Integration: AudioSuite speed ramp: Fit To plays the first two thirds for Slow Down and the last for Speed Up

crates/dsp/src/offline.rs

@@ -465,7 +465,8 @@ pub enum SpeedRampChange {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum SpeedRampSelection {
     /// The result is as long as the input. Two thirds of the input are played: the first two
-    /// thirds for Speed Up, the last two thirds for Slow Down.
+    /// thirds for Slow Down, the last two thirds for Speed Up, so each joins its neighbour
+    /// without a jump.
     FitTo,
     /// All of the input is played, and the result is 150 % of its length.
     Extend,
@@ -482,7 +483,7 @@ pub fn speed_ramp(ch: &[Vec<f32>], change: SpeedRampChange, selection: SpeedRamp
         SpeedRampSelection::Extend => ((len * 1.5).round(), 0.0, len),
         SpeedRampSelection::FitTo => {
             let played = len * 2.0 / 3.0;
-            (len, if change == SpeedRampChange::SlowDown { len - played } else { 0.0 }, played)
+            (len, if change == SpeedRampChange::SlowDown { 0.0 } else { len - played }, played)
         }
     };
     let out_len = (total as usize).min(MAX_OFFLINE_SAMPLES);

crates/dsp/tests/it/speed_ramp.rs

@@ -24,21 +24,22 @@ fn speed_ramp_fit_to_keeps_the_length_and_plays_two_thirds() {
     let x = vec![ramp(len)];
     let slow = offline::speed_ramp(&x, SpeedRampChange::SlowDown, SpeedRampSelection::FitTo, false);
     assert_eq!(slow[0].len(), len);
-    // Slow Down plays the last two thirds: it starts at normal speed one third in and stops at the end.
+    // Slow Down plays the first two thirds: it starts at normal speed at the start and stops at the end.
     for u in [0.0, 0.25, 0.5, 0.75, 0.9] {
         let n = (u * len as f64) as usize;
-        near(slow[0][n], 1.0 / 3.0 + 2.0 / 3.0 * slow_down_pos(u), &format!("slow down at {u}"));
+        near(slow[0][n], 2.0 / 3.0 * slow_down_pos(u), &format!("slow down at {u}"));
     }
-    let step = |a: &[f32], n: usize| f64::from(a[n + 1] - a[n]) * len as f64;
+    // The speed over 50 samples: a one-sample step inside the input also shows the interpolation's ripple.
+    let step = |a: &[f32], n: usize| f64::from(a[n + 50] - a[n]) * len as f64 / 50.0;
     assert!((step(&slow[0], 100) - 1.0).abs() < 0.01, "slow down starts at normal speed: {}", step(&slow[0], 100));
     assert!(step(&slow[0], len - 200) < 0.02, "slow down ends near a stop: {}", step(&slow[0], len - 200));
 
-    // Speed Up plays the first two thirds: it starts from a stop and reaches normal speed at the end.
+    // Speed Up plays the last two thirds: it starts from a stop and reaches normal speed at the end.
     let fast = offline::speed_ramp(&x, SpeedRampChange::SpeedUp, SpeedRampSelection::FitTo, false);
     assert_eq!(fast[0].len(), len);
     for u in [0.1, 0.25, 0.5, 0.75, 0.99] {
         let n = (u * len as f64) as usize;
-        near(fast[0][n], 2.0 / 3.0 * speed_up_pos(u), &format!("speed up at {u}"));
+        near(fast[0][n], 1.0 / 3.0 + 2.0 / 3.0 * speed_up_pos(u), &format!("speed up at {u}"));
     }
     assert!(step(&fast[0], 200) < 0.02, "speed up starts near a stop: {}", step(&fast[0], 200));
     assert!((step(&fast[0], len - 100) - 1.0).abs() < 0.01, "speed up ends at normal speed: {}", step(&fast[0], len - 100));

crates/engine/src/cmd/audiosuite.rs

@@ -274,10 +274,10 @@ mod tests {
             (c.start, c.length, c.name, audio)
         };
         // The defaults: Slow Down, Fit To, Fades On. Halfway through, the read position is 11/16 of the
-        // way through the last two thirds, and the gain is the speed there, 3/4.
+        // way through the first two thirds, and the gain is the speed there, 3/4.
         let (start, length, name, audio) = run(json!({}));
         assert_eq!((start, length, name.as_str()), (480, 4800, "ramp-speeramp"));
-        let want = (1.0 / 3.0 + 2.0 / 3.0 * 11.0 / 16.0) * 0.75;
+        let want = (2.0 / 3.0 * 11.0 / 16.0) * 0.75;
         assert!((audio[2400] - want).abs() < 2e-3, "slow down halfway: got {}, want {want}", audio[2400]);
         assert!(audio[4790].abs() < 0.01, "the fade-out ends near silence, got {}", audio[4790]);
         // Speed Up with Extend and no fades: all of the clip is played over 150 % of its length, and the