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

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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
This commit is contained in:
Max Kellermann
2019-07-05 21:01:01 +02:00
parent a784c8b1ae
commit af99f9fc90
9 changed files with 174 additions and 22 deletions
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2
test/software_volume.cxx
View File

@@ -50,7 +50,7 @@ try {
audio_format = ParseAudioFormat(argv[1], false);
PcmVolume pv;
const auto out_sample_format = pv.Open(audio_format.format);
const auto out_sample_format = pv.Open(audio_format.format, false);
if (out_sample_format != audio_format.format)
fprintf(stderr, "Converting to %s\n",

45
test/test_pcm_volume.cxx
View File

@@ -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];