ao_wasapi: Slightly improve timer accuracy

Use QueryPerformanceCounter to improve the accuracy of
IAudioClock::GetPosition.

While this is mainly for "realtime correctness" (usually the delay is a
single sample or less), there are cases where IAudioClock::GetPosition
takes a long time to return from its call (though the documentation doesn't
define what a "long time" is), so correcting its value might be important in
case the documented possible delay happens.

1 files changed, 12 insertions, 3 deletions

diff --git a/audio/out/ao_wasapi.c b/audio/out/ao_wasapi.c
index fdca69c20c..39199159b3 100644
--- a/audio/out/ao_wasapi.c
+++ b/audio/out/ao_wasapi.c
@@ -121,6 +121,7 @@ typedef struct wasapi_state {
     IAudioClock *pAudioClock;
     UINT64 clock_frequency; /* scale for the "samples" returned by the clock */
     UINT64 sample_count; /* the amount of samples per channel written to a GetBuffer buffer */
+    LARGE_INTEGER qpc_frequency; /* frequency of windows' high resolution timer */
 
     int opt_exclusive;
     int opt_list;
@@ -528,6 +529,8 @@ static int init_clock(struct wasapi_state *state) {
     hr = IAudioClock_GetFrequency(state->pAudioClock, &state->clock_frequency);
     EXIT_ON_ERROR(hr);
 
+    QueryPerformanceFrequency(&state->qpc_frequency);
+
     state->sample_count = 0;
 
     MP_VERBOSE(state, "IAudioClock::GetFrequency gave a frequency of %"PRIu64".\n", (uint64_t) state->clock_frequency);
@@ -1362,19 +1365,25 @@ static int play(struct ao *ao, void **data, int samples, int flags)
 }
 
 static float get_device_delay(struct wasapi_state *state) {
-    /* where we pray that this hasn't desynced */
+    /* where we pray that sample_count hasn't desynced */
     mp_memory_barrier();
     UINT64 sample_count = state->sample_count;
-    UINT64 position;
+    UINT64 position, qpc_position;
     HRESULT hr;
 
-    switch (hr = IAudioClock_GetPosition(state->pAudioClock, &position, NULL)) {
+    switch (hr = IAudioClock_GetPosition(state->pAudioClock, &position, &qpc_position)) {
         case S_OK: case S_FALSE:
             break;
         default:
             MP_ERR(state, "IAudioClock::GetPosition returned %s\n", explain_err(hr));
     }
 
+    LARGE_INTEGER qpc_count;
+    QueryPerformanceCounter(&qpc_count);
+    UINT64 qpc_diff = (qpc_count.QuadPart * 10000000 / state->qpc_frequency.QuadPart) - qpc_position;
+
+    position += state->clock_frequency * qpc_diff / 10000000;
+
     /* convert position to the same base as sample_count */
     position = position * state->format.Format.nSamplesPerSec / state->clock_frequency;