refactor: excitation

[cm-ayf]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.99%. Comparing base (506c136) to head (662ec74).

Files with missing lines	Patch %	Lines
src/vocoder/excitation.rs	94.64%	3 Missing ⚠️
src/vocoder/mod.rs	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   68.43%   68.99%   +0.56%     
==========================================
  Files          39       39              
  Lines        1682     1661      -21     
==========================================
- Hits         1151     1146       -5     
+ Misses        531      515      -16     

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