Fix #49: Revise the README doc

Change example to illustrate that the interpolated sample tapers to 0 at the
end (to fill the expected length) and to ensure that it does not use an old API.

Author: andrewcsmith

README.md

@@ -72,8 +72,10 @@ assert_eq!(added, vec![[0.4], [-0.5], [-0.3]]);
 
 // Scale the playback rate by `0.5`.
 let foo = [[0.0], [1.0], [0.0], [-1.0]];
-let frames: Vec<_> = foo.iter().cloned().scale_hz(0.5).collect();
-assert_eq!(&frames[..], &[[0.0], [0.5], [1.0], [0.5], [0.0], [-0.5], [-1.0]][..]);
+let mut source = foo.iter().cloned();
+let interp = Linear::from_source(&mut source).unwrap();
+let frames: Vec<_> = source.scale_hz(interp, 0.5).collect();
+assert_eq!(&frames[..], &[[0.0], [0.5], [1.0], [0.5], [0.0], [-0.5], [-1.0], [-0.5]][..]);
 ```
 
 The **signal** module also provides a series of **Signal** source types,
@@ -120,9 +122,12 @@ of sample types. Traits include:
 - `FromFrameSliceMut`, `ToFrameSliceMut`, `DuplexFrameSliceMut`,
 - `DuplexSlice`, `DuplexSliceMut`,
 
-The **rate** module provides a **Converter** type, for converting and
+The **interpolate** module provides a **Converter** type, for converting and
 interpolating the rate of **Signal**s. This can be useful for both sample rate
-conversion and playback rate multiplication.
+conversion and playback rate multiplication. **Converter**s can use a range of
+interpolation methods, with Floor, Linear, and Sinc interpolation provided in
+the library. (NB: Sinc interpolation currently requires heap allocation, as it
+uses VecDeque.)
 
 Using in a `no_std` environment
 -------------------------------

tests/interpolate.rs

@@ -3,6 +3,7 @@
 extern crate sample;
 
 use sample::interpolate::{Converter, Floor, Linear};
+use sample::Signal;
 
 #[test]
 fn test_floor_converter() {
@@ -42,3 +43,12 @@ fn test_linear_converter() {
     assert_eq!(conv.next(), None);
 }
 
+#[test]
+fn test_scale_playback_rate() {
+    // Scale the playback rate by `0.5`
+    let foo = [[0.0], [1.0], [0.0], [-1.0]];
+    let mut source = foo.iter().cloned();
+    let interp = Linear::from_source(&mut source).unwrap();
+    let frames: Vec<_> = source.scale_hz(interp, 0.5).collect();
+    assert_eq!(&frames[..], &[[0.0], [0.5], [1.0], [0.5], [0.0], [-0.5], [-1.0], [-0.5]][..]);
+}