Add --cut-slope option for filter steepness (default 12 dB/oct); cascade biquads for steeper slopes; update docs

README.md

@@ -7,7 +7,7 @@ A CLI tool for processing WAV files to generate impulse responses (IR) from swee
 - **Fast FFT-based deconvolution** for accurate IR extraction
 - **Automatic input conversion:** Accepts any WAV sample rate, bit depth, or channel count
 - **Optional output IR length:** Specify output IR length in milliseconds with --length-ms
-- **Optional low-cut and high-cut filtering:** Apply 2nd-order Butterworth filters to the recorded sweep before IR extraction (--lowcut, --highcut)
+- **Optional low-cut and high-cut filtering:** Apply Butterworth filters to the recorded sweep before IR extraction (--lowcut, --highcut, --cut-slope)
 - **Automatic fade-out:** Linear fade-out at the end of the IR to avoid clicks (default 5 ms, configurable with --fade-ms)
 - **96kHz 24-bit WAV file support** for high-quality audio processing
 - **Multiple output formats** with configurable sample rates and bit depths
@@ -79,6 +79,14 @@ You can apply a low-cut (high-pass) and/or high-cut (low-pass) filter to the rec
 
 This applies a 40 Hz low-cut (high-pass) and 18 kHz high-cut (low-pass) filter to the recorded sweep.
 
+You can control the filter steepness (slope) with `--cut-slope` (in dB/octave, default 12). For example:
+
+```sh
+./valhallir-deconvolver --sweep sweep.wav --recorded recorded.wav --output ir.wav --lowcut 40 --highcut 18000 --cut-slope 24
+```
+
+This applies a 40 Hz low-cut and 18 kHz high-cut, both with a 24 dB/octave slope (steeper than the default 12).
+
 ### Different Output Formats
 
 Generate IRs in different sample rates and bit depths:
@@ -141,6 +149,7 @@ Generate IRs in different sample rates and bit depths:
 | `--fade-ms` | Fade-out duration in milliseconds at end of IR (default 5) | 5 | No |
 | `--lowcut` | Low-cut filter (high-pass) cutoff frequency in Hz (recorded sweep) | - | No |
 | `--highcut` | High-cut filter (low-pass) cutoff frequency in Hz (recorded sweep) | - | No |
+| `--cut-slope` | Filter slope in dB/octave (12, 24, 36, ...; default 12) | 12 | No |
 
 ## File Requirements
 
@@ -173,8 +182,9 @@ Generate IRs in different sample rates and bit depths:
 - You can change the fade duration with `--fade-ms`
 
 ### Filtering
-- You can apply a 2nd-order Butterworth low-cut (high-pass) and/or high-cut (low-pass) filter to the recorded sweep before IR extraction
+- You can apply a Butterworth low-cut (high-pass) and/or high-cut (low-pass) filter to the recorded sweep before IR extraction
 - Use `--lowcut` and/or `--highcut` to specify cutoff frequencies in Hz
+- Use `--cut-slope` to control the filter steepness (12 dB/octave = gentle, 24+ = steeper)
 
 ### Deconvolution Process
 1. **FFT-based deconvolution** of recorded signal by sweep signal

main.go

@@ -73,6 +73,11 @@ func main() {
 				Name:  "lowcut",
 				Usage: "Low-cut filter (high-pass) cutoff frequency in Hz (applied to recorded sweep, optional)",
 			},
+			&cli.IntFlag{
+				Name:  "cut-slope",
+				Usage: "Cut filter slope in dB/octave (12, 24, 36, 48, ...; default 12)",
+				Value: 12,
+			},
 		},
 		Action: func(c *cli.Context) error {
 			// Read sweep WAV file
@@ -95,13 +100,17 @@ func main() {
 			recSampleRate := recordedData.SampleRate
 			highcutHz := c.Float64("highcut")
 			lowcutHz := c.Float64("lowcut")
+			cutSlope := c.Int("cut-slope")
+			if cutSlope < 12 || cutSlope%12 != 0 {
+				return fmt.Errorf("cut-slope must be a positive multiple of 12 (got %d)", cutSlope)
+			}
 			if lowcutHz > 0 {
-				log.Printf("Applying low-cut (high-pass) filter to recorded sweep: %.2f Hz", lowcutHz)
+				log.Printf("Applying low-cut (high-pass) filter to recorded sweep: %.2f Hz, slope: %d dB/oct", lowcutHz, cutSlope)
-				recordedFiltered = convolve.ApplyHighpassButterworth(recordedFiltered, recSampleRate, lowcutHz)
+				recordedFiltered = convolve.CascadeLowcut(recordedFiltered, recSampleRate, lowcutHz, cutSlope)
 			}
 			if highcutHz > 0 {
-				log.Printf("Applying high-cut (low-pass) filter to recorded sweep: %.2f Hz", highcutHz)
+				log.Printf("Applying high-cut (low-pass) filter to recorded sweep: %.2f Hz, slope: %d dB/oct", highcutHz, cutSlope)
-				recordedFiltered = convolve.ApplyLowpassButterworth(recordedFiltered, recSampleRate, highcutHz)
+				recordedFiltered = convolve.CascadeHighcut(recordedFiltered, recSampleRate, highcutHz, cutSlope)
 			}
 
 			log.Println("Performing deconvolution...")

pkg/convolve/convolve.go

@@ -425,3 +425,31 @@ func ApplyHighpassButterworth(data []float64, sampleRate int, cutoffHz float64)
 	}
 	return out
 }
+
+// CascadeLowcut applies the low-cut (high-pass) filter multiple times for steeper slopes.
+// slopeDb: 12, 24, 36, ... (dB/octave)
+func CascadeLowcut(data []float64, sampleRate int, cutoffHz float64, slopeDb int) []float64 {
+	if slopeDb < 12 {
+		slopeDb = 12
+	}
+	n := slopeDb / 12
+	out := data
+	for i := 0; i < n; i++ {
+		out = ApplyHighpassButterworth(out, sampleRate, cutoffHz)
+	}
+	return out
+}
+
+// CascadeHighcut applies the high-cut (low-pass) filter multiple times for steeper slopes.
+// slopeDb: 12, 24, 36, ... (dB/octave)
+func CascadeHighcut(data []float64, sampleRate int, cutoffHz float64, slopeDb int) []float64 {
+	if slopeDb < 12 {
+		slopeDb = 12
+	}
+	n := slopeDb / 12
+	out := data
+	for i := 0; i < n; i++ {
+		out = ApplyLowpassButterworth(out, sampleRate, cutoffHz)
+	}
+	return out
+}