Updated flat sigmoid to match birdnet_analyzer (#25)

* Updated flat sigmoid to match birdnet_analyzer

* add changelog

---------

Co-authored-by: Stefan Taubert <23339395+stefantaubert@users.noreply.github.com>

Author: Josef-Haupt

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added convenience functions to export embeddings by @Josef-Haupt
 - Added some code documentation
 
+### Changed
+
+- Updated flat sigmoid to match birdnet_analyzer by @Josef-Haupt
+
 ### Bugfixes
 
 - Fixed issue with one test on Python 3.11 & 3.12

src/birdnet/utils/helper.py

@@ -316,9 +316,10 @@ def fillup_with_silence(
 
 
 def flat_sigmoid_logaddexp_fast(
-  x: npt.NDArray, sensitivity: float, clip_val: float = 15.0
+  x: npt.NDArray, sensitivity: float, clip_val: float = 15.0, bias: float = 1.0
 ) -> npt.NDArray:
-  y = sensitivity * np.clip(x, -clip_val, clip_val)
+  transformed_bias = (bias - 1.0) * 10.0
+  y = sensitivity * np.clip(x + transformed_bias, -clip_val, clip_val)
 
   positive_mask = y >= 0
   abs_y = np.abs(y)