# create lowpass filter
function create_lowpass_filter([System.Double]$samplerate, [System.Double]$freq, [System.Double]$reso){
    $s = [Math]::Tan( $freq * 3.14159265 / $samplerate )
    $t = $s / $reso
    $u = $s*$s + $t + 1.0
    $a0 = $s*$s/$u
    $a1 = 2.0*$s*$s / $u
    $a2 = $s*$s/$u
    $b1 = -2.0*($s*$s - 1.0)/$u
    $b2 = -($s*$s - $t + 1.0)/$u
    return @{
        a0 =$a0; a1 =$a1; a2 =$a2; b1 =$b1; b2 =$b2;
        in_z = 0.0; in_zz = 0.0; out_z = 0.0; out_zz = 0.0}
}

# filter execute
function filter_execute([System.Double]$in, $f){
    $denormal_canceler = 1.0e-100
    $out = $f.a0*$in + $f.a1*$f.in_z + $f.a2*$f.in_zz + $f.b1*$f.out_z + $f.b2*$f.out_zz
    $f.in_zz = $f.in_z + $denormal_canceler
    $f.in_z = $in + $denormal_canceler
    $f.out_zz = $f.out_z + $denormal_canceler
    $f.out_z = $out + $denormal_canceler
    return $out
}

# SoX .dat format example
#
#; Sample Rate 48000
#; Channels 2
#               0      -0.45745626      -0.45745626
#   2.0833333e-05       -0.3917995       -0.3917995
#   4.1666667e-05     -0.067243849      -0.06724384
#        6.25e-05       0.32710202       0.32710202

# parameter
$freq = 1000.0
$reso = 0.707

# read header
$header = (Get-Content -TotalCount 2 "fa.dat")
$samplerate, $channels = $header | foreach { ($_ -split "\s+")[-1]}
if( $channels -ne 2 ){ return }

# create filter
$filter1 = create_lowpass_filter $samplerate $freq $reso
$filter2 = create_lowpass_filter $samplerate $freq $reso

# start
$writer = [System.IO.File]::CreateText("fa2.dat")
$header | foreach{ $writer.WriteLine($_) }
[System.IO.File]::ReadLines("fa.dat") `
    | select -Skip 2 `
    | foreach { ,(-split $_) } `
    | foreach { $_[0] + " " + (filter_execute $_[1] $filter1) + " " + (filter_execute $_[2] $filter2) }`
    | foreach { $writer.WriteLine($_) }
$writer.Close()