Using a first-order filter for DC rejection
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.
diff --git a/src/opus_encoder.c b/src/opus_encoder.c
index fb2c821..80e2e70 100644
--- a/src/opus_encoder.c
+++ b/src/opus_encoder.c
@@ -385,20 +385,16 @@
int c, i;
int shift;
- /* Approximates -round(log2(4.*cutoff_Hz/Fs)) */
- shift=celt_ilog2(Fs/(cutoff_Hz*3));
+ /* Approximates -round(log2(6.3*cutoff_Hz/Fs)) */
+ shift=celt_ilog2(Fs/(cutoff_Hz*4));
for (c=0;c<channels;c++)
{
for (i=0;i<len;i++)
{
- opus_val32 x, tmp, y;
+ opus_val32 x, y;
x = SHL32(EXTEND32(in[channels*i+c]), 14);
- /* First stage */
- tmp = x-hp_mem[2*c];
+ y = x-hp_mem[2*c];
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));
}
}
@@ -409,55 +405,39 @@
{
int i;
float coef, coef2;
- coef = 4.0f*cutoff_Hz/Fs;
+ coef = 6.3f*cutoff_Hz/Fs;
coef2 = 1-coef;
if (channels==2)
{
- float m0, m1, m2, m3;
+ float m0, m2;
m0 = hp_mem[0];
- m1 = hp_mem[1];
m2 = hp_mem[2];
- m3 = hp_mem[3];
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];
x1 = in[2*i+1];
- /* First stage */
- tmp0 = x0-m0;
- tmp1 = x1-m2;
+ out0 = x0-m0;
+ out1 = x1-m2;
m0 = coef*x0 + VERY_SMALL + coef2*m0;
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+1] = out1;
}
hp_mem[0] = m0;
- hp_mem[1] = m1;
hp_mem[2] = m2;
- hp_mem[3] = m3;
} else {
- float m0, m1;
+ float m0;
m0 = hp_mem[0];
- m1 = hp_mem[1];
for (i=0;i<len;i++)
{
- opus_val32 x, tmp, y;
+ opus_val32 x, y;
x = in[i];
- /* First stage */
- tmp = x-m0;
+ y = x-m0;
m0 = coef*x + VERY_SMALL + coef2*m0;
- /* Second stage */
- y = tmp - m1;
- m1 = coef*tmp + VERY_SMALL + coef2*m1;
out[i] = y;
}
hp_mem[0] = m0;
- hp_mem[1] = m1;
}
}
#endif
