pcm/Volume: convert S16 to S24 to preserve quality and reduce noise

Applying software volume to S16 samples means several bits of
precision are lost; at 25% volume, two bits are lost.  Additionally,
dithering adds some noise.

The problem gets worse when you apply the software volume code twice:
for the software mixer volume, and again for the replay gain.  This
loses some more precision and adds even more dithering noise, which
can become audible (see
https://github.com/MusicPlayerDaemon/MPD/issues/542).

By converting everything to 24 bit, we need to shift only two bits to
the right instead of ten, losing nearly no precision, and dithering is
not needed.  Even if the output device is unable to play S24 directly,
we can convert back to S16 with only one stage of dithering.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/542

1 files changed, 43 insertions, 2 deletions

diff --git a/test/test_pcm_volume.cxx b/test/test_pcm_volume.cxx
index 536fee9e3..35ad845de 100644
--- a/test/test_pcm_volume.cxx
+++ b/test/test_pcm_volume.cxx
@@ -36,7 +36,7 @@ TestVolume(G g=G())
 	typedef typename Traits::value_type value_type;
 
 	PcmVolume pv;
-	EXPECT_EQ(pv.Open(F), F);
+	EXPECT_EQ(pv.Open(F, false), F);
 
 	constexpr size_t N = 509;
 	static value_type zero[N];
@@ -77,6 +77,47 @@ TEST(PcmTest, Volume16)
 	TestVolume<SampleFormat::S16>();
 }
 
+TEST(PcmTest, Volume16to32)
+{
+	constexpr SampleFormat F = SampleFormat::S16;
+	typedef int16_t value_type;
+	RandomInt<value_type> g;
+
+	PcmVolume pv;
+	EXPECT_EQ(pv.Open(F, true), SampleFormat::S24_P32);
+
+	constexpr size_t N = 509;
+	static value_type zero[N];
+	const auto _src = TestDataBuffer<value_type, N>(g);
+	const ConstBuffer<void> src(_src, sizeof(_src));
+
+	pv.SetVolume(0);
+	auto dest = pv.Apply(src);
+	EXPECT_EQ(src.size * 2, dest.size);
+	EXPECT_EQ(0, memcmp(dest.data, zero, sizeof(zero)));
+
+	pv.SetVolume(PCM_VOLUME_1);
+	dest = pv.Apply(src);
+	EXPECT_EQ(src.size * 2, dest.size);
+	auto s = ConstBuffer<int16_t>::FromVoid(src);
+	auto d = ConstBuffer<int32_t>::FromVoid(dest);
+	for (size_t i = 0; i < N; ++i)
+		EXPECT_EQ(d[i], s[i] << 8);
+
+	pv.SetVolume(PCM_VOLUME_1 / 2);
+	dest = pv.Apply(src);
+	EXPECT_EQ(src.size * 2, dest.size);
+
+	s = ConstBuffer<int16_t>::FromVoid(src);
+	d = ConstBuffer<int32_t>::FromVoid(dest);
+	for (unsigned i = 0; i < N; ++i) {
+		const int32_t expected = (s[i] << 8) / 2;
+		EXPECT_EQ(d[i], expected);
+	}
+
+	pv.Close();
+}
+
 TEST(PcmTest, Volume24)
 {
 	TestVolume<SampleFormat::S24_P32>(RandomInt24());
@@ -90,7 +131,7 @@ TEST(PcmTest, Volume32)
 TEST(PcmTest, VolumeFloat)
 {
 	PcmVolume pv;
-	pv.Open(SampleFormat::FLOAT);
+	pv.Open(SampleFormat::FLOAT, false);
 
 	constexpr size_t N = 509;
 	static float zero[N];