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Using a first-order filter for DC rejection

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A second-order DC rejection filter is uselsss unless we have complex
poles. However, complex poles means we have to compute the filter as a
single pass (rather than two casdaded first-order filters), which has
numerical issues that would require a higher complexity to solve.
So rather than waste cycles with a second-order filter (with a longer
impulse response), we just go with a first-order filter.
This commit is contained in:
Jean-Marc Valin 2018-03-12 11:39:08 -04:00
parent e1c0770a49
commit a4b5282f94
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GPG key ID: 5E5DD9A36F9189C8
1 changed files with 12 additions and 32 deletions
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44
src/opus_encoder.c
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@ -385,20 +385,16 @@ static void dc_reject(const opus_val16 *in, opus_int32 cutoff_Hz, opus_val16 *ou
int c, i; int c, i;
int shift; int shift;
/* Approximates -round(log2(4.*cutoff_Hz/Fs)) */ /* Approximates -round(log2(6.3*cutoff_Hz/Fs)) */
shift=celt_ilog2(Fs/(cutoff_Hz*3)); shift=celt_ilog2(Fs/(cutoff_Hz*4));
for (c=0;c<channels;c++) for (c=0;c<channels;c++)
{ {
for (i=0;i<len;i++) for (i=0;i<len;i++)
{ {
opus_val32 x, tmp, y; opus_val32 x, y;
x = SHL32(EXTEND32(in[channels*i+c]), 14); x = SHL32(EXTEND32(in[channels*i+c]), 14);
/* First stage */ y = x-hp_mem[2*c];
tmp = x-hp_mem[2*c];
hp_mem[2*c] = hp_mem[2*c] + PSHR32(x - hp_mem[2*c], shift); hp_mem[2*c] = hp_mem[2*c] + PSHR32(x - hp_mem[2*c], shift);
/* Second stage */
y = tmp - hp_mem[2*c+1];
hp_mem[2*c+1] = hp_mem[2*c+1] + PSHR32(tmp - hp_mem[2*c+1], shift);
out[channels*i+c] = EXTRACT16(SATURATE(PSHR32(y, 14), 32767)); out[channels*i+c] = EXTRACT16(SATURATE(PSHR32(y, 14), 32767));
} }
} }
@ -409,55 +405,39 @@ static void dc_reject(const opus_val16 *in, opus_int32 cutoff_Hz, opus_val16 *ou
{ {
int i; int i;
float coef, coef2; float coef, coef2;
coef = 4.0f*cutoff_Hz/Fs; coef = 6.3f*cutoff_Hz/Fs;
coef2 = 1-coef; coef2 = 1-coef;
if (channels==2) if (channels==2)
{ {
float m0, m1, m2, m3; float m0, m2;
m0 = hp_mem[0]; m0 = hp_mem[0];
m1 = hp_mem[1];
m2 = hp_mem[2]; m2 = hp_mem[2];
m3 = hp_mem[3];
for (i=0;i<len;i++) for (i=0;i<len;i++)
{ {
opus_val32 x0, x1, tmp0, tmp1, out0, out1; opus_val32 x0, x1, out0, out1;
x0 = in[2*i+0]; x0 = in[2*i+0];
x1 = in[2*i+1]; x1 = in[2*i+1];
/* First stage */ out0 = x0-m0;
tmp0 = x0-m0; out1 = x1-m2;
tmp1 = x1-m2;
m0 = coef*x0 + VERY_SMALL + coef2*m0; m0 = coef*x0 + VERY_SMALL + coef2*m0;
m2 = coef*x1 + VERY_SMALL + coef2*m2; m2 = coef*x1 + VERY_SMALL + coef2*m2;
/* Second stage */
out0 = tmp0 - m1;
out1 = tmp1 - m3;
m1 = coef*tmp0 + VERY_SMALL + coef2*m1;
m3 = coef*tmp1 + VERY_SMALL + coef2*m3;
out[2*i+0] = out0; out[2*i+0] = out0;
out[2*i+1] = out1; out[2*i+1] = out1;
} }
hp_mem[0] = m0; hp_mem[0] = m0;
hp_mem[1] = m1;
hp_mem[2] = m2; hp_mem[2] = m2;
hp_mem[3] = m3;
} else { } else {
float m0, m1; float m0;
m0 = hp_mem[0]; m0 = hp_mem[0];
m1 = hp_mem[1];
for (i=0;i<len;i++) for (i=0;i<len;i++)
{ {
opus_val32 x, tmp, y; opus_val32 x, y;
x = in[i]; x = in[i];
/* First stage */ y = x-m0;
tmp = x-m0;
m0 = coef*x + VERY_SMALL + coef2*m0; m0 = coef*x + VERY_SMALL + coef2*m0;
/* Second stage */
y = tmp - m1;
m1 = coef*tmp + VERY_SMALL + coef2*m1;
out[i] = y; out[i] = y;
} }
hp_mem[0] = m0; hp_mem[0] = m0;
hp_mem[1] = m1;
} }
} }
#endif #endif