lpf.go

package main

import (
	"math"
)

type LowPassFilter struct {
	window []float64
}

func NewLowPassFilter(bandwidth int64) *LowPassFilter {

	//this is basically a low pass filter based on the Hamming (Hanning?) window
	//it doesn't really work too well

	const a = 0.50
	var inverseBandwidth = 1. / (float64(bandwidth+1) * .5)
	var filter = new(LowPassFilter)
	filter.window = make([]float64, bandwidth)
	var sum float64 = 0
	for i := 0; i < int(bandwidth); i++ {
		filter.window[i] = (a + (1.-a)*math.Cos(math.Pi*(float64(i)-float64(bandwidth-1)*0.5)*inverseBandwidth)) * 0.25
		sum += filter.window[i]
	}
	/*
		//this is to make sure that the lowPassed will never be louder than original
		//oops, doesn't work
		var inverseSum = 1./sum
		for i:=0; i<int(bandwidth); i++ {
			filter.window[i] *= inverseSum
		}
	*/
	return filter
}

func (filter LowPassFilter) Filter(input []int32) []int32 {
	var output = make([]int32, len(input))
	for i := 0; i < len(output); i++ {
		var accumulator float64 = 0
		for j := 0; j < len(filter.window); j++ {
			if i-len(filter.window)+1+j >= 0 {
				sample := float64(input[i-len(filter.window)+1+j]) / float64(2<<31)
				accumulator += filter.window[j] * sample
			}
		}
		output[i] = int32(accumulator * float64(2<<31))
	}
	return output
}