Have GetActiveChannelCount return the system channels instead of host device channels

src/audio_core/sink/cubeb_sink.cpp

@@ -253,8 +253,9 @@ CubebSink::~CubebSink() {
 #endif
 }
 
-SinkStream* CubebSink::AcquireSinkStream(Core::System& system, u32 system_channels,
+SinkStream* CubebSink::AcquireSinkStream(Core::System& system, u32 system_channels_,
                                          const std::string& name, StreamType type) {
+    system_channels = system_channels_;
     SinkStreamPtr& stream = sink_streams.emplace_back(std::make_unique<CubebSinkStream>(
         ctx, device_channels, system_channels, output_device, input_device, name, type, system));
 

src/audio_core/sink/sdl2_sink.cpp

@@ -168,8 +168,9 @@ SDLSink::SDLSink(std::string_view target_device_name) {
 
 SDLSink::~SDLSink() = default;
 
-SinkStream* SDLSink::AcquireSinkStream(Core::System& system, u32 system_channels,
+SinkStream* SDLSink::AcquireSinkStream(Core::System& system, u32 system_channels_,
                                        const std::string&, StreamType type) {
+    system_channels = system_channels_;
     SinkStreamPtr& stream = sink_streams.emplace_back(std::make_unique<SDLSinkStream>(
         device_channels, system_channels, output_device, input_device, type, system));
     return stream.get();

src/audio_core/sink/sink.h

@@ -85,9 +85,21 @@ public:
      */
     virtual void SetSystemVolume(f32 volume) = 0;
 
+    /**
+     * Get the number of channels the game has set, can be different to the host hardware's support.
+     * Either 2 or 6.
+     *
+     * @return Number of device channels.
+     */
+    u32 GetSystemChannels() const {
+        return system_channels;
+    }
+
 protected:
     /// Number of device channels supported by the hardware
     u32 device_channels{2};
+    /// Number of channels the game is sending
+    u32 system_channels{2};
 };
 
 using SinkPtr = std::unique_ptr<Sink>;

src/audio_core/sink/sink_stream.cpp

@@ -40,29 +40,38 @@ void SinkStream::AppendBuffer(SinkBuffer& buffer, std::span<s16> samples) {
 
     if (system_channels == 6 && device_channels == 2) {
         // We're given 6 channels, but our device only outputs 2, so downmix.
-        static constexpr std::array<f32, 4> down_mix_coeff{1.0f, 0.707f, 0.251f, 0.707f};
+        // Front = 1.0
+        // Center = 0.596
+        // Back = 0.707
+        // LFE = 0.354
+        // 1.0 + 0.596 + 0.707 + 0.354 = 2.657, 1/2.657 = 0.37636f downscale coefficient
+        static constexpr std::array<f32, 4> down_mix_coeff{0.37636f, 0.22431056f, 0.13323144f,
+                                                           0.26608652f};
 
         for (u32 read_index = 0, write_index = 0; read_index < samples.size();
              read_index += system_channels, write_index += device_channels) {
+            const auto fl =
+                static_cast<f32>(samples[read_index + static_cast<u32>(Channels::FrontLeft)]);
+            const auto fr =
+                static_cast<f32>(samples[read_index + static_cast<u32>(Channels::FrontRight)]);
+            const auto c =
+                static_cast<f32>(samples[read_index + static_cast<u32>(Channels::Center)]);
+            const auto lfe =
+                static_cast<f32>(samples[read_index + static_cast<u32>(Channels::LFE)]);
+            const auto bl =
+                static_cast<f32>(samples[read_index + static_cast<u32>(Channels::BackLeft)]);
+            const auto br =
+                static_cast<f32>(samples[read_index + static_cast<u32>(Channels::BackRight)]);
+
             const auto left_sample{
-                ((Common::FixedPoint<49, 15>(
+                static_cast<s32>((fl * down_mix_coeff[0] + c * down_mix_coeff[1] +
-                      samples[read_index + static_cast<u32>(Channels::FrontLeft)]) *
+                                  lfe * down_mix_coeff[2] + bl * down_mix_coeff[3]) *
-                      down_mix_coeff[0] +
+                                 volume)};
-                  samples[read_index + static_cast<u32>(Channels::Center)] * down_mix_coeff[1] +
-                  samples[read_index + static_cast<u32>(Channels::LFE)] * down_mix_coeff[2] +
-                  samples[read_index + static_cast<u32>(Channels::BackLeft)] * down_mix_coeff[3]) *
-                 volume)
-                    .to_int()};
 
             const auto right_sample{
-                ((Common::FixedPoint<49, 15>(
+                static_cast<s32>((fr * down_mix_coeff[0] + c * down_mix_coeff[1] +
-                      samples[read_index + static_cast<u32>(Channels::FrontRight)]) *
+                                  lfe * down_mix_coeff[2] + br * down_mix_coeff[3]) *
-                      down_mix_coeff[0] +
+                                 volume)};
-                  samples[read_index + static_cast<u32>(Channels::Center)] * down_mix_coeff[1] +
-                  samples[read_index + static_cast<u32>(Channels::LFE)] * down_mix_coeff[2] +
-                  samples[read_index + static_cast<u32>(Channels::BackRight)] * down_mix_coeff[3]) *
-                 volume)
-                    .to_int()};
 
             samples[write_index + static_cast<u32>(Channels::FrontLeft)] =
                 static_cast<s16>(std::clamp(left_sample, min, max));

src/core/hle/service/audio/audren_u.cpp

@@ -359,7 +359,7 @@ private:
 
     void GetActiveChannelCount(HLERequestContext& ctx) {
         const auto& sink{system.AudioCore().GetOutputSink()};
-        u32 channel_count{sink.GetDeviceChannels()};
+        u32 channel_count{sink.GetSystemChannels()};
 
         LOG_DEBUG(Service_Audio, "(STUBBED) called. Channels={}", channel_count);