98 lines
2.3 KiB
C++
98 lines
2.3 KiB
C++
/* halfband polyphase IIR filter
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coefficients designed with halfband program:
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https://gist.github.com/3be345efb6c97d757398#file-halfband-c
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parameters: 16 coefficients, 0.1 transition band
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stopband: ~-150dB
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overall delay: ~8 samples
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*/
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#define copy(dst, src) memcpy(dst, src, sizeof(double)*8)
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// all should be initialized to 0
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typedef struct {
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double ao[8], bo[8];
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double at[8], bt[8];
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double x1, x2, x3;
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int i;
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} halfband_t;
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INNER void
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halfband_a(double a[8], double ao[8], double x0, double x2)
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{
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a[0] = x2 + (x0 - ao[0])*0.006185967461045014;
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a[1] = ao[0] + (a[0] - ao[1])*0.054230780876613788;
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a[2] = ao[1] + (a[1] - ao[2])*0.143280861566087270;
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a[3] = ao[2] + (a[2] - ao[3])*0.262004358403954640;
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a[4] = ao[3] + (a[3] - ao[4])*0.398796973552973666;
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a[5] = ao[4] + (a[4] - ao[5])*0.545323651071132232;
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a[6] = ao[5] + (a[5] - ao[6])*0.698736833646440347;
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a[7] = ao[6] + (a[6] - ao[7])*0.862917812650502936;
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}
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INNER void
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halfband_b(double b[8], double bo[8], double x1, double x3)
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{
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b[0] = x3 + (x1 - bo[0])*0.024499027624721819;
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b[1] = bo[0] + (b[0] - bo[1])*0.094283481125726432;
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b[2] = bo[1] + (b[1] - bo[2])*0.199699579426327684;
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b[3] = bo[2] + (b[2] - bo[3])*0.328772348316831664;
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b[4] = bo[3] + (b[3] - bo[4])*0.471167216679969414;
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b[5] = bo[4] + (b[4] - bo[5])*0.621096845120503893;
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b[6] = bo[5] + (b[5] - bo[6])*0.778944517099529166;
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b[7] = bo[6] + (b[6] - bo[7])*0.952428157718303137;
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}
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INNER double
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halfband(halfband_t *h, double x0)
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{
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double a[8], b[8];
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halfband_a(a, h->ao, x0, h->x2);
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halfband_b(b, h->bo, h->x1, h->x3);
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copy(h->ao, h->at);
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copy(h->bo, h->bt);
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copy(h->at, a);
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copy(h->bt, b);
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h->x3 = h->x2;
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h->x2 = h->x1;
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h->x1 = x0;
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return (a[7] + b[7])*0.5;
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}
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INNER double
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decimate(halfband_t *h, double x0)
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{
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double c[8];
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if ((h->i = !h->i)) {
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halfband_b(c, h->bo, x0, h->x2);
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copy(h->bo, c);
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h->x2 = h->x1;
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h->x1 = x0;
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return 0;
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} else {
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halfband_a(c, h->ao, x0, h->x2);
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copy(h->ao, c);
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h->x2 = h->x1;
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h->x1 = x0;
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return (c[7] + h->bo[7])*0.5;
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}
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}
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// note: do not zero-stuff! send the input each time.
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INNER double
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interpolate(halfband_t *h, double x0)
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{
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double c[8];
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if ((h->i = !h->i)) {
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halfband_a(c, h->ao, x0, h->x1);
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copy(h->ao, c);
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return c[7];
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} else {
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halfband_b(c, h->bo, x0, h->x1);
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copy(h->bo, c);
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h->x1 = x0;
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return c[7];
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}
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}
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#undef copy
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