Improve playback consistency for realtime audio during inconsistent framerates

Implements and uses `GetStableFrameTimeScale` instead of `GetLastFrameTimeScale` for realtime audio playback

src/audio_core/hle/hle.cpp

@@ -419,7 +419,7 @@ void DspHle::Impl::AudioTickCallback(s64 cycles_late) {
     // Reschedule recurrent event
     const double time_scale =
         Settings::values.enable_realtime_audio
-            ? std::clamp(Core::System::GetInstance().GetLastFrameTimeScale(), 1.0, 3.0)
+            ? std::clamp(Core::System::GetInstance().GetStableFrameTimeScale(), 1.0, 3.0)
             : 1.0;
     s64 adjusted_ticks = static_cast<s64>(audio_frame_ticks / time_scale - cycles_late);
     core_timing.ScheduleEvent(adjusted_ticks, tick_event);

src/core/core.cpp

@@ -392,8 +392,8 @@ PerfStats::Results System::GetLastPerfStats() {
     return perf_stats ? perf_stats->GetLastStats() : PerfStats::Results{};
 }
 
-double System::GetLastFrameTimeScale() {
-    return perf_stats->GetLastFrameTimeScale();
+double System::GetStableFrameTimeScale() {
+    return perf_stats->GetStableFrameTimeScale();
 }
 
 void System::Reschedule() {

src/core/core.h

@@ -193,7 +193,7 @@ public:
 
     [[nodiscard]] PerfStats::Results GetLastPerfStats();
 
-    double GetLastFrameTimeScale();
+    double GetStableFrameTimeScale();
 
     /**
      * Gets a reference to the emulated CPU.

src/core/perf_stats.cpp

@@ -129,6 +129,17 @@ double PerfStats::GetLastFrameTimeScale() const {
     return duration_cast<DoubleSecs>(previous_frame_length).count() / FRAME_LENGTH;
 }
 
+double PerfStats::GetStableFrameTimeScale() const {
+    std::scoped_lock lock{object_mutex};
+
+    constexpr double FRAME_LENGTH_MILLIS = (1.0 / SCREEN_REFRESH_RATE) * 1000;
+    const short num_frames = std::min(50ul, current_index + 1);
+    const double sum = std::accumulate(perf_history.begin() + current_index - num_frames,
+                                       perf_history.begin() + current_index, 0.0);
+    const double stable_frame_length = sum / num_frames;
+    return stable_frame_length / FRAME_LENGTH_MILLIS;
+}
+
 void FrameLimiter::WaitOnce() {
     if (frame_advancing_enabled) {
         // Frame advancing is enabled: wait on event instead of doing framelimiting

src/core/perf_stats.h

@@ -82,6 +82,12 @@ public:
      */
     double GetLastFrameTimeScale() const;
 
+    /**
+     * Has the same functionality as GetLastFrameTimeScale, but uses the mean frame time over the
+     * last 50 frames rather than only the frame time of the previous frame.
+     */
+    double GetStableFrameTimeScale() const;
+
     void AddArticBaseTraffic(u32 bytes) {
         artic_transmitted += bytes;
     }