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tube: better oversampling filter to reduce aliasing

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Connor Olding 2014-02-08 16:49:05 -08:00
parent 1550230dc5
commit 1b1e4353fb
2 changed files with 27 additions and 41 deletions
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4
README.md
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@ -37,8 +37,8 @@ white noise generator. loud, full-range, 0dBFS. don't say i didn't warn you.
__crap\_tube (0x50F7BA11)__
static waveshaper with 4x oversampling, sounds kinda like a tube i guess?
be aware that the oversampling is a bit naive and attenuates the signal
past 17kHz, assuming a 44.1kHz sample rate.
be aware that its oversampling currently boosts top-most frequencies by
~2dB before attenuating.
### crap delay test

64
crap/tube.h
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@ -11,8 +11,8 @@
#define COPYRIGHT "MIT"
#define PARAMETERS 2
#define HIST_SIZE 36
#define HIST_SIZE_2 18
#define HIST_SIZE_2 (2 + 2*10)
#define HIST_SIZE (HIST_SIZE_2*2)
typedef struct {
double desired, actual, speed;
@ -59,43 +59,29 @@ distort(double x)
// a0 is already factored into the rest of the coefficients
#define LOWPASS(i, b0, b1, a1, a2) \
y = b0*x + b1*h[i*2 + 0] + b0*h[i*2 + 1] \
- a1*h[i*2 + 2] - a2*h[i*2 + 3]; \
- a1*h[i*2 + 2] - a2*h[i*2 + 3]; \
BQSHIFT(i); \
x = y;
static double
upsample(double h[HIST_SIZE_2], double x)
oversample(double h[HIST_SIZE_2], double x)
{
/* designed with <https://coewww.rutgers.edu/~orfanidi/hpeq/>
fs = 44100; OS = 4; Gs = -120; f2 = 18248/fs/OS; f2s = 22050/fs/OS;
N = hpeqord(-Inf,0,Gs,-6,2*pi*f2s,2*pi*f2,2) % 20.9971490662433
*/
double y;
// -3 at 18600/44100/2, -96 stopband
// designed with <https://coewww.rutgers.edu/~orfanidi/hpeq/>
LOWPASS(0, +0.71327159,+0.00688573,-0.45391337,+0.88734229);
LOWPASS(1, +0.63347126,+0.05572752,-0.36946634,+0.69213639);
LOWPASS(2, +0.55963645,+0.13990391,-0.26487901,+0.52405582);
LOWPASS(3, +0.49037095,+0.24706928,-0.14763065,+0.37544183);
LOWPASS(4, +0.42692239,+0.36379839,-0.02763286,+0.24527604);
LOWPASS(5, +0.37268890,+0.47433865,+0.08224090,+0.13747554);
LOWPASS(6, +0.33241251,+0.56148939,+0.16727062,+0.05904378);
LOWPASS(7, +0.31079382,+0.60975767,+0.21392163,+0.01742368);
BQSHIFT(8);
return y;
}
static double
downsample(double h[HIST_SIZE_2], double x)
{
double y;
// -3 at 17544/44100/4, -96 stopband
// same idea as upsample()
LOWPASS(0, +0.62136966,-0.87573986,-1.56336581,+0.93036527);
LOWPASS(1, +0.56540370,-0.77393348,-1.44258778,+0.79946170);
LOWPASS(2, +0.49824084,-0.63630306,-1.31114921,+0.67132784);
LOWPASS(3, +0.41949184,-0.46466704,-1.16600279,+0.54031944);
LOWPASS(4, +0.33172375,-0.26684785,-1.00993399,+0.40653364);
LOWPASS(5, +0.24269774,-0.06242297,-0.85492245,+0.27789496);
LOWPASS(6, +0.16673206,+0.11379847,-0.72421195,+0.17147454);
LOWPASS(7, +0.12199271,+0.21811002,-0.64769184,+0.10978728);
BQSHIFT(8);
LOWPASS(0, +0.65855672,-0.92949215,-1.56012668,+0.94774797);
LOWPASS(1, +0.61716958,-0.85685555,-1.47099672,+0.84848033);
LOWPASS(2, +0.56959704,-0.76263107,-1.37527450,+0.75183750);
LOWPASS(3, +0.51503506,-0.64652564,-1.27052746,+0.65407194);
LOWPASS(4, +0.45328074,-0.50918096,-1.15569311,+0.55307363);
LOWPASS(5, +0.38524858,-0.35361043,-1.03185791,+0.44874463);
LOWPASS(6, +0.31367929,-0.18703461,-0.90341296,+0.34373693);
LOWPASS(7, +0.24382090,-0.02260789,-0.77918770,+0.24422161);
LOWPASS(8, +0.18353302,+0.12028336,-0.67260228,+0.15995168);
LOWPASS(9, +0.14205849,+0.21898074,-0.59952639,+0.10262410);
BQSHIFT(10);
return y;
}
@ -106,14 +92,14 @@ process_one(double x, double drive, double wet)
}
static double
oversample(personal *data, double x, int right)
process_os(personal *data, double x, int right)
{
double *h0 = (!right) ? data->history_L : data->history_R;
double *h1 = h0 + HIST_SIZE_2;
double y;
#define doit(SAMP) \
downsample(h1, process_one(4*upsample(h0, SAMP), \
oversample(h1, process_one(4*oversample(h0, SAMP), \
smooth(&data->drive), smooth(&data->wet)))
y = doit(x);
doit(0);
@ -132,8 +118,8 @@ process(personal *data,
{
disable_denormals();
for (unsigned long pos = 0; pos < count; pos++) {
out_L[pos] = oversample(data, in_L[pos], 0);
out_R[pos] = oversample(data, in_R[pos], 1);
out_L[pos] = process_os(data, in_L[pos], 0);
out_R[pos] = process_os(data, in_R[pos], 1);
}
}
@ -145,8 +131,8 @@ process_double(personal *data,
{
disable_denormals();
for (unsigned long pos = 0; pos < count; pos++) {
out_L[pos] = oversample(data, in_L[pos], 0);
out_R[pos] = oversample(data, in_R[pos], 1);
out_L[pos] = process_os(data, in_L[pos], 0);
out_R[pos] = process_os(data, in_R[pos], 1);
}
}