Version 0.9

Added: Filter type "IIR", which allows to specify a user-defined IIR filter of arbitrary order with custom coefficients.
Improved: For playback devices, the APO is now installed both as GFX (post-mix) and LFX (pre-mix) APO. Normally the filtering happens in GFX, but the command "Stage" allows to select LFX to apply effects based on the number of input channels.
Added: Expression language. The commands "If", "ElseIf", "Else", "EndIf", "Eval" and inline expressions allow to change the filter behaviour based on runtime conditions.
Added: Command "Copy", which copies audio data between channels.
Added: Command "Delay", which delays the audio on the currently selected channels.
Improved: Configuration files are now locked exclusively for writing while reading them.
Improved: There is now a transition (10 ms) from old to new configuration after loading.
Improved: Substantial internal restructuring to improve extensibility and maintainability. Filtering commands now work exactly in the order they were specified.
Fixed: When the microphone input was in stereo but the application requested a mono stream, the audio was heavily distorted.
Fixed: The sine sweep generated by the Benchmark application had floating-point precision issues leading to severe artifacts when certain filters were applied.

Setup/config/iir_lowpass.txt

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+# This shows how to create a lowpass biquad filter using the IIR filter type
+# The result should be equivalent to the command
+# Filter: ON LP Fc 3000 Hz
+
+Do: `freq = 3000`
+Do: `Q = 1/sqrt(2)`
+Do: `omega = 2 * pi * freq / sampleRate`
+Do: `sn = sin(omega)`
+Do: `cs = cos(omega)`
+Do: `alpha = sin(omega) / (2 * Q)`
+Do: `a0 = (1 - cs) / 2`
+Do: `a1 = 1 - cs`
+Do: `a2 = (1 - cs) / 2`
+Do: `b0 = 1 + alpha`
+Do: `b1 = -2 * cs`
+Do: `b2 = 1 - alpha`
+
+Filter: ON IIR Order 2 Coefficients `a0` `a1` `a2` `b0` `b1` `b2`

Setup/config/selective_delay.txt

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+# Copy channels to virtual channels to filter them while keeping the originals
+Copy: L2=L R2=R
+# Select channels for filtering
+Channel: L2 R2
+# Limit frequency range for delay
+Filter: ON HP Fc 200 Hz
+Filter: ON LP Fc 2000 Hz
+# Apply delay
+Delay: 500 ms
+# Add the delayed signal back to the original channels
+Copy: L=0.5*L+0.5*L2 R=0.5*R+0.5*R2