avfilter/af_aiir: add missing normalization of biquads gains

libavfilter/af_aiir.c

@@ -499,6 +499,7 @@ static int decompose_zp2biquads(AVFilterContext *ctx, int channels)
             double a[6] = { 0 };
             double min_distance = DBL_MAX;
             double max_mag = 0;
+            double factor;
             int i;
 
             for (i = 0; i < iir->nb_ab[0]; i++) {
@@ -593,12 +594,24 @@ static int decompose_zp2biquads(AVFilterContext *ctx, int channels)
             iir->ab[1][2 * nearest_zero.a] = iir->ab[1][2 * nearest_zero.a + 1] = NAN;
             iir->ab[1][2 * nearest_zero.b] = iir->ab[1][2 * nearest_zero.b + 1] = NAN;
 
-            iir->biquads[current_biquad].a[0] = 1.0;
-            iir->biquads[current_biquad].a[1] = a[2] / a[4];
-            iir->biquads[current_biquad].a[2] = a[0] / a[4];
-            iir->biquads[current_biquad].b[0] = (b[4] / a[4]) * (current_biquad ? 1.0 : iir->g);
-            iir->biquads[current_biquad].b[1] = (b[2] / a[4]) * (current_biquad ? 1.0 : iir->g);
-            iir->biquads[current_biquad].b[2] = (b[0] / a[4]) * (current_biquad ? 1.0 : iir->g);
+            iir->biquads[current_biquad].a[0] = a[4];
+            iir->biquads[current_biquad].a[1] = a[2];
+            iir->biquads[current_biquad].a[2] = a[0];
+            iir->biquads[current_biquad].b[0] = b[4];
+            iir->biquads[current_biquad].b[1] = b[2];
+            iir->biquads[current_biquad].b[2] = b[0];
+
+            factor = (iir->biquads[current_biquad].a[0] +
+                      iir->biquads[current_biquad].a[1] +
+                      iir->biquads[current_biquad].a[2]) / 4;
+
+            iir->biquads[current_biquad].b[0] *= factor;
+            iir->biquads[current_biquad].b[1] *= factor;
+            iir->biquads[current_biquad].b[2] *= factor;
+
+            iir->biquads[current_biquad].b[0] *= (current_biquad ? 1.0 : iir->g);
+            iir->biquads[current_biquad].b[1] *= (current_biquad ? 1.0 : iir->g);
+            iir->biquads[current_biquad].b[2] *= (current_biquad ? 1.0 : iir->g);
 
             av_log(ctx, AV_LOG_VERBOSE, "a=%f %f %f:b=%f %f %f\n",
                    iir->biquads[current_biquad].a[0],