rolled my own downsampler

gb-emu/src/main.rs

@@ -132,15 +132,15 @@ fn create_audio_output() -> (AudioOutput, Stream) {
 
     let sample_rate = config.sample_rate().0;
 
-    let (output, mut rx) = AudioOutput::new(sample_rate);
+    let (output, mut rx) = AudioOutput::new(sample_rate as f32);
 
     let stream = device
         .build_output_stream(
             &config.config(),
             move |data: &mut [f32], _info: &cpal::OutputCallbackInfo| {
-                for v in data {
+                for v in data.chunks_exact_mut(2) {
                     match executor::block_on(rx.pop()) {
-                        Some(a) => *v = a,
+                        Some(a) => v.copy_from_slice(&a),
                         None => panic!("Audio queue disconnected!"),
                     }
                 }

lib/Cargo.toml

@@ -7,7 +7,6 @@ edition = "2021"
 
 [dependencies]
 rand = "0.8.5"
-samplerate = "0.2.4"
 async-ringbuf = "0.1.2"
 futures = "0.3.26"
 once_cell = "1.17.1"

lib/src/connect/mod.rs

@@ -24,18 +24,18 @@ pub trait Renderer {
 }
 
 pub struct AudioOutput {
-    pub sample_rate: u32,
+    pub sample_rate: f32,
-    pub send_rb: AsyncHeapProducer<f32>,
+    pub send_rb: AsyncHeapProducer<[f32; 2]>,
 }
 
 impl AudioOutput {
-    pub fn new(sample_rate: u32) -> (Self, AsyncHeapConsumer<f32>) {
+    pub fn new(sample_rate: f32) -> (Self, AsyncHeapConsumer<[f32; 2]>) {
-        let rb_len = sample_rate as usize / 60;
+        let rb_len = sample_rate as usize / (60 * 2);
 
-        let rb = AsyncHeapRb::<f32>::new(rb_len);
+        let rb = AsyncHeapRb::<[f32; 2]>::new(rb_len);
         let (mut send_rb, rx) = rb.split();
 
-        executor::block_on(send_rb.push_iter(vec![0.; rb_len - 1].into_iter())).unwrap();
+        executor::block_on(send_rb.push_iter(vec![[0.; 2]; rb_len - 1].into_iter())).unwrap();
 
         (
             Self {

lib/src/processor/memory/mmio/apu.rs

@@ -1,19 +1,21 @@
-use self::types::{Channels, DacSample, Mixer, VinEnable, Volume};
+use self::{
+    downsampler::Downsampler,
+    types::{Channels, DacSample, Mixer, VinEnable, Volume},
+};
 use crate::{
     connect::AudioOutput,
-    constants::CLOCK_SPEED,
     processor::memory::Address,
     util::{get_bit, set_or_clear_bit},
 };
 use futures::executor;
 use itertools::izip;
-use samplerate::{ConverterType, Samplerate};
 
 mod channels;
+mod downsampler;
 mod types;
 
 impl DacSample {
-    fn mixed(&self, mixer: &Mixer) -> Vec<f32> {
+    fn mixed(&self, mixer: &Mixer) -> [f32; 2] {
         let left = (self.one * mixer.ch1.left.scale())
             + (self.two * mixer.ch2.left.scale())
             + (self.three * mixer.ch3.left.scale())
@@ -22,7 +24,7 @@ impl DacSample {
             + (self.two * mixer.ch2.right.scale())
             + (self.three * mixer.ch3.right.scale())
             + (self.four * mixer.ch4.right.scale());
-        vec![
+        [
             self.mix_channel(left, mixer.vol_left),
             self.mix_channel(right, mixer.vol_right),
         ]
@@ -40,7 +42,7 @@ pub struct Apu {
     mixer: Mixer,
     div_apu: u8,
     buffer: Vec<DacSample>,
-    converter: Samplerate,
+    converter: Downsampler,
     output: AudioOutput,
 }
 
@@ -48,14 +50,6 @@ const CYCLES_PER_FRAME: usize = 70224;
 
 impl Apu {
     pub fn new(output: AudioOutput) -> Self {
-        let converter = Samplerate::new(
-            ConverterType::ZeroOrderHold,
-            CLOCK_SPEED as u32,
-            output.sample_rate,
-            2,
-        )
-        .unwrap();
-
         Self {
             apu_enable: true,
             channels: Channels::default(),
@@ -63,7 +57,7 @@ impl Apu {
             mixer: Mixer::default(),
             div_apu: 0,
             buffer: vec![],
-            converter,
+            converter: Downsampler::new(output.sample_rate),
             output,
         }
     }
@@ -111,16 +105,12 @@ impl Apu {
     }
 
     fn next_audio(&mut self) {
-        let converted = self
+        let converted = self.converter.process(
-            .converter
+            self.buffer
-            .process(
-                &self
-                    .buffer
                 .drain(..)
-                    .flat_map(|v| v.mixed(&self.mixer))
+                .map(|v| v.mixed(&self.mixer))
-                    .collect::<Vec<f32>>(),
+                .collect::<Vec<[f32; 2]>>(),
-            )
+        );
-            .unwrap();
         executor::block_on(self.output.send_rb.push_slice(&converted)).unwrap();
     }
 

lib/src/processor/memory/mmio/apu/downsampler.rs

@@ -0,0 +1,29 @@
+use crate::constants::CLOCK_SPEED;
+
+const TIME_PER_CYCLE: f32 = 1. / CLOCK_SPEED as f32;
+
+pub(super) struct Downsampler {
+    ratio: f32,
+    time_accum: f32,
+}
+
+impl Downsampler {
+    pub fn new(sample_rate: f32) -> Self {
+        Self {
+            ratio: 1. / sample_rate,
+            time_accum: 0.,
+        }
+    }
+
+    pub fn process(&mut self, signal: Vec<[f32; 2]>) -> Vec<[f32; 2]> {
+        let mut output = vec![];
+        for val in signal {
+            self.time_accum += TIME_PER_CYCLE;
+            if self.time_accum >= self.ratio {
+                self.time_accum = 0.;
+                output.push(val);
+            }
+        }
+        output
+    }
+}