Improve sample rounding and clean up noise shaping leftovers (#771)

Author: roderickvd

Cargo.lock

@@ -20,6 +20,7 @@ version = "0.2.0"
 [dependencies]
 futures-executor = "0.3"
 futures-util = { version = "0.3", default_features = false, features = ["alloc"] }
+libmath = "0.2"
 log = "0.4"
 byteorder = "1.4"
 shell-words = "1.0.0"

playback/Cargo.toml

@@ -32,18 +32,19 @@ impl Converter {
 
     // Denormalize and dither
     pub fn scale(&mut self, sample: f32, factor: i64) -> f32 {
+        let dither = match self.ditherer {
+            Some(ref mut d) => d.noise(),
+            None => 0.0,
+        };
+
         // From the many float to int conversion methods available, match what
         // the reference Vorbis implementation uses: sample * 32768 (for 16 bit)
-        let int_value = sample * factor as f32;
+        let int_value = sample * factor as f32 + dither;
 
-        // https://doc.rust-lang.org/nomicon/casts.html: casting float to integer
-        // rounds towards zero, then saturates. Ideally halves should round to even to
-        // prevent any bias, but since it is extremely unlikely that a float has
-        // *exactly* .5 as fraction, this should be more than precise enough.
-        match self.ditherer {
-            Some(ref mut d) => int_value + d.noise(int_value),
-            None => int_value,
-        }
+        // Casting float to integer rounds towards zero by default, i.e. it
+        // truncates, and that generates larger error than rounding to nearest.
+        // Absolute lowest error is gained from rounding ties to even.
+        math::round::half_to_even(int_value.into(), 0) as f32
     }
 
     // Special case for samples packed in a word of greater bit depth (e.g.

playback/src/convert.rs

@@ -32,7 +32,7 @@ pub trait Ditherer {
     where
         Self: Sized;
     fn name(&self) -> &'static str;
-    fn noise(&mut self, sample: f32) -> f32;
+    fn noise(&mut self) -> f32;
 }
 
 impl fmt::Display for dyn Ditherer {
@@ -64,7 +64,7 @@ impl Ditherer for TriangularDitherer {
         "Triangular"
     }
 
-    fn noise(&mut self, _sample: f32) -> f32 {
+    fn noise(&mut self) -> f32 {
         self.distribution.sample(&mut self.cached_rng)
     }
 }
@@ -87,7 +87,7 @@ impl Ditherer for GaussianDitherer {
         "Gaussian"
     }
 
-    fn noise(&mut self, _sample: f32) -> f32 {
+    fn noise(&mut self) -> f32 {
         self.distribution.sample(&mut self.cached_rng)
     }
 }
@@ -113,7 +113,7 @@ impl Ditherer for HighPassDitherer {
         "Triangular, High Passed"
     }
 
-    fn noise(&mut self, _sample: f32) -> f32 {
+    fn noise(&mut self) -> f32 {
         let new_noise = self.distribution.sample(&mut self.cached_rng);
         let high_passed_noise = new_noise - self.previous_noises[self.active_channel];
         self.previous_noises[self.active_channel] = new_noise;