Support audio-only FFmpeg inputs. Somewhat wonky, though.

[nageru] / stereocompressor.cpp

diff --git a/stereocompressor.cpp b/stereocompressor.cpp
index 0d87aabe86867673a66c378c84add47c977ac365..d9f1142fa3595b08ccaadd9431cb70fd7a602288 100644 (file)
--- a/stereocompressor.cpp
+++ b/stereocompressor.cpp
@@ -1,16 +1,75 @@
-#include <math.h>
+#include "stereocompressor.h"
+
 #include <assert.h>
 #include <algorithm>
+#include <cmath>
 
-#include "stereocompressor.h"
+using namespace std;
 
 namespace {
 
+// Implement a less accurate but faster pow(x, y). We use the standard identity
+//
+//    x^y = exp(y * ln(x))
+//
+// with the ranges:
+//
+//    x in 1..(1/threshold)
+//    y in -1..0
+//
+// Assume threshold goes from 0 to -40 dB. That means 1/threshold = 100,
+// so input to ln(x) can be 1..100. Worst case for end accuracy is y=-1.
+// To get a good minimax approximation (not the least wrt. continuity
+// at x=1), I had to make a piecewise linear function for the two ranges:
+//
+//   with(numapprox):
+//   f1 := minimax(ln, 1..6, [3, 3], x -> 1/x, 'maxerror');
+//   f2 := minimax(ln, 6..100, [3, 3], x -> 1/x, 'maxerror');
+//   f := x -> piecewise(x < 6, f1(x), f2(x));
+//
+// (Continuity: Error is down to the 1e-6 range for x=1, difference between
+// f1 and f2 range at the crossover point is in the 1e-5 range. The cutoff
+// point at x=6 is chosen to get maxerror pretty close between f1 and f2.)
+//
+// Maximum output of ln(x) here is of course ln(100) ~= 4.605. So we can find
+// an approximation for exp over the range -4.605..0, where we care mostly
+// about the relative error:
+//
+//   g := minimax(exp, -ln(100)..0, [3, 3], x -> 1/exp(x), 'maxerror');
+//
+// We can find the worst-case error in dB from this through a simple plot:
+//
+//   dbdiff := (x, y) -> abs(20 * log10(x / y));
+//   plot(dbdiff(g(-f(x)), 1/x), x=1..100);
+//
+// which readily shows the error never to be above ~0.001 dB or so
+// (actually 0.00119 dB, for the case of x=100). y=-1 remains the worst case,
+// it would seem.
+//
+// If we cared even more about speed, we could probably fuse y into
+// the coefficients for ln_nom and postgain into the coefficients for ln_den.
+// But if so, we should probably rather just SIMD the entire thing instead.
+inline float fastpow(float x, float y)
+{
+       float ln_nom, ln_den;
+       if (x < 6.0f) {
+               ln_nom = -0.059237648f + (-0.0165117771f + (0.06818859075f + 0.007560968243f * x) * x) * x;
+               ln_den = 0.0202509098f + (0.08419174188f + (0.03647189417f + 0.001642577975f * x) * x) * x;
+       } else {
+               ln_nom = -0.005430534f + (0.00633589178f + (0.0006319155549f + 0.4789541675e-5f * x) * x) * x;
+               ln_den = 0.0064785099f + (0.003219629109f + (0.0001531823694f + 0.6884656640e-6f * x) * x) * x;
+       }
+       float v = y * ln_nom / ln_den;
+       float exp_nom = 0.2195097621f + (0.08546059868f + (0.01208501759f + 0.0006173448113f * v) * v) * v;
+       float exp_den = 0.2194980791f + (-0.1343051968f + (0.03556072737f - 0.006174398513f * v) * v) * v;
+       return exp_nom / exp_den;
+}
+
 inline float compressor_knee(float x, float threshold, float inv_threshold, float inv_ratio_minus_one, float postgain)
 {
        assert(inv_ratio_minus_one <= 0.0f);
-       if (x > threshold && inv_ratio_minus_one < 0.0f) {
-               return postgain * pow(x * inv_threshold, inv_ratio_minus_one);
+       if (x > threshold) {
+               return postgain * fastpow(x * inv_threshold, inv_ratio_minus_one);
        } else {
                return postgain;
        }
@@ -34,6 +93,17 @@ void StereoCompressor::process(float *buf, size_t num_samples, float threshold,
        float *left_ptr = buf;
        float *right_ptr = buf + 1;
 
+       if (inv_ratio_minus_one >= 0.0) {
+               for (size_t i = 0; i < num_samples; ++i) {
+                       *left_ptr *= makeup_gain;
+                       left_ptr += 2;
+
+                       *right_ptr *= makeup_gain;
+                       right_ptr += 2;
+               }
+               return;
+       }
+
        float peak_level = this->peak_level;
        float compr_level = this->compr_level;
 
@@ -42,9 +112,9 @@ void StereoCompressor::process(float *buf, size_t num_samples, float threshold,
                if (fabs(*right_ptr) > peak_level) peak_level = float(fabs(*right_ptr));
 
                if (peak_level > compr_level) {
-                       compr_level = std::min(compr_level * attack_increment, peak_level);
+                       compr_level = min(compr_level * attack_increment, peak_level);
                } else {
-                       compr_level = std::max(compr_level * release_increment, 0.0001f);
+                       compr_level = max(compr_level * release_increment, 0.0001f);
                }
 
                float scalefactor_with_gain = compressor_knee(compr_level, threshold, inv_threshold, inv_ratio_minus_one, makeup_gain);
@@ -55,7 +125,7 @@ void StereoCompressor::process(float *buf, size_t num_samples, float threshold,
                *right_ptr *= scalefactor_with_gain;
                right_ptr += 2;
 
-               peak_level = std::max(peak_level * peak_increment, 0.0001f);
+               peak_level = max(peak_level * peak_increment, 0.0001f);
        }
 
        // Store attenuation level for debug/visualization.