notes
```The nominal implementation for biquads is the Direct Form I variant. But the Direct Form
II is actually more suited for calculating the biquad since it needs only 2 memory
locations, and the multiplications can be pipelined better by the compiler. In release
build, I've noted a considerable speedup when compared to DF I. When processing stereo,
the code below was ~ 2X faster. Until I develop a SIMD biquad that is faster, this will
do.
```
code
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33``` ```// b0, b1, b2, a1, a2 calculated via r.b-j's cookbook // formulae. // m1, m2 are the memory locations // dn is the de-denormal coeff (=1.0e-20f) void processBiquad(const float* in, float* out, unsigned length) { for(unsigned i = 0; i < length; ++i) { register float w = in[i] - a1*m1 - a2*m2 + dn; out[i] = b1*m1 + b2*m2 + b0*w; m2 = m1; m1 = w; } dn = -dn; } void processBiquadStereo(const float* inL, const float* inR, float* outL, float* outR, unsigned length) { for(unsigned i = 0; i < length; ++i) { register float wL = inL[i] - a1*m1L - a2*m2L + dn; register float wR = inR[i] - a1*m1R - a2*m2R + dn; outL[i] = b1*m1L + b2*m2L + b0*wL; m2L = m1L; m1L = wL; outR[i] = b1*m1R + b2*m2R + b0*wR; m2R = m1R; m1R = wR; } dn = -dn; } ```

```true, this structure is faster. but it is also (even) more sensitive to coefficients changes, so it becomes unstable quite fast compaerd to the DF I form. I'd really like to know if there's a way to change coefficients and at the same time time changing the history of the filter for avoiding unstability.
```Use direct form I (single accumulation point) when using fixed-point processors. For floating point, use direct form II transposed, which has superior numerical characteristics to direct form II (non-transposed).