diff --git a/src/vocoder/coefficients.rs b/src/vocoder/coefficients.rs
index 22c130c..2462788 100644
--- a/src/vocoder/coefficients.rs
+++ b/src/vocoder/coefficients.rs
@@ -57,7 +57,13 @@ impl CoefficientsT for GeneralizedCoefficients {
     }
 }
 
-pub trait CoefficientsT: Buffer {
+impl Generalized for GeneralizedCoefficients {
+    fn gamma(&self) -> f64 {
+        self.gamma
+    }
+}
+
+pub trait CoefficientsT: Buffer + Sized {
     type Cep: CepstrumT;
 
     fn to_cep(&self, alpha: f64) -> Self::Cep;
@@ -76,10 +82,45 @@ pub trait CoefficientsT: Buffer {
         let ir = self.b2mc(alpha).freqt(576 - 1, -alpha).c2ir(576);
         ir.iter().map(|x| x * x).sum()
     }
+
+    fn start(&mut self, target: Self, fperiod: usize) -> CoefficientsSession<'_, Self> {
+        let increment = self
+            .iter()
+            .zip(&target[..self.len()])
+            .map(|(current, target)| (target - current) / fperiod as f64)
+            .collect();
+        CoefficientsSession {
+            current: self,
+            increment,
+            target,
+        }
+    }
 }
 
-impl Generalized for GeneralizedCoefficients {
-    fn gamma(&self) -> f64 {
-        self.gamma
+pub struct CoefficientsSession<'a, T: CoefficientsT> {
+    current: &'a mut T,
+    increment: Vec<f64>,
+    target: T,
+}
+
+impl<'a, T: CoefficientsT> CoefficientsSession<'a, T> {
+    pub fn advance(&mut self) {
+        for (current, inc) in self.current.iter_mut().zip(&self.increment) {
+            *current += inc;
+        }
+    }
+}
+
+impl<T: CoefficientsT> Deref for CoefficientsSession<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.current
+    }
+}
+
+impl<T: CoefficientsT> Drop for CoefficientsSession<'_, T> {
+    fn drop(&mut self) {
+        self.current.copy_from_slice(&self.target);
     }
 }
diff --git a/src/vocoder/mod.rs b/src/vocoder/mod.rs
index 83c00d0..8c80161 100644
--- a/src/vocoder/mod.rs
+++ b/src/vocoder/mod.rs
@@ -17,6 +17,7 @@ mod stage;
 
 use self::{
     cepstrum::{CepstrumT, MelCepstrum},
+    coefficients::CoefficientsT,
     excitation::Excitation,
     generalized::Generalized,
     lsp::LineSpectralPairs,
@@ -116,28 +117,21 @@ impl Vocoder {
                 let mut cepstrum = MelCepstrum::new(spectrum, self.alpha);
                 cepstrum.postfilter_mcp(self.beta);
                 let cc = cepstrum.mc2b();
-                let cinc: Vec<_> = cc
-                    .iter()
-                    .zip(coefficients.iter())
-                    .map(|(cc, c)| (cc - c) / self.fperiod as f64)
-                    .collect();
 
+                let mut coefficients = coefficients.start(cc, self.fperiod);
                 self.excitation.start(p, self.fperiod);
 
-                (0..self.fperiod).for_each(|i| {
+                for r in &mut rawdata[..self.fperiod] {
                     let mut x = self.excitation.get(lpf);
                     if x != 0.0 {
                         x *= coefficients[0].exp();
                     }
-                    filter.df(&mut x, self.alpha, coefficients);
-                    for i in 0..coefficients.len() {
-                        coefficients[i] += cinc[i];
-                    }
-                    rawdata[i] = x * self.volume;
-                });
+                    filter.df(&mut x, self.alpha, &coefficients);
+                    coefficients.advance();
+                    *r = x * self.volume;
+                }
 
                 self.excitation.end(p);
-                *coefficients = cc;
             }
             Stage::NonZero {
                 stage,
@@ -153,26 +147,19 @@ impl Vocoder {
                 for i in 1..cc.len() {
                     cc[i] *= gamma;
                 }
-                let cinc: Vec<_> = cc
-                    .iter()
-                    .zip(coefficients.iter())
-                    .map(|(cc, c)| (cc - c) / self.fperiod as f64)
-                    .collect();
 
+                let mut coefficients = coefficients.start(cc, self.fperiod);
                 self.excitation.start(p, self.fperiod);
 
-                (0..self.fperiod).for_each(|i| {
+                for r in &mut rawdata[..self.fperiod] {
                     let mut x = self.excitation.get(lpf);
                     x *= coefficients[0];
-                    filter.df(&mut x, self.alpha, coefficients);
-                    for i in 0..coefficients.len() {
-                        coefficients[i] += cinc[i];
-                    }
-                    rawdata[i] = x * self.volume;
-                });
+                    filter.df(&mut x, self.alpha, &coefficients);
+                    coefficients.advance();
+                    *r = x * self.volume;
+                }
 
                 self.excitation.end(p);
-                *coefficients = cc;
             }
         }
     }