diff --git a/app/Main.hs b/app/Main.hs
index 44f8fe9..9a50c8e 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -107,6 +107,7 @@ audioCallback samples cursorRef fmt buffer = case fmt of
         let bufferLen = V.length buffer
             nToCopy = min bufferLen cursor
         when (cursor < bufferLen) $ do
+            -- TODO That's not correct
             V.set buffer 0
         V.copy (V.slice 0 nToCopy buffer) (V.slice 0 nToCopy samples)
         -- If more samples are ready
diff --git a/src/Nes/APU/State/Filter/Chain.hs b/src/Nes/APU/State/Filter/Chain.hs
index 5e9fa46..27c30bd 100644
--- a/src/Nes/APU/State/Filter/Chain.hs
+++ b/src/Nes/APU/State/Filter/Chain.hs
@@ -48,11 +48,14 @@ filterChainConsumeSample sample fc = do
     V.modifyM (filters fc) (consume sample) 0
     firstFilter <- V.read (filters fc) 0
     _ <-
-        V.ifoldM
-            ( \prev currIdx curr -> do
-                newCurr <- filterChainConsumeIteration prev (dt fc) curr
-                V.write (filters fc) currIdx newCurr
-                return newCurr
+        V.ifoldM'
+            ( \prev currIdx curr ->
+                if currIdx == 0
+                    then return curr
+                    else do
+                        !newCurr <- filterChainConsumeIteration prev (dt fc) curr
+                        V.write (filters fc) currIdx newCurr
+                        return newCurr
             )
             firstFilter
             (filters fc)
diff --git a/src/Nes/APU/State/Filter/Iir.hs b/src/Nes/APU/State/Filter/Iir.hs
index fda3cb4..062c567 100644
--- a/src/Nes/APU/State/Filter/Iir.hs
+++ b/src/Nes/APU/State/Filter/Iir.hs
@@ -21,9 +21,11 @@ data IirFilter = MkIirF
     , previousOutput :: {-# UNPACK #-} !Sample
     , previousInput :: {-# UNPACK #-} !Sample
     , delta :: {-# UNPACK #-} !Float
-    , outputF :: !(IirFilter -> Sample)
+    , pass :: {-# UNPACK #-} !IirFilterPass
     }
 
+data IirFilterPass = Identity | LowPass | HighPass deriving (Eq)
+
 identityIirFilter :: IirFilter
 identityIirFilter =
     MkIirF
@@ -31,7 +33,7 @@ identityIirFilter =
         , previousInput = 0
         , previousOutput = 0
         , delta = 0
-        , outputF = previousInput
+        , pass = Identity
         }
 
 highPassIirFilter :: SampleRate -> Cutoff -> IirFilter
@@ -41,7 +43,7 @@ highPassIirFilter sampleRate cutoff =
         , previousOutput = 0
         , previousInput = 0
         , delta = 0
-        , outputF = \f -> alpha f * previousOutput f + alpha f * delta f
+        , pass = HighPass
         }
   where
     period = 1 / sampleRate
@@ -54,7 +56,7 @@ lowPassIirFilter sampleRate cutoff =
         , previousOutput = 0
         , previousInput = 0
         , delta = 0
-        , outputF = \f -> previousOutput f + alpha f * delta f
+        , pass = LowPass
         }
   where
     period = 1 / sampleRate
@@ -62,7 +64,11 @@ lowPassIirFilter sampleRate cutoff =
 
 instance (Monad m) => Filter m IirFilter where
     {-# INLINE output #-}
-    output f = return $ outputF f f
+    output f = return $ case pass f of
+        Identity -> previousInput f
+        LowPass -> previousOutput f + alpha f * delta f
+        HighPass -> alpha f * previousOutput f + alpha f * delta f
+
     {-# INLINE consume #-}
     consume sample f = do
         prevOut <- output @m f
diff --git a/test/nestest/Spec.hs b/test/nestest/Spec.hs
index da4dce9..a355a1e 100644
--- a/test/nestest/Spec.hs
+++ b/test/nestest/Spec.hs
@@ -45,7 +45,7 @@ spec = it "Trace should match logfile" $ do
     let st = newCPUState{programCounter = 0xc000}
     -- TODO why is the tick count set to 7 ? Reset?
     _ <- try @IOException $ runProgram' st (bus{cycles = 7}) (trace traceRef)
-    actualTrace <- fmap fixStackValue . beforeUnhandledOpcode . toRawTrace <$> readIORef traceRef
+    actualTrace <- fmap fixStackValue . beforeAPUAccess . toRawTrace <$> readIORef traceRef
     -- BS.writeFile "actual.log" $ BSC.unlines actualTrace
     length actualTrace `shouldBe` length expectedTrace
     forM_ [0 .. length expectedTrace - 1] $ \i -> do
@@ -170,8 +170,8 @@ getCPUStateTrace = withCPUState $ \st ->
         (unByte $ unSR $ status st)
         (unByte $ registerS st)
 
-beforeUnhandledOpcode :: [ByteString] -> [ByteString]
-beforeUnhandledOpcode =
+beforeAPUAccess :: [ByteString] -> [ByteString]
+beforeAPUAccess =
     takeWhile
         (\line -> not $ "C68B" `BS.isPrefixOf` line)
 
@@ -179,8 +179,7 @@ loadExpectedRawTrace :: IO RawTrace
 loadExpectedRawTrace = do
     fileContent <- BS.readFile "test/assets/rom_trace.log"
     let rawTrace = fixStackValue <$> BSC.lines fileContent
-    -- TODO When project is finished, we shouldn't have to do the following filters
-    return $ withoutPPUCycles <$> beforeUnhandledOpcode rawTrace
+    return $ withoutPPUCycles <$> beforeAPUAccess rawTrace
   where
     withoutPPUCycles bs =
         let beforePPU = fst . BS.breakSubstring " PPU:" $ bs