Wavecap Filters

WaveCap Audio Filter Tuning Skill

Use this skill to tune audio pre-processing filters that improve speech intelligibility before transcription.

Configuration Location

Audio filter settings are in the whisper: section of the config file:

• User config: /Users/thw/Projects/WaveCap/state/config.yaml
• Default config: /Users/thw/Projects/WaveCap/backend/default-config.yaml

Filter Parameters

Highpass Filter (removes low-frequency rumble)

whisper:
  highpassCutoffHz: 250  # Default: 250 Hz

When to adjust:

• Increase if you hear wind noise, engine rumble, or low-frequency hum
• Decrease if voices sound thin or tinny

Lowpass Filter (removes high-frequency hiss)

whisper:
  lowpassCutoffHz: 3800  # Default: 3800 Hz

When to adjust:

• Decrease if there's persistent high-frequency hiss or static
• Increase if speech sounds muffled or lacks clarity

De-emphasis (compensates for radio pre-emphasis)

whisper:
  deemphasisTimeConstantMicros: 75  # Default: 75 microseconds

When to adjust:

• If audio sounds too bright/harsh, increase the value
• If audio sounds too dull, decrease the value

Automatic Gain Control (AGC)

whisper:
  agcTargetRms: 0.04  # Default: 0.04 (4% of full scale)

When to adjust:

• Increase if audio is too quiet and Whisper misses words
• Decrease if audio clips or sounds distorted
• Set to null if source audio is already well-normalized

View Current Settings

grep -A20 "whisper:" /Users/thw/Projects/WaveCap/state/config.yaml | grep -E "(highpass|lowpass|deemphasis|agc)"

Common Tuning Scenarios

Noisy Environment (wind, engines, background noise)

whisper:
  highpassCutoffHz: 350      # More aggressive low-cut
  lowpassCutoffHz: 3500      # Reduce hiss
  agcTargetRms: 0.05         # Boost speech above noise

Clean Studio/Dispatch Audio

whisper:
  highpassCutoffHz: 150      # Preserve natural bass
  lowpassCutoffHz: 4500      # Keep clarity
  agcTargetRms: 0.03         # Light normalization

Weak/Distant Radio Signal

whisper:
  highpassCutoffHz: 300      # Remove noise floor
  lowpassCutoffHz: 3800      # Standard
  agcTargetRms: 0.07         # Aggressive boost

Broadcastify/Web Streams (already processed)

whisper:
  highpassCutoffHz: 200      # Light filtering
  lowpassCutoffHz: 4000      # Preserve quality
  agcTargetRms: 0.04         # Standard

Apply Changes

After editing state/config.yaml, restart the service:

launchctl stop com.wavecap.server && sleep 2 && launchctl start com.wavecap.server

Analyze Current Audio Quality

Check RMS levels of recent recordings to inform AGC tuning:

cd /Users/thw/Projects/WaveCap/backend && source .venv/bin/activate && python3 << 'EOF'
import numpy as np
from pathlib import Path
import struct

def get_wav_rms(filepath):
    with open(filepath, 'rb') as f:
        f.read(44)  # Skip header
        data = f.read()
        samples = np.frombuffer(data, dtype=np.int16).astype(np.float32) / 32768.0
        return np.sqrt(np.mean(samples**2))

recordings = sorted(Path("/Users/thw/Projects/WaveCap/state/recordings").glob("*.wav"),
                   key=lambda x: x.stat().st_mtime, reverse=True)[:10]
print("Recent recordings RMS levels:")
for r in recordings:
    rms = get_wav_rms(r)
    print(f"  {r.name}: {rms:.4f} ({rms*100:.1f}%)")
EOF

Tips

• Start with default values and make small adjustments (10-20% changes)
• Test with a variety of recordings before committing to changes
• AGC is the most impactful setting for quiet/loud sources
• Highpass filter is most important for outdoor/mobile radio
• Keep notes on what works for different stream sources