Merge branch '2.3' of git@github.com:mixxxdj/mixxx.git

# Conflicts:
#	src/util/sandbox.cpp

1 files changed, 17 insertions, 7 deletions

diff --git a/src/sources/soundsourceflac.cpp b/src/sources/soundsourceflac.cpp
index a6624a7e82..4e97e15f0a 100644
--- a/src/sources/soundsourceflac.cpp
+++ b/src/sources/soundsourceflac.cpp
@@ -399,16 +399,26 @@ namespace {
 // https://bugs.launchpad.net/mixxx/+bug/1769717
 // https://hydrogenaud.io/index.php/topic,61792.msg559045.html#msg559045
 
-// We will shift decoded samples left by (32 - m_bitsPerSample) to
-// get rid of the garbage in the most significant bits before scaling
-// to the range [-CSAMPLE_PEAK, CSAMPLE_PEAK - epsilon] with
-// epsilon = 1 / 2 ^ bitsPerSample.
-constexpr CSAMPLE kSampleScaleFactor = CSAMPLE_PEAK /
+// We multiply the decoded samples by 2 ^ (32 - m_bitsPerSample) to
+// get rid of the garbage in the most significant bits which get shifted
+// out to the left. The resulting, upscaled integer value is then scaled
+// down to the floating point range [-CSAMPLE_PEAK, CSAMPLE_PEAK - epsilon]
+// with epsilon = 1 / 2 ^ bitsPerSample.
+//
+// We have to negate the nominator to compensate for the negative denominator!
+// Otherwise the phase would be inverted: https://bugs.launchpad.net/mixxx/+bug/1933001
+constexpr CSAMPLE kSampleScaleFactor = -CSAMPLE_PEAK /
         (static_cast<FLAC__int32>(1) << std::numeric_limits<FLAC__int32>::digits);
 
 inline CSAMPLE convertDecodedSample(FLAC__int32 decodedSample, int bitsPerSample) {
-    DEBUG_ASSERT(std::numeric_limits<FLAC__int32>::is_signed);
-    return (decodedSample << ((std::numeric_limits<FLAC__int32>::digits + 1) - bitsPerSample)) * kSampleScaleFactor;
+    static_assert(std::numeric_limits<FLAC__int32>::is_signed);
+    static_assert(kSampleScaleFactor > CSAMPLE_ZERO, "preserve phase of decoded signal");
+    // Multiples by 2 ^ (32 - bitsPerSample)
+    const auto upscaleShift = (std::numeric_limits<FLAC__int32>::digits + 1) - bitsPerSample;
+    const auto upscaledSample = decodedSample << upscaleShift;
+    // upscaledSample is now a 32-bit signed integer that needs to scaled
+    // to the range [-CSAMPLE_PEAK, CSAMPLE_PEAK - epsilon]
+    return upscaledSample * kSampleScaleFactor;
 }
 
 } // anonymous namespace