            return out;
+        }
+    };
+}

src/systems/namco.zig

@@ -38,6 +38,7 @@ const z80 = @import("chips").z80;
 const common = @import("common");
 const memory = common.memory;
 const clock = common.clock;
+const filter = common.filter;
 const pin = common.bitutils.pin;
 const AudioCallback = common.glue.AudioCallback;
 const AudioOptions = common.glue.AudioOptions;
@@ -386,6 +387,11 @@ pub fn Type(comptime sys: System) type {
             sample_counter: i32,
             volume: f32,
             voices: [3]AudioVoice = [_]AudioVoice{.{}} ** 3,
+            filter: filter.Type(.{
+                .enable_dcadjust = false,
+                .enable_lowpass_filter = true,
+                .dcadjust_buf_len = 2,
+            }) = .{},
             num_samples: u32,
             sample_pos: u32,
             callback: AudioCallback,
@@ -918,7 +924,7 @@ pub fn Type(comptime sys: System) type {
                     }
                 }
                 sm *= snd.volume * 0.333333;
-                snd.sample_buffer[snd.sample_pos] = sm;
+                snd.sample_buffer[snd.sample_pos] = self.audio.filter.put(sm);
                 snd.sample_pos += 1;
                 if (snd.sample_pos == snd.num_samples) {
                     if (snd.callback) |cb| {