feat(tui): add interactive 5-band per-user equalizer

pkg/audio/biquad.go

@@ -0,0 +1,135 @@
+package audio
+
+import (
+	"math"
+)
+
+// BiquadFilter represents a second-order IIR filter.
+// Formulas from RBJ Audio-EQ-Cookbook.
+type BiquadFilter struct {
+	// Coefficients
+	b0, b1, b2, a1, a2 float64
+
+	// State (history)
+	x1, x2, y1, y2 float64
+}
+
+// NewPeakingEQ creates a peaking EQ filter (boost/cut at specific frequency)
+// rate: sample rate (e.g. 48000)
+// freq: center frequency in Hz
+// q: quality factor (width of the bell)
+// dbGain: gain in decibels (e.g. +3.0, -6.0)
+func NewPeakingEQ(rate, freq, q, dbGain float64) *BiquadFilter {
+	f := &BiquadFilter{}
+	f.Configure(rate, freq, q, dbGain)
+	return f
+}
+
+// Configure recalculates coefficients
+func (f *BiquadFilter) Configure(rate, freq, q, dbGain float64) {
+	// Intermediate variables
+	A := math.Pow(10, dbGain/40)
+	omega := 2 * math.Pi * freq / rate
+	sn := math.Sin(omega)
+	cs := math.Cos(omega)
+	alpha := sn / (2 * q)
+
+	// Coefficients
+	b0 := 1 + alpha*A
+	b1 := -2 * cs
+	b2 := 1 - alpha*A
+	a0 := 1 + alpha/A
+	a1 := -2 * cs
+	a2 := 1 - alpha/A
+
+	// Normalize by a0
+	invA0 := 1 / a0
+	f.b0 = b0 * invA0
+	f.b1 = b1 * invA0
+	f.b2 = b2 * invA0
+	f.a1 = a1 * invA0
+	f.a2 = a2 * invA0
+}
+
+// Process processes a single sample
+func (f *BiquadFilter) Process(in float64) float64 {
+	// Difference equation:
+	// y[n] = b0*x[n] + b1*x[n-1] + b2*x[n-2] - a1*y[n-1] - a2*y[n-2]
+	out := f.b0*in + f.b1*f.x1 + f.b2*f.x2 - f.a1*f.y1 - f.a2*f.y2
+
+	// Update history
+	f.x2 = f.x1
+	f.x1 = in
+	f.y2 = f.y1
+	f.y1 = out
+
+	return out
+}
+
+// Reset clears the filter memory
+func (f *BiquadFilter) Reset() {
+	f.x1, f.x2, f.y1, f.y2 = 0, 0, 0, 0
+}
+
+// EQChain manages a cascade of filters (our 5 bands)
+type EQChain struct {
+	Filters []*BiquadFilter
+}
+
+// NewEQChain creates the standard 5-band EQ chain
+func NewEQChain(sampleRate float64) *EQChain {
+	return &EQChain{
+		Filters: []*BiquadFilter{
+			NewPeakingEQ(sampleRate, 100, 1.0, 0),  // SUB (Reduced from 1000 to proper bass freq)
+			NewPeakingEQ(sampleRate, 350, 1.0, 0),  // LOW
+			NewPeakingEQ(sampleRate, 1000, 1.0, 0), // MID
+			NewPeakingEQ(sampleRate, 3000, 1.0, 0), // HI
+			NewPeakingEQ(sampleRate, 8000, 1.0, 0), // AIR
+		},
+	}
+}
+
+// SetGain sets the gain for a specific band index (0-4)
+func (e *EQChain) SetGain(bandIdx int, dbGain float64) {
+	if bandIdx < 0 || bandIdx >= len(e.Filters) {
+		return
+	}
+
+	rate := 48000.0 // Assuming fixed rate for now
+	// Frequencies map to our standard bands
+	freqs := []float64{100, 350, 1000, 3000, 8000}
+
+	e.Filters[bandIdx].Configure(rate, freqs[bandIdx], 1.0, dbGain)
+}
+
+// Reset clears history of all filters
+func (e *EQChain) Reset() {
+	for _, f := range e.Filters {
+		f.Reset()
+	}
+}
+
+// ProcessBlock processes a slice of samples in-place (or returns new slice)
+// We'll return a new float buffer for FFT analysis anyway
+func (e *EQChain) Process(samples []int16) []int16 {
+	out := make([]int16, len(samples))
+
+	for i, s := range samples {
+		val := float64(s)
+
+		// Run through cascade
+		for _, f := range e.Filters {
+			val = f.Process(val)
+		}
+
+		// Clip
+		if val > 32767 {
+			val = 32767
+		} else if val < -32768 {
+			val = -32768
+		}
+
+		out[i] = int16(val)
+	}
+	return out
+}