Add ability to reverse global stereo output

Closes #307.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Ability to reverse global left and right audio channels (new configuration file option) to work around broken hardware (issue #307).
+
 ### Changed
 
 - Update to libmt32emu v2.7.0.

include/config.def

@@ -53,6 +53,7 @@ BEGIN_SECTION(audio)
 CFG(output_device,		TAudioOutputDevice,		AudioOutputDevice,			TAudioOutputDevice::PWM				)
 CFG(sample_rate,		int,				AudioSampleRate,			48000						)
 CFG(chunk_size,			int,				AudioChunkSize,				256						)
+CFG(reversed_stereo,		bool,				AudioReversedStereo,			false						)
 END_SECTION
 
 BEGIN_SECTION(control)

sdcard/mt32-pi.cfg

@@ -147,6 +147,13 @@ sample_rate = 48000
 # Values: 2-2048 (256*)
 chunk_size = 256
 
+# Set whether the stereo channels should be swapped or not.
+#
+# Use this option to work around wrongly-wired audio hardware.
+#
+# Values: on, off*
+reversed_stereo = off
+
 # -----------------------------------------------------------------------------
 # Control options
 # -----------------------------------------------------------------------------

src/mt32pi.cpp

@@ -562,6 +562,7 @@ void CMT32Pi::AudioTask()
 
 	// Circle's "fast path" for I2S 24-bit really expects 32-bit samples
 	const bool bI2S = m_pConfig->AudioOutputDevice == CConfig::TAudioOutputDevice::I2S;
+	const bool bReversedStereo = m_pConfig->AudioReversedStereo;
 	const u8 nBytesPerSample = bI2S ? sizeof(s32) : (sizeof(s8) * 3);
 	const u8 nBytesPerFrame = 2 * nBytesPerSample;
 
@@ -578,12 +579,26 @@ void CMT32Pi::AudioTask()
 
 		m_pCurrentSynth->Render(FloatBuffer, nFrames);
 
+		if (bReversedStereo)
+		{
+			// Convert to signed 24-bit integers with channel swap
+			for (size_t i = 0; i < nFrames * nChannels; i += nChannels)
+			{
+				s32* const pLeftSample = reinterpret_cast<s32*>(IntBuffer + i * nBytesPerSample);
+				s32* const pRightSample = reinterpret_cast<s32*>(IntBuffer + (i + 1) * nBytesPerSample);
+				*pLeftSample = FloatBuffer[i + 1] * Sample24BitMax;
+				*pRightSample = FloatBuffer[i] * Sample24BitMax;
+			}
+		}
+		else
+		{
 			// Convert to signed 24-bit integers
 			for (size_t i = 0; i < nFrames * nChannels; ++i)
 			{
 				s32* const pSample = reinterpret_cast<s32*>(IntBuffer + i * nBytesPerSample);
 				*pSample = FloatBuffer[i] * Sample24BitMax;
 			}
+		}
 
 		const int nResult = m_pSound->Write(IntBuffer, nWriteBytes);
 		if (nResult != static_cast<int>(nWriteBytes))