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

README.md

@@ -7,6 +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 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
@@ -68,6 +69,24 @@ By default, a 5 ms linear fade-out is applied to the end of the IR to avoid clic
 
 This applies a 10 ms fade-out at the end of the IR.
 
+### Filtering the Recorded Sweep
+
+You can apply a low-cut (high-pass) and/or high-cut (low-pass) filter to the recorded sweep before IR extraction. This is useful for removing rumble, DC, or high-frequency noise:
+
+```sh
+./valhallir-deconvolver --sweep sweep.wav --recorded recorded.wav --output ir.wav --lowcut 40 --highcut 18000
+```
+
+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:
@@ -128,6 +147,9 @@ Generate IRs in different sample rates and bit depths:
 | `--trim-threshold` | Silence threshold for trimming (0.0-1.0) | 0.001 | No |
 | `--length-ms` | Output IR length in milliseconds (trim or zero-pad) | - | No |
 | `--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
 
@@ -159,6 +181,11 @@ Generate IRs in different sample rates and bit depths:
 - By default, a 5 ms linear fade-out is applied to the end of the IR (and MPT IR) to avoid clicks
 - You can change the fade duration with `--fade-ms`
 
+### Filtering
+- 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
 2. **Regularization** to prevent division by zero

main.go

@@ -65,6 +65,19 @@ func main() {
 				Usage: "Fade-out duration in milliseconds to apply at the end of the IR (default 5)",
 				Value: 5.0,
 			},
+			&cli.Float64Flag{
+				Name:  "highcut",
+				Usage: "High-cut filter (low-pass) cutoff frequency in Hz (applied to recorded sweep, optional)",
+			},
+			&cli.Float64Flag{
+				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
@@ -82,8 +95,26 @@ func main() {
 			log.Printf("Sweep: %d samples, %d channels", len(sweepData.PCMData), sweepData.Channels)
 			log.Printf("Recorded: %d samples, %d channels", len(recordedData.PCMData), recordedData.Channels)
 
+			// Optionally filter the recorded sweep
+			recordedFiltered := recordedData.PCMData
+			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, slope: %d dB/oct", lowcutHz, cutSlope)
+				recordedFiltered = convolve.CascadeLowcut(recordedFiltered, recSampleRate, lowcutHz, cutSlope)
+			}
+			if highcutHz > 0 {
+				log.Printf("Applying high-cut (low-pass) filter to recorded sweep: %.2f Hz, slope: %d dB/oct", highcutHz, cutSlope)
+				recordedFiltered = convolve.CascadeHighcut(recordedFiltered, recSampleRate, highcutHz, cutSlope)
+			}
+
 			log.Println("Performing deconvolution...")
-			ir := convolve.Deconvolve(sweepData.PCMData, recordedData.PCMData)
+			ir := convolve.Deconvolve(sweepData.PCMData, recordedFiltered)
 			log.Printf("Deconvolution result: %d samples", len(ir))
 
 			log.Println("Trimming silence...")

pkg/convolve/convolve.go

@@ -341,3 +341,115 @@ func FadeOutLinear(data []float64, fadeSamples int) []float64 {
 	}
 	return out
 }
+
+// ApplyLowpassButterworth applies a 2nd-order Butterworth low-pass filter to the data.
+// cutoffHz: cutoff frequency in Hz, sampleRate: sample rate in Hz.
+func ApplyLowpassButterworth(data []float64, sampleRate int, cutoffHz float64) []float64 {
+	if cutoffHz <= 0 || cutoffHz >= float64(sampleRate)/2 {
+		return data
+	}
+	// Biquad coefficients
+	w0 := 2 * math.Pi * cutoffHz / float64(sampleRate)
+	cosw0 := math.Cos(w0)
+	sinw0 := math.Sin(w0)
+	Q := 1.0 / math.Sqrt(2) // Butterworth Q
+	alpha := sinw0 / (2 * Q)
+
+	b0 := (1 - cosw0) / 2
+	b1 := 1 - cosw0
+	b2 := (1 - cosw0) / 2
+	a0 := 1 + alpha
+	a1 := -2 * cosw0
+	a2 := 1 - alpha
+
+	// Normalize
+	b0 /= a0
+	b1 /= a0
+	b2 /= a0
+	a1 /= a0
+	a2 /= a0
+
+	// Apply filter (Direct Form I)
+	out := make([]float64, len(data))
+	var x1, x2, y1, y2 float64
+	for i := 0; i < len(data); i++ {
+		x0 := data[i]
+		y0 := b0*x0 + b1*x1 + b2*x2 - a1*y1 - a2*y2
+		out[i] = y0
+		x2 = x1
+		x1 = x0
+		y2 = y1
+		y1 = y0
+	}
+	return out
+}
+
+// ApplyHighpassButterworth applies a 2nd-order Butterworth high-pass filter to the data.
+// cutoffHz: cutoff frequency in Hz, sampleRate: sample rate in Hz.
+func ApplyHighpassButterworth(data []float64, sampleRate int, cutoffHz float64) []float64 {
+	if cutoffHz <= 0 || cutoffHz >= float64(sampleRate)/2 {
+		return data
+	}
+	// Biquad coefficients
+	w0 := 2 * math.Pi * cutoffHz / float64(sampleRate)
+	cosw0 := math.Cos(w0)
+	sinw0 := math.Sin(w0)
+	Q := 1.0 / math.Sqrt(2) // Butterworth Q
+	alpha := sinw0 / (2 * Q)
+
+	b0 := (1 + cosw0) / 2
+	b1 := -(1 + cosw0)
+	b2 := (1 + cosw0) / 2
+	a0 := 1 + alpha
+	a1 := -2 * cosw0
+	a2 := 1 - alpha
+
+	// Normalize
+	b0 /= a0
+	b1 /= a0
+	b2 /= a0
+	a1 /= a0
+	a2 /= a0
+
+	// Apply filter (Direct Form I)
+	out := make([]float64, len(data))
+	var x1, x2, y1, y2 float64
+	for i := 0; i < len(data); i++ {
+		x0 := data[i]
+		y0 := b0*x0 + b1*x1 + b2*x2 - a1*y1 - a2*y2
+		out[i] = y0
+		x2 = x1
+		x1 = x0
+		y2 = y1
+		y1 = y0
+	}
+	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
+}