Rename filter flags: --highpass to --lowcut, --lowpass to --highcut; clarify docs and usage

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 2nd-order Butterworth filters to the recorded sweep before IR extraction (--lowcut, --highcut)
 - **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 +69,16 @@ 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.
+
 ### Different Output Formats
 
 Generate IRs in different sample rates and bit depths:
@@ -128,6 +139,8 @@ 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 |
 
 ## File Requirements
 
@@ -159,6 +172,10 @@ 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 2nd-order 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
+
 ### Deconvolution Process
 1. **FFT-based deconvolution** of recorded signal by sweep signal
 2. **Regularization** to prevent division by zero