go-ts/pkg/audio/biquad.go

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
}