Specify number of channels in the data passed to SoundAssetWriter, and pad channels that aren't there.

2 files changed, 11 insertions, 8 deletions

diff --git a/src/sound_asset_writer.cc b/src/sound_asset_writer.cc
index a0414aba..f789eb3b 100644
--- a/src/sound_asset_writer.cc
+++ b/src/sound_asset_writer.cc
@@ -200,29 +200,30 @@ LIBDCP_ENABLE_WARNINGS
 
 
 void
-SoundAssetWriter::write (float const * const * data, int frames)
+SoundAssetWriter::write(float const * const * data, int data_channels, int frames)
 {
 	DCP_ASSERT (!_finalized);
 	DCP_ASSERT (frames > 0);
 
+	auto const asset_channels = _asset->channels();
+	DCP_ASSERT (data_channels <= asset_channels);
+
 	static float const clip = 1.0f - (1.0f / pow (2, 23));
 
 	if (!_started) {
 		start ();
 	}
 
-	int const ch = _asset->channels ();
-
 	for (int i = 0; i < frames; ++i) {
 
 		byte_t* out = _state->frame_buffer.Data() + _frame_buffer_offset;
 
-		/* Write one sample per channel */
-		for (int j = 0; j < ch; ++j) {
+		/* Write one sample per asset channel */
+		for (int j = 0; j < asset_channels; ++j) {
 			int32_t s = 0;
 			if (j == 13 && _sync) {
 				s = _fsk.get();
-			} else {
+			} else if (j < data_channels) {
 				/* Convert sample to 24-bit int, clipping if necessary. */
 				float x = data[j][i];
 				if (x > clip) {
@@ -236,7 +237,7 @@ SoundAssetWriter::write (float const * const * data, int frames)
 			*out++ = (s & 0xff00) >> 8;
 			*out++ = (s & 0xff0000) >> 16;
 		}
-		_frame_buffer_offset += 3 * ch;
+		_frame_buffer_offset += 3 * asset_channels;
 
 		DCP_ASSERT (_frame_buffer_offset <= int(_state->frame_buffer.Capacity()));
 
diff --git a/src/sound_asset_writer.h b/src/sound_asset_writer.h
index 0435b85f..6d8191c7 100644
--- a/src/sound_asset_writer.h
+++ b/src/sound_asset_writer.h
@@ -70,9 +70,11 @@ public:
 	~SoundAssetWriter();
 
 	/** @param data Pointer an array of float pointers, one for each channel.
+	 *  @param channels Number of channels in data; if this is less than the channels in the asset
+	 *  the remaining asset channels will be padded with silence.
 	 *  @param frames Number of frames i.e. number of floats that are given for each channel.
 	 */
-	void write (float const * const *, int);
+	void write(float const * const * data, int channels, int frames);
 
 	bool finalize () override;