Changeset 1335

Timestamp:

02/13/08 11:00:30 (6 months ago)

Author:

rossb

Message:

added explanatory comment re DS API behavior

Files:

• portaudio/branches/v19-devel/src/hostapi/dsound/pa_win_ds.c (modified) (3 diffs)

portaudio/branches/v19-devel/src/hostapi/dsound/pa_win_ds.c

@@ -1914 +1914 @@
         long            bytesExpected;
         long            buffersWrapped;
+
         QueryPerformanceCounter( &currentTime );
         elapsedTime.QuadPart = currentTime.QuadPart - stream->previousPlayTime.QuadPart;
         stream->previousPlayTime = currentTime;
+
         /* How many bytes does DirectSound say have been played. */
         bytesPlayed = playCursor - stream->previousPlayCursor;
         if( bytesPlayed < 0 ) bytesPlayed += stream->outputBufferSizeBytes; // unwrap
         stream->previousPlayCursor = playCursor;
+
         /* Calculate how many bytes we would have expected to been played by now. */
         bytesExpected = (long) ((elapsedTime.QuadPart * stream->outputBufferSizeBytes) / stream->perfCounterTicksPerBuffer.QuadPart);
@@ -1934 +1937 @@
     numBytesEmpty = playCursor - stream->outputBufferWriteOffsetBytes;
     if( numBytesEmpty < 0 ) numBytesEmpty += stream->outputBufferSizeBytes; // unwrap offset
+
     /* Have we underflowed? */
     if( numBytesEmpty > (stream->outputBufferSizeBytes - playWriteGap) )
@@ -1941 +1945 @@
             stream->outputUnderflowCount += 1;
         }
+
+        /*
+            From MSDN:
+                The write cursor indicates the position at which it is safe  
+            to write new data to the buffer. The write cursor always leads the
+            play cursor, typically by about 15 milliseconds' worth of audio
+            data.
+                It is always safe to change data that is behind the position 
+            indicated by the lpdwCurrentPlayCursor parameter.
+        */
+
         stream->outputBufferWriteOffsetBytes = writeCursor;
         numBytesEmpty = stream->outputBufferSizeBytes - playWriteGap;