5 #include "stereocompressor.h"
11 // Implement a less accurate but faster pow(x, y). We use the standard identity
13 // x^y = exp(y * ln(x))
17 // x in 1..(1/threshold)
20 // Assume threshold goes from 0 to -40 dB. That means 1/threshold = 100,
21 // so input to ln(x) can be 1..100. Worst case for end accuracy is y=-1.
22 // To get a good minimax approximation (not the least wrt. continuity
23 // at x=1), I had to make a piecewise linear function for the two ranges:
26 // f1 := minimax(ln, 1..6, [3, 3], x -> 1/x, 'maxerror');
27 // f2 := minimax(ln, 6..100, [3, 3], x -> 1/x, 'maxerror');
28 // f := x -> piecewise(x < 6, f1(x), f2(x));
30 // (Continuity: Error is down to the 1e-6 range for x=1, difference between
31 // f1 and f2 range at the crossover point is in the 1e-5 range. The cutoff
32 // point at x=6 is chosen to get maxerror pretty close between f1 and f2.)
34 // Maximum output of ln(x) here is of course ln(100) ~= 4.605. So we can find
35 // an approximation for exp over the range -4.605..0, where we care mostly
36 // about the relative error:
38 // g := minimax(exp, -ln(100)..0, [3, 3], x -> 1/exp(x), 'maxerror');
40 // We can find the worst-case error in dB from this through a simple plot:
42 // dbdiff := (x, y) -> abs(20 * log10(x / y));
43 // plot(dbdiff(g(-f(x)), 1/x), x=1..100);
45 // which readily shows the error never to be above ~0.001 dB or so
46 // (actually 0.00119 dB, for the case of x=100). y=-1 remains the worst case,
49 // If we cared even more about speed, we could probably fuse y into
50 // the coefficients for ln_nom and postgain into the coefficients for ln_den.
51 // But if so, we should probably rather just SIMD the entire thing instead.
52 inline float fastpow(float x, float y)
56 ln_nom = -0.059237648 + (-0.0165117771 + (0.06818859075 + 0.007560968243 * x) * x) * x;
57 ln_den = 0.0202509098 + (0.08419174188 + (0.03647189417 + 0.001642577975 * x) * x) * x;
59 ln_nom = -0.005430534 + (0.00633589178 + (0.0006319155549 + 0.4789541675e-5 * x) * x) * x;
60 ln_den = 0.0064785099 + (0.003219629109 + (0.0001531823694 + 0.6884656640e-6 * x) * x) * x;
62 float v = y * ln_nom / ln_den;
63 float exp_nom = 0.2195097621 + (0.08546059868 + (0.01208501759 + 0.0006173448113 * v) * v) * v;
64 float exp_den = 0.2194980791 + (-0.1343051968 + (0.03556072737 - 0.006174398513 * v) * v) * v;
65 return exp_nom / exp_den;
68 inline float compressor_knee(float x, float threshold, float inv_threshold, float inv_ratio_minus_one, float postgain)
70 assert(inv_ratio_minus_one <= 0.0f);
71 if (x > threshold && inv_ratio_minus_one < 0.0f) {
72 return postgain * fastpow(x * inv_threshold, inv_ratio_minus_one);
80 void StereoCompressor::process(float *buf, size_t num_samples, float threshold, float ratio,
81 float attack_time, float release_time, float makeup_gain)
83 float attack_increment = float(pow(2.0f, 1.0f / (attack_time * sample_rate + 1)));
84 if (attack_time == 0.0f) attack_increment = 100000; // For instant attack reaction.
86 const float release_increment = float(pow(2.0f, -1.0f / (release_time * sample_rate + 1)));
87 const float peak_increment = float(pow(2.0f, -1.0f / (0.003f * sample_rate + 1)));
89 float inv_ratio_minus_one = 1.0f / ratio - 1.0f;
90 if (ratio > 63) inv_ratio_minus_one = -1.0f; // Infinite ratio.
91 float inv_threshold = 1.0f / threshold;
93 float *left_ptr = buf;
94 float *right_ptr = buf + 1;
96 float peak_level = this->peak_level;
97 float compr_level = this->compr_level;
99 for (size_t i = 0; i < num_samples; ++i) {
100 if (fabs(*left_ptr) > peak_level) peak_level = float(fabs(*left_ptr));
101 if (fabs(*right_ptr) > peak_level) peak_level = float(fabs(*right_ptr));
103 if (peak_level > compr_level) {
104 compr_level = min(compr_level * attack_increment, peak_level);
106 compr_level = max(compr_level * release_increment, 0.0001f);
109 float scalefactor_with_gain = compressor_knee(compr_level, threshold, inv_threshold, inv_ratio_minus_one, makeup_gain);
111 *left_ptr *= scalefactor_with_gain;
114 *right_ptr *= scalefactor_with_gain;
117 peak_level = max(peak_level * peak_increment, 0.0001f);
120 // Store attenuation level for debug/visualization.
121 scalefactor = compressor_knee(compr_level, threshold, inv_threshold, inv_ratio_minus_one, 1.0f);
123 this->peak_level = peak_level;
124 this->compr_level = compr_level;