audio_core: Preserve front channel volume after 6 to 2 downmix

Many games report 6 channel output while only providing data for 2. We only output 2-channel audio regardless, and in the downmixing, front left/right only provide 36% of their volume. This is done assuming all of the other channels also contain valid data, but in many games they don't. This PR alters the downmixing to preserve front left/right, so volume is not lost.

This improves volume in Link's Awakening, New Super Mario Bros U, Disgaea 6, Super Kirby Clash.

1 files changed, 9 insertions, 6 deletions

diff --git a/src/audio_core/sink_context.cpp b/src/audio_core/sink_context.cpp
index a69543696..cc55b290c 100644
--- a/src/audio_core/sink_context.cpp
+++ b/src/audio_core/sink_context.cpp
@@ -15,10 +15,17 @@ std::size_t SinkContext::GetCount() const {
 void SinkContext::UpdateMainSink(const SinkInfo::InParams& in) {
     ASSERT(in.type == SinkTypes::Device);
 
-    has_downmix_coefs = in.device.down_matrix_enabled;
-    if (has_downmix_coefs) {
+    if (in.device.down_matrix_enabled) {
         downmix_coefficients = in.device.down_matrix_coef;
+    } else {
+        downmix_coefficients = {
+            1.0f,   // front
+            0.707f, // center
+            0.0f,   // lfe
+            0.707f, // back
+        };
     }
+
     in_use = in.in_use;
     use_count = in.device.input_count;
     buffers = in.device.input;
@@ -34,10 +41,6 @@ std::vector<u8> SinkContext::OutputBuffers() const {
     return buffer_ret;
 }
 
-bool SinkContext::HasDownMixingCoefficients() const {
-    return has_downmix_coefs;
-}
-
 const DownmixCoefficients& SinkContext::GetDownmixCoefficients() const {
     return downmix_coefficients;
 }